TABL . VIII. Electric See also:

• VOLUME

Ethyl ether 1.175 to 3.760 W. Kohlrausch. Benzene 4.700 Absolutely pure water ap- 25.0 at 18° C. Value estimated proximates probably to by F. See also:

• KOHL

For instance, one equivalent gramme molecule of See also:

• SODIUM [symbol Na, from Lat. natrium; atomic weight 23.00 (0=16)]

. 34,000 34• 46° See also:

• MANGANESE (symbol Mn; atomic weight, 54.93 (0=16)], a metallic chemical element. Its dioxide (pyrolusite)

If, in the first See also:

• PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

The lengths and mean diameters of the wires were carefully measured, and their resistance was then taken at certain known temperatures obtained by immersing the coils in boiling See also:

• ANILINE, PHENYLADIINE, or AMINOBENZENE, (C6H5NH2)

The resistivity curves of the magnetic metals are also remarkable for the change of curvature they exhibit at the magnetic See also:

• CRITICAL (in alphabetical order of authors)

A fact of considerable See also:

• INTEREST

427). The temperature coefficient of pure copper is an important See also:

• CONSTANT, BENJAMIN (1845-1902)

In the Absolute Methods the resistivity is determined without reference to any other substance, but with reference only to the fundamental standards of length, mass and time. Immense labour has been expended in investigations concerned with the See also:

• PRODUCTION (Lat. productionem, from producere, to pro-duce)

Thick copper rods are connected to the terminals of the wire in the interior of the case, and brought to the outside, being carefully insulated at the same time from one another and from the case. The coil so constructed can be placed under water or paraffin oil, the temperature of whi,.ch can be exactly observed during the See also:

• PROCESS

The chemical phenomena are considered in the See also:

• ARTICLE (from Lat. articulus, a joint)

The essence of the law is the proportionality between C and E, which means that the ratio E/C is a constant. See also:

• BUTE
• BUTE, JOHN STUART, 3RD EARL OF (1713-1792)

The electromotive force of polarization is due to the deposition of films of the products of chemical decomposition on the surface of the electrodes, and only reaches its full value when a continuous film is formed. If the current be stopped before such a film is completed, the reverse electromotive force is less than its full value. A given current flowing for a given time deposits a definite amount of substance on the electrodes; and therefore the amount per unit See also:

• AREA

Any convenient source of alternating current may be used. The currents from the secondary circuit of a small See also:

• INDUCTION (from Lat. inducere, to lead into; cf. Gr. ilrayu yli)

321). The current from one or two voltaic cells is led to an ebonite See also:

• DRUM (early forms drome or dromme, a word common to many Teut. languages, cf. Dan. tromme, Ger. Trommel: the word is ultimately the same as " trumpet," and is probably onomatopoeic in origin; it appears late in Eng. about the middle of the 16th century)

The absolute resistances of certain solutions have been determined by Kohlrausch by comparison with mercury, and, by using one of these solutions in any cell, the constant of that cell may be found once for all. From the observed resistance of any given solution in the cell the resistance of a centimetre cube—the so-called specific resistance—may be calculated. The reciprocal of this, or the conductivity, is a more generally useful constant; it is conveniently expressed in terms of a unit equal to the reciprocal of an ohm. Thus Kohlrausch found that a solution of See also:

• POTASSIUM [symbol K (from kalium), atomic weight 39.114 0=16)]

The other two arms are made of coils of wire of equal resistance, and metallic resistance is added to the shorter tube till the bridge is balanced. Direct currents of somewhat high electromotive force are used to work the bridge. Equal currents then flow through the two tubes; the effects of polarization and dilution must be the same in each, and the resistance added to the shorter tube must be equal to the resistance of a See also:

• COLUMN (Lat. columna)

Whatever be the contact potential difference between zinc and its solution, it is the same at both ends, and thus the potential difference between the zinc rods is equal to that between the liquid at the two ends of the tube. This potential difference may be measured without passing any appreciable current through the tapping electrodes, and thus the resistance of the liquid deduced. Equivalent Conductivity of Solutions.—As is the case in the other properties of solutions, the phenomena are much more simple when the concentration is small than when it is great, and a study of dilute solutions is therefore the best way of getting an insight into the essential principles of the subject. The See also:

• FOUNDATION (Lat. fundatio, from fundare, to found)

The curve for the neutral salt comes to a limiting value; that for the acid attains a maximum at a certain very small concentration, and falls again when the dilution carried farther. It has usually been considered that this destruc- tion of conductivity is due to chemical action between the acid and the residual impurities in the water. At such great dilution these impurities are present in quantities comparable with the amount of acid which they convert into a less highly conducting neutral salt. In the case of acids, then, the maxi- mum must be taken as the limiting value. The decrease in equivalent conductivity at great dilution is, however, so constant that this explanation seems insufficient. The true cause of the phenomenon may perhaps be connected with the fact that the bodies in which it occurs, acids and alkalis, contain the ions, hydrogen in the one case, hydroxyl in the other, which are present in the solvent, water, and have, perhaps because of this relation, velocities higher than those of any other ions. The values of` the molecular conductivities of all neutral salts are, at great dilution, of the same order of magnitude, while those of acids at their See also:

• MAXIMA

T. Trouton (B.A. Report, 1886, p. 312). Now, Ohm's Law implies that no work is done by the current in overcoming reversible electromotive forces such as those of polarization. Thus the molecular inter-change of ions, which must occur in order that the products may be able to work their way through the liquid and appear at the electrodes, continues throughout the solution whether a current is flowing or not. The influence of the current on the ions is merely directive, and, when it flows, streams of electrified ions travel in opposite directions, and, if the applied electromotive force is enough to overcome the See also:

• LOCAL

A cation, i.e. an ion giving up its charge at the cathode, as the electrode at which the current leaves the solution is called, carries a positive charge of electricity; an anion, travelling in the opposite direction, carries a negative charge. It will now be seen that the quantity of electricity flowing per second, i.e. the currentthrough the solution, depends on (I) the number of the ions concerned, (2) the charge on each ion, and (3) the velocity with which the ions travel past each other. Now, the number of ions is given by the concentration of the solution, for even if all the ions are not actively engaged in carrying the current at the same instant, they must, on any dynamical See also:

• IDEA (Gr. Ibia, connected with i&eiv, to see; cf. Lat. species from specere, to look at)

Now let us consider the amounts of potassium and chlorine liberated at the electrodes by this current. At the cathode, if the chlorine ions were at See also:

• REST (O. Eng. rest, reste, bed, cognate with other Teutonic forms, e.g. Ger. Rast, Riiste, rest, and probably Gothic Rasta, league, i.e. resting or stopping place)

In order to obtain the absolute velocities u and v, we must find some other relation between them. Let us resolve u into 2 (u+v) in one direction, say to the right, and 1(u—v) to the left. Similarly v can be resolved into i(v+u) to the left and (s—u) to the right. On pairing these velocities we have a combined movement of the ions to the right, with a speed of 2(u—v) and a See also:

• DRIFT (from "drive ")

I ; 106, pp. 337 and 513) was able to deduce the ratio of the two velocities or simple salts when no complex ions are present, and many further m experiments have been made on the subject (see Das Leitvern:ogen der Elektrolyte). By combining the results thus obtained with the sum of the velocities, as determined from the conductivities, Kohlrausch calculated the absolute velocities of different ions under stated conditions. Thus, in the case of the solution of potassium chloride considered above, Hittorf's experiments show us that the ratio of the velocity of the anion to that of the cation in this solution,is .51 : •49. The absolute velocity of the potassium ion under unit potential gradient is therefore 0.000567 cm. per sec., and that of the chlorine ion 0.000592 cm. per sec. Similar calculations can be made for solutions of other concentrations, and of different substances. Table IX. shows Kohlrausch's values for the ionic velocities of three chlorides of alkali metals at 18° C., calculated for a potential gradient of r volt per cm.; the See also:

• NUMBERS, BOOK OF
• NUMBERS, PARTITION OF

Na . 45 I Li . 36 NO3 . 6 ,, NH4 . 66 OH 162 H . 320 C2H3O2 36 Ag • 57 C3H602 33 Having obtained these numbers we can deduce the conductivity of the dilute solution of any salt, and the comparison of the calculated with the observed values furnished the first See also:

• CONFIRMATION (Lat. confirmatio, from confirmare, to establish, make firm)

If we have a potential gradient of 1 volt per centimetre the electric force is 1os in C.G.S. units. The charge of electricity on I gram-equivalent of any ion is 1/•0001036=9653 units, hence the See also:

• MECHANICAL

27 „ 390 „ NO3 . . 15 , 25 NH4 15 ,, 83 9, OH 5.4 ,, 32 H . . . 3•I „ 310 C2H3O2. . 27 „ 46 „ Ag 17 16 C,H3O2 30 ,, 41 „ Since the ions move with See also:

• UNIFORM

By this method the velocity of the hydrogen ion through a jelly solution under a known potential gradient was observed to about o•oo26 cm. per sec., a number of the same order as that required by Kohlrausch's theory. Direct determinations of the velocities of a few other ions have been made by W. C. D. Whetham (Phil. Trans, vol. 184, A, p. 337; vol. 186, A, p. 507; Phil. See also:

• HAG

In one case, for example, decinormal solutions of potassium carbonate and potassium bichromate were used. The colour of the latter is due to the presence of the bichromate See also:

• GROUP

192, A, p. 331). He placed the gelatine solution of a salt, potassium chloride, for example, in a horizontal glass tube, and found the rate of See also:

• MIGRATION

See also:

• DENISON
• DENISON, GEORGE ANTHONY (1805-1896)
• DENISON, GEORGE TAYLOR (1839- )

The velocity of elementary ions is found to be a periodic function of the atomic weight, similar elements lying on corresponding portions of a curve drawn to See also:

• EXPRESS (through the French from the past participle of the Lat. exprimere, to press out, by transference used of representing objects in painting or sculpture, or of thoughts, &c. in words)

. . H7C402 3o-8 - 3.5 Valerie . . . H9C602 28.8 - 2.0 Caprionic„ . . . H110602 27'4 - 1.4 Nature of Electrolytes.—We have as yet said nothing about the fundamental cause of electrolytic activity, nor considered why, for example, a solution of potassium chloride is a good conductor, while a solution of See also:

• SUGAR

The velocity with which they work their way through the liquid must then increase as such molecular rearrangements become more frequent, and will therefore depend on the number of solute molecules, i.e. on theconcentration. On this supposition the observed constancy of velocity would be impossible. We shall therefore adopt as a working hypothesis the theory, confirmed by other phenomena (see ELECTROLYSIS), that an electrolyte consists of dissociated ions. It will be noticed that neither the evidence in favour of the dissociation theory which is here considered, nor that described in the article ELECTROLYSIS, requires more than the effective dissociation of the ions from each other. They may well be connected in some way with solvent molecules, and there are several indications that an ion consists of an electrified part of the molecule of the dissolved salt with an attendant See also:

• ATMOSPHERE (Gr. fir µ6r, vapour; orkaipa, a sphere)

1026). Such relations suggest that the frictional resistance to the motion of an ion is due to the ordinary viscosity of the liquid, and that the ion is analogous to a See also:

• BODY

Kohlrausch and L. Holborn (Leipzig, 1898), and The Theory of Solution and Electrolysis, by W. C. D. Whetham (See also:

• CAMBRIDGE
• CAMBRIDGE, EARLS AND DUKES OF
• CAMBRIDGE, RICHARD OWEN (1717-1802)

A. See also:

• COULOMB, CHARLES AUGUSTIN (1736-1806)

Thus in 1872 E. G. Warburg (Pogg. Ann., 1872, 145, P. 578) found that the leak through hydrogen was only about one-See also:

• HALF

Zeit., 1900, 2, p. 116) and C. T. R. See also:

• WILSON, ALEXANDER (1766-1813)
• WILSON, HENRY (1812–1875)
• WILSON, HORACE HAYMAN (1786–1860)
• WILSON, JAMES (1742—1798)
• WILSON, JAMES (1835— )
• WILSON, JAMES HARRISON (1837– )
• WILSON, JOHN (1627-1696)
• WILSON, JOHN (178 1854)
• WILSON, ROBERT (d. 1600)
• WILSON, SIR DANIEL (1816–1892)
• WILSON, SIR ROBERT THOMAS (1777—1849)
• WILSON, SIR WILLIAM JAMES ERASMUS
• WILSON, THOMAS (1663-1755)
• WILSON, THOMAS (c. 1525-1581)
• WILSON, WOODROW (1856— )

See also:

• RUTHERFORD, MARK
• RUTHERFORD, WILLIAM GUNION (1853–1907)

Wilson's results on this point are Gas. Relative Rate of Leak. Rate of Leak. Sp. Gr. Aim I •oo I H . .184 2.7 shown in the following table (Proc. Roy. Soc., 1901, 6o, p. 277) The rate of leak of electricity through gas contained in a closed See also:

• CO2

Hag. [6], 13, p. 265) have shown, with the time of the See also:

• DAY (O. Eng. dreg, Ger. Tag; according to the New English Dictionary, " in no way related to the Lat. dies")
• DAY, JOHN (1574-1640?)
• DAY, THOMAS (1748-1789)

This explains the very interesting effect discovered by J. See also:

• ELSTER

5. A See also:

• PUMA

The fact that the conductivity of the gas is removed by filtering shows that it is due to something mixed with the gas which is removed from it by filtration, and since the conductivity is also removed by an electric field, the cause of the conductivity must be charged with electricity so as to be driven to the sides of the tube by the electric force. Since the gas as a whole is not electrified either positively or negatively, there must be both negative and positive charges in the gas, the amount of electricity of one sign being equal to that of the other. We are thus led to the conclusion that the conductivity of the gas is due to electrified particles being mixed up with the gas, some of these particles having charges of positive electricity, others of negative. These electrified particles are called ions, and the process by which the gas is made a conductor is called the ionization of the gas. We shall show later that the charges and masses of the ions can be determined, and that the gaseous ions are not identical with those met with in the electrolysis of solutions. One very characteristic property of conduction of electricity through a gas is the relation between the current through the gas and the electric force which gave rise to it. This relation is not in general that expressed by Ohm's law, which always, as far as our present knowledge extends, expresses the relation for conduction through metals and electrolytes. With gases, on the other hand, it is only when the current is very small that Ohm's law is true. If we represent graphically by means of a curve the relation between the current passing between two parallel metal plates separated by ionized gas and the difference of potential between the plates, the curve is of the See also:

• CHARACTER (Gr. xapareri7p, from xap&crew, to scratch)

7; curves of this kind have been obtained by von Schweidler (Wien. Ber., 1899, ro8, p. 273), and J. E. S. Townsend Phil. Mag., 1901 [61, 1, p. 198). We shall discuss later the causes of the rise in the current with large potential differences, when we consider ionization by collision. The general features of the earlier part of the curve are readily explained on the ionization hypothesis. On this view the Rontgen rays or other ionizing agent acting on the gas between the plates, produces positive and negative ions at a definite rate. Let us sup-pose that q positive and q negative ions are by this means produced per second between the plates; these under the electric force will tend to move, the positive ones to the negative plate, the negative ones to the positive.

Some of these ions will reach the plate, others before reaching the plate will get so near one of the opposite sign that the attraction between them will cause them to unite and form an electrically neutral system; when they do this they end theirexistence as ions. The current between the plates is proportional to the number of ions which reach the plates per second. Now it is evident that we cannot go on taking more ions out of the gas than are produced; thus we cannot, when the current is steady, have more than q positive ions driven to the negative plate per second, and the same number of negative ions to the positive. If each of the positive ions carries a charge of e units of positive electricity, and if there is an equal and opposite charge on each negative ion, then the maximum amount of electricity which can be given to the plates per second is qe, and this is equal to the saturation current. Thus if we measure the saturation current, we get a direct measure of the 60 50 ,a0 30 20 10 0 100 200 300 400 500 600 700 800 900 1000 1100 1100 1500 1900 1500 FIG. 7. ionization, and this does not require us to know the value of any quantity except the constant charge on the ion. If we attempted to deduce the amount of ionization by measurements of the current before it was saturated, we should require to know in addition the velocity with which the ions move under a given electric force, the time that elapses between the liberation of an ion and its combination with one of the opposite sign, and the potential difference between the plates. Thus if we wish to measure the amount of ionization in a gas we should be careful to see that the current is saturated. The difference between conduction through gases and through metals is shown in a striking way when we use potential differences large enough to produce the saturation current. Suppose we have got a potential difference between the plates more than sufficient to produce the saturation current, and let us increase the distance between the plates. If the gas were to act like a metallic conductor this would diminish the current, because the greater length would involve a greater resistance in the circuit.

In the case we are considering the separation of the plates will increase the current, because now there is a larger volume of gas exposed to the rays; there are therefore more ions produced, and as the saturation current is proportional to the number of ions the saturation current is increased. If the potential difference between the plates were much less than that required to saturate the current, then increasing the distance would diminish the current; the gas for such potential differences obeys Ohm's law and the behaviour of the gaseous resistance is therefore similar to that of a metallic one. In order to produce the saturation current the electric field must be strong enough to drive each ion to the electrode before it has time to enter into combination with one of the opposite sign. Thus when the plates in the preceding example are far apart, it will take a larger potential difference to produce this current than when the plates are close together. The potential difference required to saturate the current will increase as the square of the distance between the plates, for if the ions are to be delivered in a given time to the plates their speed must be proportional to the distance between the plates. But the speed is proportional to the electric force acting on the ion; hence the electric force must be proportional to the distance between the plates, and as in a uniform field the potential difference is equal to the electric force multiplied by the distance between the plates, the potential difference will vary as the square of this distance. The potential difference required to produce saturation will, other circumstances being the same, increase with the amount of ionization, for when the number of ions is large and they are crowded together, the time which will elapse before a positive one combines with a negative will be smaller than when the number of ions is small. The ions have therefore to be removed more quickly from the gas when the ionization is great than when it is small; thus they must move' at' a higher speed and must therefore be acted upon by a larger force. E. M. F. FIG.

6. When the ions are not removed from the gas, they will increase until the number of ions of one sign which combine with ions of the opposite sign in any time is equal to the number produced by the ionizing agent in that time. We can easily calculate the number of free ions at any time after the ionizing agent has commenced to act. Let q be the number of ions (positive or negative) produced in one cubic centimetre of the gas per second by the ionizing agent, nt, n2, the number of free positive and negative ions respectively per cubic centimetre of the gas. The number of collisions between positive and negative ions per second in one cubic centimetre of the gas is proportional to nine. If a certain fraction of the collisions between the positive and negative ions result in the formation of an electrically neutral system, the number of ions which disappear per second on a cubic centimetre will be equal to See also:

• ANT
• ANT (O. Eng. aemete, from Teutonic a, privative, and maitan, cut or bite off, i.e. " the biter off "; aemete in Middle English became differentiated in dialect use to (mete, then amte, and so ant, and also to emete, whence the synonym " emmet," now only u

We may use equation (1) to determine the rate at which the ions disappear when the ionizing agent is removed. Putting q=o in that equation we get do/at = — an2. Hence n=no/(1+n0at) (3), where no is the number of ions when t=o. Thus the number of ions falls to one-half its initial value in the time I/noa. The quantity a is called the coefficient of recombination, and its value for different gases has been determined by Rutherford (Phil. Mag. 1897 [5], 44, p. 422), Townsend (Phil. Trans., 1900, 193, p. 129), McClung (Phil. Mag., 1902 [6], 3, p. 283), Langevin (Ann. chim. phys.

[7], 28, p. 289), Retschinsky (Ann. d. Phys., 1905, 17, p. 518), Hendred (Phys. Rev., 1905, 21, p. 314). The values of a/e, e being the charge on an ion in electrostatic measure as determined by these observers for different gases, is given in the following table: Townsend. McClung.1Langevin. Retschinsky. Hendred. Air . 3420 3380 3200 4140 3500 02 3380 CO2 .

3500 3490 3400 H2 3020 2940 The gases in these experiments were carefully dried and free from dust; the apparent value of a is much increased when dust or small drops of water are present in the gets, for then the ions get caught by the dust particles, the mass of a particle is so great compared with that of an ion that they are practically immovable under the action of the electric field, and so the ions clinging to them escape detection when electrical methods are used. Taking e as 3.5 X Io-i0, we see that a is about 1.2X lo-6, so that the number of recombinations in unit time between n positive and n negative ions in unit volume is I.2 X lo-6n2. The kinetic theory of gases shows that if we have n molecules of air per cubic centimetre, the number of collisions per second is 1-2X lo--10n2 at a temperature of o° C. Thus we see that the number of recombinations between oppositely charged ions is enormously greater than the number of collisions between the same number of neutral molecules. We shall see that the difference in size between the ion and the molecule is not nearly sufficient to account for the difference between the collisions in the two cases; the difference is due to the force between the oppositely charged ions, which drags ions into collisions which but for this force would have missed each other. Several methods have been used to measure a. In one method air, exposed to some ionizing agent at one end of a long tube, isslowly sucked through the tube and the saturation current measured at different points along the tube. These currents are proportional to the values of n at the place of observation: if we know the distance of this place from the end of the tube when the gas was ionized and the velocity of the stream of gas, we can find tin equation (3), and knowing the value of n we can deduce the value of a from the equation 1/n1 — i/n2 = a(t~ —t2), where ni, n2 are the values of n at the times t1, t2 respectively. In this method the tubes ought to be so wide that the loss of ions by diffusion to the sides of the tube is negligible. There are other methods which involve the knowledge of the speed with which the ions move under the action of known electric farces; we shall defer the See also:

• CONSIDERATION (from Lat. considerare, to look at closely, examine, generally taken to be from con-, and the base seen in sidus, sideris, a star, the word being supposed to be originally an astrological or astronomical term)

This probably explains the large values of a obtained by Retschinsky, who ionized the gas by the a rays from radium, a method which produces very patchy ionization. Variation of a with the Pressure of the Gas.—All observers agree that there is little variation in a with the pressures for pressures of between 5 and I atmospheres; at lower pressures, however, the value of a seems to diminish with the pressure: thus Langevin (Ann. chim. phys., 1903, 28, p. 287) found that at a pressure of } of an atmosphere the value of a was about $ of its value at atmospheric pressure. Variation of a with the Temperature.—Erikson (Phil. Meg., Aug. 1909) has shown that the value of a for air increases as the temperature diminishes, and that at the temperature of liquid air -18o° C., it is more than twice as great as at +12° C. Since, as we have seen, the recombination is due to the coming together of the positive and negative ions under the influence of the electrical attraction between them, it follows that a large electric force sufficient to overcome this attraction would keep the ions apart and hence diminish the coefficient of recombination. Simple considerations, however, will show that it would require exceedingly strong electric See also:

• FIELDS, JAMES THOMAS (1817-1881)

129). The method used was to suck the ionized gas through narrow tubes; by measuring the loss of both the positive and negative ions after the gases had passed through a known length of tube, and allowing for the loss by recombination, the loss by diffusion and hence the coefficient of diffusion could be determined. The following tables give the values of the coefficients of diffusion D on the C.G.S. system of units as determined by Townsend: Gas. D for+ions. D for—ions. Mean Value • Ratio of D for of D. —to D for+ions. 0r ' 025 '0396 • 0323 P58 CO2 .023 .026 •0245 1.13 H2 •123 •190 •156 1.54 Gas. D for+ions. D for—ions. Mean Value Ratio of D for of D. —to D for+ions.

Air •032 '037 .0335 1.09 02 •o288 •0358 .0323 I.24 CO2 .0245 •0255 .025 1.04 H2 .128 •142 •135 1.11 It is interesting to compare with these coefficients the values of D when various gases diffuse through each other. D for hydrogen through air is •634, for oxygen through air •177, for the vapour of isobutyl See also:

• AMIDE

The ions produced by chemical reactions and in flames are much less See also:

• MOBILE

289), See also:

• PHILLIPS
• PHILLIPS, ADELAIDE (1833-1882)
• PHILLIPS, EDWARD (163o-1696)
• PHILLIPS, JOHN (1800—1874)
• PHILLIPS, SAMUEL (1814—1854)
• PHILLIPS, STEPHEN (1868– )
• PHILLIPS, THOMAS (1770-1845)
• PHILLIPS, WENDELL (1811-1884)

Camb. Phil. Soc. 9, p. 401). H. A. Wilson (Phil. Trans. 192, p.409) ,See also:

• MARX, HEINRICH KARL (1818-1883)

4, 11, p. 558; Ann. Chim. Phys. 7, 30, p. 5) and Gold (Proc. Roy. Soc. 79, p. 43) have investigated the velocities of ions produced by putting various salts into flames; McClelland (Phil. Mag. 46, p.

29) the velocity of the ions in gases sucked from the neighbourhood of flames and arcs; Townsend (Proc. Camb. Phil. Soc. 9, p. 345) and Bloch (loc. cit.) the velocity of ions produced by chemical reaction; and Chattock (Phil. Mag. [5], 48, p. 401) the velocity of the ions produced when electricity escapes from a sharp See also:

• NEEDLE (O. Eng. ncedl; the word appears in various forms in Teutonic languages, Ger. Nadel, Dutch naal, the root being ne-, to sew, cf. Ger. nahen, and probably Lat. nere, to spin, Gr. vi)ats, spinning)

If we increase the strength of the field between the plates, and hence the upward velocity of the positive ion, until the positive ions just begin to reach the upper plate, we know that with this strength of field the velocity of the positive ion is equal to V. By this method, which has been used by Rutherford, Zeleny and H. A. Wilson, the velocity of ions in fields of various strengths has been determined. The arrangement used by Zeleny is represented in fig. 8. P and Q are square brass plates. They are bored through their centres, and to the openings the tubes R and S are attached, the space between the plates being covered in so as to form a closed box. K is a piece of wire See also:

• GAUZE

The changes in the potential of T are due to ions giving up their charges to it. With a given velocity of air-blast the potential of T was found not to change unless the difference of potential between P and Q exceeded a critical value. The field corresponding to this critical value thus made the ions move with the known velocity of the blast. -tAilrli that the deflexion of theelectrometer connected with DD is less than it was when A and B were at the same potential, because some of FIG 9 the positive ions in their passage through BAB are driven against the plates B. If u is the velocity along the lines of force in the uniform electric field between A and B, and t the time it takes for the gas to pass through BAB, then all the positive ions within a distance ut of the plates B will be driven up against these plates, and thus if the positive ions are equally distributed through the gas, the number of positive ions which emerge from the system when the electric field is on will See also:

• BEAR
• BEAR, BLACK
• BEAR, BROWN
• BEAR, GRIZZLY
• BEAR, ISABELLINE
• BEAR, WHITE

CD is a See also:

• BASE

Thus the greatest distance the ion can get from the plate is equal to 2au/pd, and if the distance between the plates is gradually reduced to this value, the plate AB will begin to lose a negative charge; hence when this happens d = 2au/pd, or u = pd2/2a, an equation by means of which we can find u. In this form the method is not applicable when ions of both signs are present. Franck and Pohl (Verh. deutsch. physik. Gesell. 1907, 9, p. 69) have by a slight modification removed this restriction. The modification consists in confining the ionization to a layer of gas below the gauze EF. If the velocity of the positive ions is to be determined, these ions are forced through the gauze by applying to the ionized gas a small constant electric force acting upwards; if negative ions are required, the constant force is reversed. After passing through the gauze the ions are acted upon by alternating forces as in Rutherford's method. Langevin (Ann. chim. phys., 1903, 28, p. 289) devised a method of measuring the velocity of the ions which has been extensively used; it has the See also:

• ADVANTAGE

Suppose that we expose the gas between two parallel plates A, B to Rontgen rays or some other ionizing agent, then stop the rays and apply a uniform electric field to the region between the plates. If the force on the positive ion is from A to B, the plate B will receive a positive charge of electricity. After the electric force has acted for a time T reverse it. B will now begin to receive negative electricity and will go on doing so until the supply of negative ions is exhausted. Let us consider how the quantity of positive electricity received by B will vary with T. To See also:

• FIX, THEODORE (180o-1846)

By observing the time when the increase in the rate of diminution of the positive charge with the time suddenly sets in we can determine Tl, and hence the velocity of the negative ions. The velocity of the ions produced by the See also:

• DISCHARGE (adapted from the O. Fr. discharge, modern decharge, from a med. Lat. discargare, to unload, dis- and earn care, to load, cf. " charge ")

There are other methods of determining the velocities of the ions, but as these depend on the theory of the conduction of electricity through a gas containing charged ions, we shall consider them in our discussion of that theory. By the use of these methods it has been shown that the velocities of the ions in a given gas are the same whether the ionization is produced by Rontgen rays, radioactive substances, ultra-violet light, or by the discharge of electricity from points. When the ionization is produced by chemical action the ions are very much less mobile, moving in the same electric field with a velocity less than one-thousandth part of the velocity of the first kind of ions. On the other hand, as we shall see later, the velocity of the negative ions in flames is enormously greater than that of even the first kind of ion under similar electric fields and at the same pressure. But when these negative ions get into the See also:

• COLD (in O. Eng. cald and ceald, a word coming ultimately from a root cognate with the Lat. gelu, gelidus, and common in the Teutonic languages, which usually have two distinct forms for the substantive and the adjective, cf. Ger. Kolte, kalt, Dutch koude

V. Observer. Air . . . 1.6 Rutherford Air (dry). . . 1.36 1.87 . . Zeleny 1.60 1.70 . . Langevin ,, 1'39 1.78 . , Phillips 1 .54 1.78 . . Wellisch Air (moist) 1.37 1.81 . . Zeleny Oxygen (dry) 1.36 1.8o ..

Oxygen (moist) . 1.29 1.52 , Carbonic acid (dry) 0.76 0.81 . ,, 0.86 0.90 .. Langevin 0.81 0.85 . . Wellisch Carbonic acid (moist) 0.82 0.75 .. Zeleny Hydrogen (dry) 6.70 7.95 Hydrogen (moist) 5.30 5.60 See also:

• NITROGEN [symbol N., atomic weight 14.01, 0=16]

. 1.0 See also:

• HELIUM (from Gr. i)Xcor, the sun)

0.30 0.31 Ethyl ether . . 0.29 0.31 I, Ethyl acetate , 0.31 0.28 Methyl bromide 0.29 0.28 ,, Methyl iodide . 0•a1 0.22 Carbon tetrachloride 0.30 0.31 ,, Ethyl iodide 0.17 0.16 . . Ions produced by Ultra-Violet Light. Air 1.4 Rutherford Hydrogen 3.9 Rutherford Carbonic acid . 0.78 Rutherford Ions in Gases sucked from Flames. Velocities varying from .04 to •23 McClelland Ions in Flames containing Salts. Negative ions . 12.9 cm./sec. Gold +ions for salts of Li, Na, 62 H. A. Wilson K, Rb, Cs .

. . 200 Marx f, 8o Moreau Ions liberated by Chemical Action. Bloch Velocities of the order of 0.0005 cm./sec. Ions from Point Discharge. Hydrogen . . 5.4 7'43 6.41 Chattock Carbonic acid . o.83 0.925 o•88 Chattock Air 1.32 1.8o 1.55 Chattock Oxygen . . . . I.30 j 1.85 1.57 Chattock It will be seen from this table that the greater mobility of the negative ions is very much more marked in the case of the lighter and simpler gases than in that of the heavier and more complicated ones; with the vapours of organic substances there seems but little difference between the mobilities of the positive and negative ions, indeed in one or two cases the positive one seems slightly but very slightly the more mobile of the two. In the case of the simple gases the difference is much greater when the gases are dry than when they are moist. It has been shown by direct experiment that the velocities are directly proportional to the electric force. Variation of Velocities with Pressure.—Until the pressure gets low the velocities of the ions, negative as well as positive, vary inversely as the pressure. Langevin (loc. cit.) was the first to show that at very low pressures the velocity of the negative ions increases more rapidly as the pressure is diminished than this law indicates.

If the nature of the ion did not change with the pressure, the kinetic theory of gases indicates that the velocity would vary inversely as the pressure, so that Langevin's results indicate a change in the nature of the negative ion when the pressure is diminished below a certain value. Langevin's results are given in the following table, where p represents the pressure measured in centimetres of mercury, V+ and V— the velocities of the positive and negative ions in air under unit electrostatic force, i.e. 300 volts per centimetre : Negative Ions. Positive Ions. p. V—. i pV—/76. p. V+. pV+/76. 7'5 656o 647 7'5 4430 437 20.0 2204 58o 20.0 1634 430 41'5 994 530 41'5 782 427 76•o 510 510 76•o 480 420 142.0 270 505 142.0 225 425 The increase in the case of pV— indicates that the structure of the negative ion gets simpler as the pressure is reduced. Wallisch in some experiments made at the See also:

• CAVENDISH
• CAVENDISH, GEORGE (1500-1562?)
• CAVENDISH, HENRY (1731-1810)
• CAVENDISH, SIR WILLIAM (c. 1505-1557)

167) investigated, using Langevin's method, the velocities of the + and — ions through air at atmospheric pressure at temperatures ranging from that of boiling liquid air to 411 ° C.; RI and See also:

• R2(rt-s2)

499), in his experiments on the conduction of flames and hot gases into which salts had been put, found that the velocity of the positive ions in flames at a temperature of 2000° C. containing the salts of the alkali metals was 62 cm./sec. under an electric force of one volt per centimetre, while the velocity of the positive ions in a stream of hot air at l000° C. containing the same salts was only 7 cm./sec. under the same force. The great effect of temperature is also shown in some experiments of McClelland (Phil. Mag. [5], 46, p. 29) on the velocities of the ions in gases drawn from See also:

• BUNSEN, CHRISTIAN CHARLES JOSIAS, BARON VON (1791-186o,)
• BUNSEN, ROBERT WILHELM VON (1811-1899)

Substituting these values we find u=1•97X104X. If the potential gradient is i volt per centimetre, X =1/300. Substituting this value for X, we find u = 66 cm./sec., for the velocity of a hydrogen molecule. We have seen that the velocity of the ion in hydrogen is only about 5 cm./sec., so that the ion moves more slowly than it; would if it were a single molecule. One way of explaining this is to suppose that the ion is bigger than the molecule, and is in fact an See also:

• AGGREGATION (from the Lat. ad, to, gregare, to collect together)

In connexion with the velocities of ions in the gases drawn from flames, we find other instances which suggest that condensation takes place round the ions. An increase in the size of the system is not, however, the only way by which the velocity might fall below that calculated for the hydrogen molecule, for we must remember that the hydrogen molecule, whose coefficient of diffusion is I.7, is not charged, while the ion is. The forces exerted by the ion on the other molecules of hydrogen are not the same as those which would be exerted by a molecule of hydrogen, and as the coefficient of diffusion depends on the forces between the molecules, the coefficient of diffusion of a charged molecule into hydrogen might be very different from that of an uncharged one. Wellisch (loc. cit.) has shown that the effect of the charge on the ion is sufficient in many cases to explain the small velocity of the ions, even if there were no aggregation. Mixture of Gases.—The ionization of a mixture of gases raises some very Interesting questions. If we ionize a mixture of two very different gases, say hydrogen and carbonic acid, and investigate the nature of the ions by measuring their velocities, the question arises, shall we find two kinds of positive and two kinds of negative ions moving with different velocities, as we should do if some of the positive ions were positively charged hydrogen molecules, while others were positively charged molecules of carbonic acid; or shall we find only one velocity for the positive ions and one for the negative? Many experiments have been made on the velocity of ions in mixtures of two gases, but as yet no evidence has been found of the existence of two different kinds of either positive or negative ions in such mixtures, although some of the methods for determining the velocities of the ions, especially Langevin's, ought to give evidence of this effect, if it existed. The experiments seem to show that the positive (and the same is true for the negative) ions in a mixture of gases are all of the same kind. This conclusion is one of considerable importance, as it would not be true if the ions consisted of single molecules of the gas from which they are produced. Recombination.—Several methods enable us to deduce the co-efficient of recombination of the ions when we know theit velocities. Perhaps the simplest of these consists in determining the relation between the current passing between two parallel plates immersed in ionized gas and the potential difference between the plates. For let q be the amount of ionization, i.e. the number of ions produced per second per unit volume of the gas, A the area of one of the plates, and d the distance between them ; then if the ionization is constant through the volume, the number of ions of one sign produced per second in the gas is qAd.

Now if i is the current per unit area of the plate, e the charge on an ion, iA/e ions of each sign are driven out of the gas by the current per second. In addition to this source of loss of ions there is the loss due to the recombination; if n is the number of positive or negative ions per unit volume, then the number which recombine per second is ant per cubic centimetre, and if n is constant through the volume of the gas, as will approximately be the case if the current through the gas is only a small fraction of the saturation current, the number of ions which disappear per second through recombination is an2.Ad. Hence, since when the gas is in a steady state the number of ions produced must be equal to the number which disappear, we have qAd =iA/e+an2.Ad, q =i/ed+an2. If u1 and u2 are the velocities with which the positive and negative ions move, nule'and nu2e are respectively the quantities of positive electricity passing in one direction through unit area of the gas per second, and of negative in the opposite direction, hence i = nule- -nu2e. If X is the electric force acting on the gas, kI and k2 the velocities of; the positive and negative ions under unit force, u1=kIX, u2 = k2X ; hence n = it (ki +k2)X e, and we have ail q__ed+(kl+k2)2e2X2 But qed is the saturation current per unit area of the plate; calling this I, we have I-i= dai2 e(kl+k2)2X2 or X2= i2.da e(I-i) (k1+k2)2 Hence if we determine corresponding values of X and i we can deduce the value of ale if we also know (ki+k2). The value of I is easily determined, as it is the current when X is very large. The preceding result only applies when i is small compared with I, as it is only in this case that the values of n and X are uniform throughout the volume of the gas. Another method which answers the same purpose is due to Langevin (Ann. Chim. Phys., 1903, 28 p. 289) ; it is as follows. Let A and B be two parallel planes immersed In a gas, and let a slab of the gas bounded by the planes a, b parallel to A and B be ionized by an instantaneous flash of Rontgen rays.

If A and B are at different electric potentials, then all the positive ions produced by the rays will be attracted by the negative plate and all the negative ions by the positive, if the electric field were exceedingly large they would reach these plates before they had time to recombine, so that each plate would receive No ions if the flash of Rontgen rays produced No positive and No negative ions. With weaker fields the number of ions received by the plates will be less as some of them will recombine before they can reach the plates. We can find the number of ions which reach the plates in this case in the following way :—In consequence of the movement of the ions the slab of ionized gas will broaden out and will consist of three portions, one in which there are nothing but positive ions,—this is on the side of the negative plate,—another on the side of the positive plate in which there are nothing but negative ions, and a portion between these in which there are both positive and negative ions; it is in this layer that recombination takes place, and here if n is the number of positive or negative ions at the time t after the flash of Rontgen rays, n =no/(I +anot). With the same notation as before, the breadth of either of the See also:

• OUTER

. (4), nee= + k2 ( ar dx) and from these equations we can, if we know the distribution electric intensity between the plates, calculate the number of posit and negative ions. In a steady state the number of positive and negative ions unit volume at a given place remains constant, hence neglecti the loss by diffusion, we have (5) d dx(kiniX)q-an1n2 . . . -d -(k2n2X) =q-anin2 (6). If ki and k2 are constant, we have from (I), (5) and (6) d'X'= 8are(q-anin2) (ki+r) . . . . (7), of ve in g an equati( i which is very useful, becaus it enables us, if we know the distri ution of X2, to find whether at any point in the gas the ionizE on is greater or less than the recombination of the ions. We see t it q-anin2, which is the excess of ionization over recombinat:n, is proportional to d2X2/dx2. Thus when the ionization exceeds the recombination, i.e. when q-anin2 is positive, the curve for X2 is convex to the axis of x, while when the recombination exceeds the ionization the curve for X2 will be See also:

• CONCAVE

Distribution of Electric Force when a Current is passing through an Ionized Gas.—Let the two plates be at right angles to the axis of x; then we may suppose that between the plates the electric intensity X is everywhere parallel to the axis of x. The velocities of both the positive and negative ions are assumed to be proportional to X. Let k1X,k2X represent these velocities respectively; let n1, n2 be respectively the number of positive and negative ions per unit volume at a point fixed by the co-ordinate x; let q be the number of positive or negative ions produced in unit time per unit volume at this point; and let the number of ions which recombine in unit volume in unit time be anin2; then if e is the charge on the ion, the volume density of the electrification is (nl-n2)e, hence dX dx =4z'(ni-n2)e (I). If I is the current through unit area of the gas and if we neglect any diffusion except that caused by the electric field, '1 Al nieklX +n2ek2X = I (2). From equations (r) and (2) we have __ I I k2dX nie k1+k2 (X'41- d) • (3), .a o C A where p = 8areki/a. Since e=4Xto—E0, a=2XIO-°, and k, for air at atmospheric pressure =450, $ is about 2.3 for air at atmospheric pressure and it becomes much greater at lower pressures. Thus Xi/Xo is always greater than unity, and the value of the ratio increases from unity to infinity as 13 increases from zero to infinity. As (3 does not involve either q or I, the ratio of XI to Xo is independent of the strength of the current and of the intensity of the ionization. No general solution of equation (8) has been found when k, is not equal to k2, but we can get an approximation to the solution when q is constant. The equations (I), (2), (3), (4) are satisfied by the values- ni = n2 = (q/a) k1n1Xe = ki+k2l~ k2n.Xe=ki+2k2I' X— \q/ _ (al e(ki-I}-k2). These solutions cannot, however, hold right up to the surface of the plates, for across each unit of area, at a point P, kiIl(ki+k2)e positive ions pass in unit time, and these must all come from the region between P and the positive plate. If A is the distance of P from this plate, this region cannot furnish more than qX positive ions, and only this number if there are no recombinations.

Hence the solution cannot hold when qX is less than kiI/(ki-I-k2)e, or where A is less than kil/(kl +k2)qe. Similarly the solution cannot hold nearer to the negative plate than the distance k2I/(ki+k2)ge. The force in these layers will be greater than that in the middle of the gas, and so the loss of ions by recombination will be smaller in comparison with the loss due to the removal of the ions by the current. If we assume that in these layers the loss of ions by recombination can be neglected, we can by the method of the next article find an expression for the value of the electric force at any point in the layer. This, in See also:

• CONJUNCTION (from Lat. conjungere, to join together)

857) has by the method of successive approximations obtained solutions of equation (8) (i.) when the current is only a small fraction of the saturation current, (ii.) when the current is nearly saturated. The results of his investigations are represented in fig. 12, which represents the distribution of electric force along the path of the current for various values of the current expressed as fractions of the saturation current. It will be seen that until the current amounts to about one-fifth of the maximum current, the type of solution is the one just indicated, i.e. the electric force is constant except in the neighbourhood of the electrodes when it increases rapidly. Though we are unable to obtain a general solution of the equation (8), there are some very important special cases in which that equation can be solved without difficulty. We shall consider two of these, the first being that when the current is saturated. In this case there is no loss of ions by recombination, so that using the same notation as before we have t dX2 I I 1 82r d2x —qx € ki+k2 — _ qk2' or 82 X2 ,2 /I r=q2- (ki+k2) —qk-+C, where C is a quantity to be determined by the condition that f 0Xdx=V, where V is the given potential difference between the plates. When the force is a minimum dX/dx =0, hence at this point lk1 l—x lk2 x = —ki+k2' ki+k2' Hence the ratio of the distances of this point from the positive and negative plates respectively is equal to the ratio of the velocities of the positive and negative ions. The other case we shall consider is the very important one in which the velocity of the negative ion is exceedingly large compared with the positive; this is the case in flames where, as Gold (Prot. Roy. Soc. 97, p.

43) has shown, the velocity of the negative ion is many thousand times the velocity of the positive; it is also very probably the case in all gases when the pressure is low. We may get the solution of this case either by putting ki/k2=o in equation (8), or independently as follows:—Using the same notation as before, we have i = n1k1X e+n2k2Xe, d (n2k2X) =q—anini, dx -4ir(n1—n2)e. In this case practically all the current is carried by the negative ions so that i =n2k2Xe, and therefore q = anin2. Thus n2=i/k2Xe, n1=gk2Xe/ai. dX 4ire2k2gX _ flirt kx = ai k2X' or dX2_8ae2k2gX2_ -8iri dx ai k2 The solution of this equation is X2a 2 z2 — 2+Ce8,rezk2gx~See also:

• A2(z)

As k1 is many thousand times k2 the force increases with great rapidity as we approach the cathode; this is a very characteristic feature of the passage of electricity through flames and hot gases. Thus in an experiment made by H. A. Wilson with a flame 18 cm. long, the drop of potential within 1 centimetre of the cathode was about five times the drop in the other-17 cm. of the tube. The relation between the current and the potential difference when the velocity of the negative ion is much greater than the positive is very easily obtained. Since the force is uniform and equal to e2eVq, until we get close to the cathode the fall of potential in this part of the discharge will be very approximately equal to 2e'V ql, where l is the distance between the electrodes. Close to the cathode, the electric force when io is not nearly equal to i is approximately given by the equation a 7 47r"kagx/ai X =e(kik2) q E and the fall of potential at the cathode is equal approximately to Jo' Xdx, that is to (al a ai e(kik2)i \q/ '41re2k2q' The potential difference between the plates is the sum of the fall of potential in the uniform part of the discharge plus the fall at the cathode, hence falai I V = q eke (ii-F. 'a 7re2q Al (kik2)) The fall of potential at the cathode is proportional to the square of the current, while the fall in the rest of the circuit is directly proportional to the current. In the case of flames or hot gases, the fall of potential at the cathode is much greater than that in the rest of the circuit, so that in such cases the current through the gas varies nearly as the square root of the potential difference. The equation we have just obtained is of the form V =Ai+Bi2, and H. A. Wilson has shown that a relation of this form represents the results of his experiments on the conduction of electricity through flames.

The expression for the fall of potential at the cathode is inversely proportional to g312, q being the number of ions produced per cubic centimetre per second close to the cathode; thus any increase in the ionization at the cathode will diminish the potential fall at the cathode, and as practically the whole potential difference between the electrodes occurs at the cathode, a diminution in the potential fall there will be much more important than a diminution in the electric force in the uniform part of the discharge, when the force is comparatively insignificant. This consideration explains a very striking phenomenon discovered many years ago by Hittorf, who found that if he put a wire carrying a See also:

• BEAD

11, p. 425) to be far greater than that of any known metal, and the increase of current produced by coating the cathodes with these oxides is exceedingly large; in some cases investigated by Tufts and See also:

• STARK, JAMES (1794-1859)
• STARK, JOHN (1728-1822)

If 1 is the distance between the plates, and V the potential difference between them, V = f 1Xdx = a'V k [(il+C)312—C212]. We shall show that when the current is far below the saturation value, C is very small compared with il, so that the preceding equation becomes V2=87r13i/k (1). To show that for small currents C is small compared with il, consider the case when the ionization is confined to a thin layer, thickness d close to the electrode, in that layer let no be the value of n, then we have q=ano2+i/ed. If Xo be the value of X when x=o, kXonoe=i, and kX02 i2 a i2 ( ) C= 87r —no2ke.87r=87rke2.g+i/ed . . ' 2. Since a/87rke is, as we have seen, less than unity, C will be small compared with il, if i/(eq+i/d) is small compared with 1. If Io is the saturation current, q = Io/ed, so that the former expression =id/(Io+i), if i is small compared with Io, this expression is small compared with d, and therefore a fortiori compared with 1, so that we are justified in this case in using equation (t). From equation (2) we see that the current increases as the square of the potential difference. Here an increase in the potential difference produces a much greater percentage increase than in conduction through metals, where the current is proportional to the potential difference. When the ionization is distributed through the gas, we have seen that the current is approximately proportional to the square root of the potential, and so increases more slowly with the potential difference than currents through metals. From equation (t) the current is inversely proportional to the cube of the distance between the electrodes, so that it falls off with great rapidity as this distance is increased. We may See also:

• NOTE
• NOTE (Lat. nota, mark, sign, from noscere, to know)

For the same potential difference the current is proportional to k, the velocity under unit electric force of the ion which carries the current. As the velocity of the negative ion is greater than that of the positive, the current when the ionization is confined to the neighbourhood of one of the electrodes will be greater when that electrode is made cathode than when it is anode. Thus the current will appear to pass more easily in one direction than in the opposite. Since the ions which carry the current have to travel all the way from one electrode to the other, any obstacle which is impervious to these ions will, if placed between the electrodes, stop the current to the electrode where there is no ionization. A plate of metal will be as effectual as one made of a non-conductor, and thus we get the remarkable result that by interposing a plate of an excellent conductor like copper or silver between the electrode, we can entirely stop the current. This experiment can easily be tried by using a hot plate as the electrode at which the ionization takes place: then if the other electrode is cold the current which passes when the hot plate is cathode can be entirely stopped by interposing a cold metal plate between the electrodes. Methods of counting the Number of Ions.—The detection of the ions and the estimation of their number in a given volume is much facilitated by the property they possess of promoting the condensation of water-drops in dust-free air supersaturated with water vapour. If such air contains no ions, then it requires about an eightfold supersaturation before any water-drops are formed; if, however, ions are present C. T. R. Wilson (Phil. Trans.

189, p. 265) has shown that a sixfold supersaturation is sufficient to cause the water vapour to condense round the ions and to fall down as raindrops. The absence of the drops when no ions are present is due to the curvature of the drop combined with the surface tension causing, as See also:

• LORD
• LORD (O. Eng. hldford, i.e. hldfweard, the warder or keeper of bread, hlIf, loaf; the word is not represented in any other Teutonic language)
• LORD, JOHN (1810-1894)

Let it be adjusted so that the expansion produces about a sixfold supersaturation; then if the gas is not exposed to any ionizing agents very few drops (and these probably due to the small amount of ionization which we have seen is always present in gases) are formed. If, however, the bulb is exposed to strong Rontgen rays expansion produces a dense See also:

• CLOUD (from the same root, if not the same word, as " clod," a word common in various forms to Teutonic languages for a mass or lump; it is first applied in the usual sense in the late 13th century; the Anglo-Saxon chid is only used in the sense of " a ma

The theory of the fall of a heavy drop of water through a viscous fluid shows that v =1ga2/µ, where a is the radius of the drop, g the See also:

• ACCELERATION (from Lat. accelerare, to hasten, celer, quick)

[5], 46, p. 528; [5], 48, p. 547; [6], 5, p• 34.6). The result of these measurements shows that the charge on the ion is the same whether the ionization is by Rontgen rays or by the, influence of ultra-violet light on a metal plate. It is the same whether the gas ionized is hydrogen, air or carbonic acid, and thus is presumably independent of the nature of the gas. The value of e formed by this method was 3.4X10-10 electrostatic units. H. A. Wilson (Phil. Meg. [6], 5, p. 429) used another method.

Drops of water, as we have seen, condense more easily on negative than on positive ions. It is possible, therefore, to adjust the expansion so that a cloud is formed on the negative but not on the positive ions. Wilson arranged the experiments so that such a cloud was formed between two horizontal plates which could be maintained at different potentials. The charged drops between the plates were acted upon by a uniform vertical force which affected their rate of fall. Let X be the vertical electric force, e the charge on the drop, vl the rate of fall of the drop when this force acts, and v the rate of fall due to gravity'alone. Then since the rate of fall is proportionate to the force on the drop, if a is the radius of the drop, and p its density, then so that Xe=V2.91r/`/ µ3 .'~(vl—v) VV gp v Thus if X, v, vl are known e can be determined. Wilson by this method found that e was 3.1 Xio-10 electrostatic units. A few of the ions carried charges 2e or 3e. Townsend has used the following method to compare the charge carried by a gaseous ion with that carried by an atom of hydrogen in the electrolysis of solution. We have u/D = Ne/I I, where D is the coefficient of diffusion of the ions through the gas, u the velocity of the ion in the same gas when acted on by unit electric force, N the number of molecules in a cubic centimetre of the gas when the pressure is H dynes per square centimetre, and e the charge in electrostatic units. This relation is obtained on the hypothesis that N ions in a cubic centimetre produce the same pressure as N uncharged molecules. We know the value of D from Townsend's experiments and the values of u from those ofZeleny.

We get the following values for NeXIo-10 Moist Gas. Dry Gas. Gas. Positive I Negative Positive Negative Ions. Ions. Ions. Ions. Air 1.28 1.29 1.46 1.31 Oxygen . . 1.34 1.27 1.63 1.36 Carbonic acid . . . 1.01 •87 '99 .93 Hydrogen . . .

. 1.24 1.18 1.63 1.25 Mean . 1.22 1.15 1.43 1.21 Xe -I- 3 rrpga3 __ yr s irpga3 v' or Xe = Jarpga3 (v, — v) /v. But v = 1ga'See also:

• PIG
• PIG (a word of obscure origin, connected with the Low Ger. and Dut. word of the same meaning, bigge)

This method of obtaining N is the only one which does not involve any assumption as to the shape of the molecules and the forces acting between them. Another method of determining the charge carried by an ion has been employed by Rutherford (Prot. Roy. Soc. 81, pp. 141, 162), in which the positively electrified particles emitted by radium are made use of. The method consists of : (1) Counting the number of a particles emitted by a given quantity of radium in a known time. (2) Measuring the electric charge emitted by this quantity in the same time. To See also:

• COUNT (Lat. comes, gen. comitis, Fr. comte, Ital. conte, Span. conde)

An electrometer placed in series with this vessel will show by its deflection when an a particle enters the chamber, and by counting the number of deflections per minute we can determine the number of a particles given out by the radium in that time. Another method of counting this number is to let the particles fall on a phosphorescent See also:

• SCREEN (usually, but very doubtfully, connected with Lat. scrinium, a box for holding books, from scribere, to write; a connexion with Ger. Schranke, barrier, has been suggested)

If p is the radius of curvature of this curve, m the mass of the ion, mv2/p must equal the normal force acting on the ion, i.e. it must be equal to Hev, or p=my/He. Thus the radius of curvature is constant; the path is therefore a circle, and if we can measure the radius of this circle we know the value of my/He. In the case of the rapidly moving negative ions projected from the cathode in a highly exhausted tube, which are known as cathode rays, the path of the ions can be readily deter-See also:

• MINED

13. an electric battery a ver- tical electric field is produced between E and D and the phosphorescent spot is deflected. By measuring this deflection we determine e/mv2. The tube is now placed in a uniform magnetic field, the lines of magnetic force being horizontal and at right angles to the plane of the See also:

• PAPER
• PAPER (Fr. papier, from Lat. papyrus)

That is, the rate at which this plane receives an electric charge will be the same whether there is a magnetic field between the plate or not, but if a is greater than 2Xm/eH2, then no particle which starts from the plate x=o will reach the plate x=a, and this plate will receive no charge. Thus the supply of electricity to the plate has been entirely stopped by the magnetic field. Thus, on this theory, if the distance between the plates is less than a certain value, the magnetic force should produce no effect on the rate at which the electrometer plate receives a charge, while if the distance is greater than this value the magnetic force would completely stop the supply of electricity to the plate. The actual phenomena are not so abrupt as this theory indicates. We find that when the plates are very near together the magnetic force produces a very slight effect, and this an sncrease in the rate of charging of the plate. On increasing the distance we come to a stage where the magnetic force produces a great diminution in the rate of charging. It does not, however, stop it abruptly, there being a considerable range of distance, in which the magnetic force diminishes but does not destroy the current. At still greater distances the current to the plate under the magnetic force is quite inappreciable compared with that when there is no magnetic force. We should get this See also:

• GRADUAL (Med. Lat. gradualis, of or belonging to steps or degrees; gradus, step)

Mag. [51, 48, p. 547) as well as for the cathode rays. It was found that the value of e/m for the negative ions was the same in all these cases, and that it was a Constant quantity independent of the nature of the gas from which the ions are produced and the means used to produce them. It was found, too, that this value was more than a thousand times the value of e/M, where e is the charge carried by an atom of hydrogen in the electrolysis of solutions, and M the mass of an atom of hydrogen. We have seen that this charge is the same as that carried by the negative ion in gases; thus since e/m is more than a thousand times e/M, it follows that M must be more than a thousand times m. Thus the mass of the negative ion is exceedingly small compared with the mass of the atom of hydrogen, the smallest mass recognized in chemistry. The production of negative ions thus involves the splitting up of the atom, as from a collection of atoms something is detached whose mass is less than that of a single atom. It is important to See also:

• NOTICE

Ges. 6, p. 700) . . . 1.7728 X 1o7 Bucherer (Ann. der Phys., 28, p. 513) . . . 1.763 X Io7 It follows from electrical theory that when the corpuscles are moving with a velocity comparable with that of light their masses increase rapidly with their velocity. This effect has been detected by See also:

• KAUFFMANN, IMARIA

28, p. 513). Conductivity Produced by Ultra-Violet Light.—So much use has been made in See also:

• RECENT

272) showed, has the effect of blowing from the neighbourhood of the plate negatively electrified gas, which has similar properties to the charged gas obtained by the separation of ions from a gas exposed to RSntgen rays or See also:

• URANIUM [symbol U, atomic weight 238.5 (0=16)]

Elster and Geitel arrange the metals in the following order for the facility with which negative electrification is discharged by light: rubidium, potassium, alloy of sodium and potassium, sodium, See also:

• LITHIUM [symbol Li, atomic weight 7•oo (0=16)]

57 '07 •04 •002 The table shows that the absorption of light by the metal has great influence on the photo-electric effect, for while potassium is more sensitive in blue light than sodium, the strong absorption of yellow light by sodium makes it more than five times more sensitive to this light than potassium. Stoletow, at an early period, called attention to the connexion between strong absorption and photo-electric effects. He showed that water, which does not absorb to any great extent either the ultra-violet or visible rays, does not show any photo-electric effect, while strongly coloured solutions, and especially solutions of fluorescent substances such as methyl See also:

• GREEN, A
• GREEN, ALEXANDER HENRY (1832—1896)
• GREEN, DUFF (1791—1875)
• GREEN, JOHN RICHARD (1837—1883)
• GREEN, MATTHEW (1696-1737)
• GREEN, THOMAS HILL (1836-1882)
• GREEN, VALENTINE (1739–1813)
• GREEN, WILLIAM HENRY (.1825–1900)

An extremely interesting fact discovered by Lenard is that the velocity with which the corpuscles are emitted from the metal is independent of the intensity of the incident light. The quantity of corpuscles increases with the intensity, but the velocity of the individual corpuscles does not. It is worthy of notice that in other cases when negative corpuscles are emitted from metals, as for example when the metals are exposed to cathode rays, See also:

• CANAL
• CANAL (from Lat. canalis, " channel " and " kennel " being doublets of the word)

But this cannot be the case if, as is usually assumed in the electromagnetic theory, the wave front consists of a uniform distribution of electric force without structure, for in this case the magnitude of the electric force is proportional to the square root of the intensity. On the emission theory of light a difficulty of this kind would not arise, for on that theory the energy in a luminiferous particle remains constant as the particle pursues its See also:

• FLIGHT

Corpuscles only emitted with o•28 0.21 0.35 the help of an external electric field . , . . I•oo I•o0 I•o0 that in liquids showing photo-electric effects there is always strong absorption; we may, however, have absorption without these effects. Phosphorescent substances, such as calcium sulphide show this effect, as also do various specimens of fluor-spar. As See also:

• PHOSPHORESCENCE

14, in which the ordinates are the cur-rents and the abscissae the potentials. It will be seen that when the pressure is exceedingly low the cur-See also:

• RENT

Ladenburg (Verh. d. deutsch. physik. Ges. 9, p. 504) that the velocity with which the corpuscles are emitted depends on the wave length of the light suggests that the energy in each bundle depends upon the wave length and increases as the wave length diminishes. These considerations illustrate the evidence afforded by photo-electric effects on the nature of light; these effects may also have a deep significance with regard to the structure of matter. The fact that the energy of the individual corpuscles is independent of the intensity of the light might be explained by the hypothesis that the energy of the corpuscles does not come from the light but from the energy stored up in the molecules of the metal exposed to the light. We may suppose that under the action of the light some of the molecules are thrown into an unstable state and explode, ejecting corpuscles; the light in this case acts only as a trigger to liberate the energy in the atom, and it is this energy and not that of the light which goes into the corpuscles. In this way the velocity of the corpuscles would be independent of the intensity of the light. But it may be asked, is this view consistent with the result obtained by Ladenburg that the velocity of the corpuscles depends upon the nature of the light? If light of a definite wave length expelled corpuscles with a definite and uniform velocity, it would be very improbable that the emission of the corpuscles is due to an See also:

• EXPLOSION

Let us suppose that there are two such systems, A and B, of which B ejects the corpuscles with the greater velocity. If B is more sensitive to the short waves, and A to the long ones, then as the wave length of the light diminishes the proportion of the corpuscles which come from B will increase, and as these are the faster, the average velocity of the corpuscles emitted will also increase. And although the potential acquired by a perfectly insulated piece of metal when exposed to ultra-violet light would depend only on the velocity of the fastest corpuscles and not upon their number, in practice perfect insulation is unattainable, and the potential actually acquired is determined by the condition that the gain of negative electricity by the metal through lack of insulation, is equal to the loss by the emission of negatively electrified corpuscles. The potential acquired will fall below that corresponding to perfect insulation by an amount depending on the number of the faster corpuscles emitted, and the potential will rise if the proportion of the rapidly moving corpuscles is increased, even though there is no increase in their velocity. It is interesting to compare other cases in which corpuscles are emitted with the case of ultra-violet light. When a metal or gas is bombarded by cathode rays it emits corpuscles and the velocity of these is found to be independent of the velocity of the cathode rays which excite them; the velocity is greater than for corpuscles emitted under ultra-violet light. Again, when bodies are exposed to Rontgen rays they emit corpuscles moving with a much greater velocity than those excited by cathode rays, but again the velocity does not depend upon the intensity of the rays although it does to some extent on their hardness. In the case of cathode and Rontgen rays, the velocity with which the corpuscles are emitted seems, as far as we know at present, to vary slightly, but only slightly, with the nature of the substance on which the rays fall. May not this indicate that the first effect of the primary rays is to detach a neutral doublet, consisting of a positive and negative charge, this doublet being the same from whatever system it is detached ? And that the doublet is unstable and explodes, expelling the negative charge with a high velocity, and the positive one, having a much larger charge, with a much smaller velocity, the momentum of the negative charge being equal to that of the positive. Up to now we have been considering the effects produced when light is incident on metals. Lenard found (and the result has been confirmed by the experiments of J.

J. Thomson and Lyman) that certain kinds of ultra-violet light ionize a gas when they pass through. The type of ultra-violet light which produces this effect is so easily absorbed that it is stopped by a layer a few millimetres thick of air at atmospheric pressure. Ionization by Collision.—When the ionization of the gas is produced by external agents such as Rontgen rays or ultra-violet light, the electric field produces a current by setting the positive ions moving in one direction, and the negative ones in the opposite; it makes use of ions already made and does not itself give rise to ionization. In many cases, however, such as in electric sparks, there are no external agents to produce ionization and the electric field has to produce the ions as well as set them in motion. When the ionization is produced by external means the smallest electric field is able to produce a current through the gas; when, however, these external means are absent no current is produced unless the strength of the electric field exceeds a certain critical value, which depends not merely upon the nature of the gas but also upon the pressure and the dimensions of the vessel in which it is contained. The variation of the electric field required to produce discharge can be completely explained if we suppose that the ionization of the gas is produced by the impact with its molecules of corpuscles, and in certain cases of positive ions, which under the influence of the electric field have acquired considerable kinetic energy. We have direct evidence that rapidly moving corpuscles are able to ionize molecules against which they strike, for the cathode rays consist of such corpuscles, and these when they pass through a gas produce large amounts of ionization. Suppose then that we have in a gas exposed to an electric field a few corpuscles. These will be set in motion by the field and will acquire an amount of energy in proportion to the product of the electric force, their charge, and the distance travelled in the direction of the electric field between two collisions with the molecules of the gas. If this energy is sufficient to give them the ionizing property possessed by cathode rays, then when a corpuscle strikes against a molecule it will detach another corpuscle; this under the action of the electric field will acquire enough energy to produce corpuscles on its own account, and so as the corpuscles move through the gas their number will increase in geometrical progression. Thus, though there were but few corpuscles to begin with, there may be great ionization after these have been driven some distance through the gas by the electric field.

The number of ions produced by collisions can be calculated by the following method. Let the electric force be parallel to the axis of x, and let n be the number of corpuscles per unit volume at a place fixed by the co-ordinate x; then in unit time these corpuscles will make.nu/A collisions with the molecules, if u is the velocity of a corpuscle and A the mean free path of a corpuscle. When the corpuscles are moving fast enough to produce ions by collision their .velocities are very much greater than those they would possess at the same temperature if they were not acted on by electrical force, and so we may regard the velocities as being parallel to the axis of x and determined by the electric force and the mean free path of the corpuscles. We have to consider how many of the nu/A collisions which take place per second will produce ions. We should expect that the ionization of a molecule would require a certain amount of energy, so that if the energy of the corpuscle See also:

• FELL
• FELL, JOHN (1625-1686)

In consequence of the collisions, f(XeX)nu/X corpuscles are produced per second; in consequence of the motion of the corpuscles, the number which leave unit volume per second is greater than those which enter it by—x(nu) ; while in a certain number of collisions a corpuscle will stick to the molecule and will thus cease to be a free corpuscle. Let the fraction of the number of collisions in which this occurs be g. Thus the gain in the number of corpuscles is f(XeX)nu/X, while the loss is ax(nu)-[ ;; hence dt =f(Xea) X--dx(nu)—S-u. When things are in a steady state do/dt=o, and we have dx(nu)=x(f(XeX)—)4)nu- If the current is so small that the electrical charges in the gas are not able to produce any appreciable See also:

• VARIATIONS
• VARIATIONS, CALCULUS
• VARIATIONS, CALCULUS OF

198). Since a varies inversely as the pressure, we see that a may be written in the form p4(X/p) or a/X=F(X/p). The following are some of the values of a found by Townsend for air. X Volts Pressure Pressure Pressure Pressure Pressure per cm. •17 mm. •38 mm. I•IO mm. 2•I mm. 4.1 mm. 20 •24 40 .65 '34 8o 1.35 I'3 .45 .13 120 1.8 2.O I•I .42 •13 16o 2•I 2.8 2.0 •9 .28 200 3.4 2.8 1.6 .5 240 2.45 3'8 4'0 2.35 •99 320 2• 5'5 o 2 • I 7 400 54'5 6.8 6•o 3.6 .0 48o 3.15 5.4 8•o 7'8 5'3 5 O 5.8 9.3 9.4 7.I 64o 3.25 6.2 Io•6 to•8 8.9 We see from this table that for a given value of X, a for small pressures increases as the pressure increases; it attains a maximum at a particular pressure, and then diminishes as the pressure increases. The increase in the pressure increases the number of collisions, but diminishes the energy acquired by the corpuscle in the electric field, and thus diminishes the change of any one collision resulting in ionization. If we suppose the field is so strong that at some particular pressure the energy acquired by the corpuscle is well above the value required to ionize at each collision, then it is evident that increasing the number of collisions will increase the amount of ionization, and therefore a, and a cannot begin to diminish until the pressure has increased to such an extent that the mean free path of a corpuscle is so small that the energy acquired by the corpuscle from the electric field falls below the value when each collision results in ionization.

The value of p, when X is given, for which a is a maximum, is proportional to X; this follows at once from the fact that a is of the form X. F(X/p). The value of X/p for which F(X/p) is a maximum is seen from the preceding table to be about 420, when X is expressed in volts per centimetre and p in millimetres of mercury. The maximum value of F(X/p) is about 1/6o. Since the current passing between two planes at a distance 1 apart is inEat or io5X1F(Xlp), and since the force between the plates is supposed to be uniform, X1 is equal to V, the potential between the plates; hence the current between the plates is ioevF(X/p), and the greatest value it can have is ioeY 160. Thus the ratio between the current between the plates when there is ionization and when there is none cannot be greater than Ev/60when V is measured in volts. This result is based on Townsend's experiments with very weak currents; we must remember, however, that when the collisions are so frequent that the effects of collisions can accumulate, a may have much larger values than when the current is small. In some experiments made by J. J. Thomson with intense currents from cathodes covered with hot lime, the increase in the current when the potential difference was 6o volts, instead of being e times the current when there was no ionization, as the preceding theory indicates, was several hundred times that value, thus indicating a great increase in a with the strength of the current. Townsend has shown that we can deduce from the values of a the mean free path of a corpuscle. For if the ionization is due to the collisions with the corpuscles, then unless one collision detaches more than one corpuscle the maximum number of corpuscles produced will be equal to the number of collisions.

When each collision results in the production of a corpuscle, a= 1/X and is independent of the strength of the electric field. Hence we see that the value of a, when it is independent of the electric field, is equal to the reciprocal of the free path. Thus from the table we infer that at a pressure of 17 mm. the mean free path is 1/325 CM.; hence at I mm. the mean free path of a corpuscle is 1/19 cm. Townsend has shown that this value of the mean free path agrees well with the value 1/21 cm. deduced from the kinetic theory of gases for a corpuscle moving through air. By measuring the values of a for hydrogen and carbonic acid gas Townsend and See also:

• KIRBY, WILLIAM (1759–1850)

This distribution of electricity will make the electric force diminish from the cathode to the place where there is as much positive as negative electricity, where it will have its minimum value, and then increase up to the anode. The expression i=ioeatapplies to the case when there is no source of ionization in the gas other than the collisions; if in addition to this there is a source of uniform ionization producing q ions per cubic centimetre, we can easily show that e i= local+ (eat— 1). With regard to the minimum energy which must be possessed by a corpuscle to enable it to produce ions by collision, 1 ownsend (loc. cit.) came to the conclusion that to ionize air the corpuscle must possess an amount of energy equal to that acquired by the fall of its charge through a potential difference of about 2 volts. This is also the value arrived at by H. A. Wilson by entirely different considerations. Stark, however, gives 17 volts as the minimum for ionization. The energy depends upon the nature of the gas ; recent experiments by See also:

• DAWES, HENRY LAURENS (1816-1903)
• DAWES, RICHARD (1708-1766)

The current, however, could be maintained indefinitely if the positive ions in their journey back to the cathode also produced ions by collisions, for then we should have a kind of regenerative process by which the supply of corpuscles could be continually renewed. To maintain the current it is not necessary that the ionization resulting from the positive ions should be anything like as great as that from the negative, as the investigation given below shows a very small amount of ionization by the positive ions will suffice to maintain the current. The existence of ionization by collision with positive ions has been proved by Townsend. Another method by which the current could be and is maintained is by the anode emitting corpuscles under the impact of the positive ions driven against it by the electric field. J. J. Thomson has shown by direct experiment that positively electrified particles when they strike against a metal plate cause the metal to emit corpuscles (J. J. Thomson, Pros. See also:

• COMB (a word common in various forms to Teut. languages, cf. Ger. Kamm, the Indo-Europ. origin of which is seen in yoµros, a peg or pin, and Sanskrit, gambhas, a tooth)

13, p. 212; See also:

• AUSTIN
• AUSTIN, ALFRED
• AUSTIN, JOHN (179o-1859)
• AUSTIN, SARAH (1793-1867)
• AUSTIN, STEPHEN FULLER (1793-1836)

Thus the number of positive ions produced in the layer is aioe"xdx. If these went straight to the cathode without a collision, each of them would have received an amount of kinetic energy Vex/d when they struck the cathode, and the energy of the group of ions would be Vex/d.aios' dx. The positive ions will, however, collide with the molecules of the gas through which they are passing, and this will diminish the energy they possess when they reach the cathode. The diminution in the energy will increase in geometrical See also:

• PRO

We see from the expression for V that when ($-a)d is very large V = ($-a)2d/kea. Thus V becomes See also:

• INFINITE (from Lat. in, not, finis, end or limit; cf. findere, to deave)

It is thus necessary to take into account the ionization of the positive ions. Let m be the number of positive ions per unit volume, and w their velocity, the number of collisions which occur in one second in one cubic centimetre of the gas will be proportional to mwp, where is the pressure of the gas. Let the number of ions which result from these collisions be 7mw; 7 will be a function of p and of the strength of the electric field. Let as before n be the number of corpuscles per cubic centimetre, u their velocity, and anu the number of ions which result in one second from the collisions between the corpuscles and the gas. The number of ions produced per second per cubic centimetre is equal to anu+7mw; hence when things are in a steady state dx(nu) = anu+7mw, and e(nu+mw) =i, where e is the charge on the ion and i the current through the gas. The solution of these equations when the field is uniform between the plates, is enu=C€(ay')x-7i/(a-7), emw = —Co(a~')x+ai/(a-7), where C is a constant of integration. If there is no emission of positive ions from the anode enu=i, when x=d. Determining C from this condition we find ( enu = Q Z Y 1 as(''-d) - y , emw = a a- - -e(a-y)(:-d) If the cathode did not emit any corpuscles owing to the See also:

• BOMBARDMENT

Thus if we have a strong field close to the cathode we might still get i a-7 the discharge though the rest of the field were comparatively weak. Such a distribution of electric force requires, however, a great accumulation of charged ions near the cathode; until these ions accumulate the field will be uniform. If the uniform field existing in the gas before the discharge begins were strong enough to make the corpuscles produce ions by collision, but not strong enough to make the positive ions act as ionizers, there would be some accumulation of ions, and the amount of this accumulation would depend upon the number of free corpuscles originally present in the gas, and upon the strength of the electric field. If the accumulation were sufficient to make the field near the cathode so strong that the positive ions could produce fresh ions either by collision with the cathode or with the gas, the discharge would pass though the gas; if not, there will be no continuous discharge. As the amount of the accumulation depends on the number of corpuscles present in the gas, we can understand how it is that after a spark has passed, leaving for a time a supply of corpuscles behind it, it is easiet to get a discharge to pass through the gas than it was before. ;y: il~•;.,, ;j~;~ The inequality of the electric field in the gas when a continuous discharge is A'.IIIIiuI'III•)• III...II •'aiill'gr~l'' passing through it is very obvious when the pressure of the gas is low. In this case the discharge presents a highly /!!!!~!!!\ differentiated appearance of which a type is represented in fig. 15• Starting Giiiillll` from the cathode we have a thin velvety luminous glow in contact with the sur- See also:

• FACE (from Lat. fades, derived either from facere, to make, or from a root fa-, meaning " appear "; cf. Gr. cbatvstv)

The magnitude of the electric force at different parts of the discharge is represented in fig. 16, where the ordinates represent the electric force at different parts of the tube, the cathode being on the right. We see that the electric force is very large indeed between the negative glow and the cathode, much larger than in any other part of the tube. It is not constant in this region, but increases as we approach the cathode. The force reaches a minimum either in the negative glow itself or in the part of the Faraday dark space just outside, after which it increases towards the positive column. In the case of a uniform positive column the electric force along it is constant until we get quite close to the anode, when a sudden change, called the " anode fall," takes place in the potential. The difference of potential between the cathode and the (a) (b) Fxa. 15.negative glow is called the " cathode potential fall " and is found to be constant for wide variations in the pressure of the gas and the current passing through. It increases, however, considerably when the current through the gas exceeds a certain critical value, depending among other things on the size of the cathode. This cathode fall of potential is shown by experiment to be very approximately equal to the minimum potential difference. The following table contains a comparison of the measurements of the cathode fall of potentials in various gases made by Warburg (Wied. Ann., 1887, 31, p.

545, and 189o, 40, Yo/!s per Cm e0 rdivimmm '° iverimmmim^_ p. 1), Capstick (Prot. Roy. Society, 1898, 63, p. 356), and See also:

• STRUTT, JEDEDIAH (1726-1797)

It is only with sparks not much longer than the critical spark length that we Could See also:

• HOPE, ANTHONY
• HOPE, THOMAS (c. 1770-1831)

. 340-350 .. .. .. 341 H2 . . . about 300 298 .. 168 302-308 02 . . . 369 .. N2 . 230 if free 232 .. 207 251 from oxygen Hg vapour 340 Helium . . ..

226 .. 261-326 See also:

• H2O (combined)

.. .. .. .. H, 300 .. 295 280 230 213 190 168 185 169 172 N2 . . 232 226 .. .. .. .. .. 207 178 125 170 He .

226 .. .. .. .. .. .. . . 8o 78.5 69 See also:

• ARGON (from the Gr. 6.-, privative, and Epyov, work; hence meaning " inert ")

16. If the electric force at a distance x from the cathode were proportional to e-Px we should have a state of things much resembling the distribution of electric force near the cathode. If we apply to this distribution the methods used above for the case when the force was uniform, we shall find that the minimum potential is less and the critical spark length greater than when the electric force is uniform. ' Potential Difference required to produce a Spark of given Length. —We may regard the region between the cathode and the negative glow as a place for the production of corpuscles, these corpuscles finding their way from this region through the negative glow. The parts of this glow towards the anode we may regard as a cathode, from which, as from a hot lime cathode, corpuscles are emitted. Let us now consider what will happen to these corpuscles shot out from the negative glow with a velocity depending on the cathode fall of potential and independent of the pressure. These corpuscles will collide with the molecules of the gas, and unless there is an external electric field to maintain their velocity they will soon come to rest and accumulate in front of the negative glow. The electric force exerted by this cloud of corpuscles will diminish the strength of the electric field in the region between the cathode and the negative glow, and thus tend to stop the discharge. To keep up the discharge we must have a sufficiently strong electric field between the negative glow and the anode to remove the corpuscles from this region as fast as they are sent into it from the cathode. If, however, there is no production of ions in the region between the negative glow and the anode, all the ions in this region will have come from near the cathode and will be negatively charged; this negative electrification will diminish the electric force on the cathode side of it and thus tend to stop the discharge. This back electric field could, however, be prevented by a little ionization in the region between the anode and glow, for this would afford a supply of positive ions, and thus afford an opportunity for the gas in this region to have in it as many positive as negative ions; in this case it would not give rise to any back electromotive force.

The ionization which produces these positive ions may, if the field is intense, be due to the collisions of corpuscles, or it may be due to radiation analogous to ultra-violet, or soft RSntgen rays, which have been shown by experiment to accompany the discharge. Thus in the most simple conditions for discharge we should have sufficient ionization to keep up the supply of positive ions, and an electric field strong enough to keep the velocity of the negative corpuscle equal to the value it has when it emerges from the negative glow. Thus the force must be such as to give a constant velocity to the corpuscle, and since the force required to move an ion with a given velocity is proportional to the pressure, this force will be proportional to the pressure of the gas. Let us call this force ap; then if 1 is the distance of the anode from the negative glow the potential difference between these points will be See also:

• ALP

486), See also:

• LIEBIG, JUSTUS VON, BARON (1803-1873)

Trans. 193, p. 377), Bouty (Comptes rendus, 131, pp. 469, 503), Earhart (Phil. Mag. [6], 1, p. 147), Carr (Phil. Trans., 1903), See also:

• RUSSELL (FAMILY)
• RUSSELL, ISRAEL COOK (1852- )
• RUSSELL, JOHN (1745-1806)
• RUSSELL, JOHN (d. 1494)
• RUSSELL, JOHN RUSSELL, 1ST EARL (1792-1878)
• RUSSELL, JOHN SCOTT (1808–1882)
• RUSSELL, LORD WILLIAM (1639–1683)
• RUSSELL, SIR WILLIAM HOWARD
• RUSSELL, THOMAS (1762-1788)
• RUSSELL, WILLIAM CLARK (1844– )

[6], to, p. 617), Kinsley (Phil. Mag. [6], 9, 692), See also:

• RITTER, HEINRICH (1791-1869)
• RITTER, KARL (1779–1859)

With sparks of these lengths it was found that it was possible to get a discharge with less than 330 volts, the minimum potential difference in air. The results of these observers show that there is no diminution in the minimum potential difference required to produce discharge until the spark length gets so small that the average electric force between the electrodes amounts to about one million volts per centimetre. When the force rises to this value a discharge takes place even though the potential difference is much less than 330 volts; in some of Earhart's experiments it was only about 2 volts. This kind of discharge is determined not by the condition that the potential difference should have a given value, but that the electric force should have a given value. Another point in which this discharge differs from the ordinary one is that it is influenced entirely by the nature of the electrodes and not by the nature or pressure of the gas between them, whereas the ordinary discharge is in many cases not affected appreciably by changes in the metal of the electrodes, but is always affected by changes in the pressure and character of the gas between them. Kinsley found that when one of these small sparks passed between the electrodes a kind of metallic bridge was formed between them, so that they were in metallic connexion, and that the distance between them had to be considerably increased before the bridge was broken. Almy (Phil. Mag., Sept. 1908), who used very small electrodes, was unable to get a discharge with less than the minimum spark potential even when the spark length was reduced to one-third of the wave length of sodium light. He suggests that the discharges obtained with larger electrodes for smaller voltages are due to the electrodes being dragged together by the electrostatic attraction between them. Constitution of the Electric Spark.—Schuster and Hemsalech (Phil. Trans.

193, p. 189), Hemsalech (Comptes Rendus, 130, p. 898; 132, p. 917; Jour. de Phys. 3. 9, p. 43, and Schenck, Astrophy. Jour. 14, p. 116) have by spectroscopic methods obtained very interesting results about the constitution of the spark. The method employed by Schuster and Hemsalech was as follows: Suppose we photograph the spectrum of a horizontal spark on a film which is on the rim of a wheel rotating about a horizontal axis with great velocity. If the luminosity travelled with infinite speed from one electrode to the other, the See also:

• IMAGE (Lat. imago, perhaps from the same root as imitari, copy, imitate)

If, however, the speed with which the luminosity travelled between the electrodes was comparable with the speed of the film, the line would be inclined to the horizontal, and by measuring the inclinations we could find the speed at which the luminosity travelled. In this way Schuster and Hemsalech showed that when an oscillating discharge passed between metallic terminals in air, the first spark passes through the air alone, no lines of the metal appearing in its spectrum. This first spark vaporizes some of the metal and the subsequent sparks passing mainly through the metallic vapour; the appearance of the lines in the film shows that the velocity of the luminous part of the vapour was finite. The velocity of the vapour of metals of low atomic weight was in general greater than that of the vapour of heavier metals. Thus the velocity of aluminium vapour was 1890 metres per second, that of zinc and cadmium only about 545. Perhaps the most interesting point in the investigation was the discovery that the velocities corresponding to different lines in the spectrum of the same metal were in some cases different. Thus with bismuth some of the lines indicated a velocity of 1420 metres per second, others a velocity of only 550, while one (A=3793) showed a still smaller velocity. These results are in accordance with a view suggested by other phenomena that many of the lines in a spectrum produced by an electrical discharge originate from systems formed during the discharge and not from the normal atom or molecule. Schuster and Hemsalech found that by inserting a coil with large self induction in the primary circuit they could obliterate the air lines in the discharge. Schenck, by observing the appearance presented when an alternating current, produced by discharging See also:

• LEYDEN, JOHN (1775-1811)

We have already described the general appearance of the discharge through gases at low pressures (see p. 657). There is, however, one form of discharge which is so striking and beautiful that it deserves more detailed consideration. In this type of discharge, known as the striated discharge, the positive column is made up of alternate bright and dark patches known as striations. Some of these are represented in fig. 17, which is taken from a paper by De la See also:

• RUE (Fr. rue, Lat. ruta, from Gr. pur)

If the discharge tube is wide atone place and narrow in another the striations will be closer together in the narrow parts than in the wide. The distance between the striations depends on the current through the tube. The relation is not a very simple one, as an increase of current sometimes increases while under other circumstances it decreases the distance between the striations (see Willows, Proc. Camb. Phil. Soc. ro, p. 302). The electric force is not uniform along the striated discharge, but is greater in the bright than in the dark parts of the striation. An example is shown in fig. 16, due to H. A. Wilson, which shows the distribution of electric force at every place. in a striated discharge.

In experiments made by J. J. Thomson (Phil. Mag., Oct. 1909), using a Wehnelt cathode, the variations in the electric force were more pronounced than those fl ,6E4L... t FE(tjdtid!'tffPE'fll:6t: (:~Rlttlif€ lf'tEliftllP{(Etf :E(tt' - -- o©o©©oOo00000000000, 1 IoIts,~etl aono manTeT See also:

• PIT (O. E. pytt, cognate with Du. put, Ger. Pfutze, &c., all ultimately adaptations of Lat. puteus, well, formed from root pu-, to cleanse, whence gurus, clean, pure)

49, p. 238), who has made a very careful study of the distribution of temperature in a discharge tube, finds that in those tubes the temperature varies in the same way as the electric force, but that this temperature (which it must be remembered is the average temperature of all the molecules and not merely of those which are taking part in the discharge) is by no means high; in no part of the discharge did the temperature in his experiments exceed roe C. Theory of the Striations.—We may regard the heaping up of the negative charges at intervals along the discharge as the fundamental feature in the striations,, and this heaping up may be explained as follows. Imagine a corpuscle projected with considerable velocity from a place where the electric field is strong, such as the neighbourhood of the cathode; as it moves towards the anode through the gas it will collide with the molecules, ionize them and lose energy and velocity. Thus unless the corpuscle is acted on by a field strong enough to supply it with the energy it loses by collision, its speed will gradually diminish. Further, when its energy falls below a certain value it will unite with a molecule and become part of a negative ion, instead of a corpuscle; at this stage there will be a sudden and 4e very large diminution in its velocity. Let us now follow the course of a stream of corpuscles starting from the cathode and approaching the anode. If the speed falls off as the stream proceeds, the corpuscles in the See also:

• REAR

The luminosity at the head of the striations is due to the recombination of the ions. These ions have acquired considerable energy from the electric field, and this energy will be available for supplying the energy radiated away as light. The recombination of ions which do not possess considerable amounts of energy does not seem to give rise to luminosity. Thus, in an ionized gas not exposed to an electric field, although we have recombination between the ions, we need not have luminosity. We have at present no exact data as to the amount of energy which must be given to an ion to make it luminous on recombination; it also certainly varies with the nature of the ion; thus even with hot Wehnelt cathodes J. J. Thomson has never been able to make the discharge through air luminous with a potential less than from 16 to 17 volts. The mercury lamps, however, in which the discharge passes through mercury vapour are luminous with a potential difference of about 12 volts. It follows that if the preceding theory be right the potential difference between two bright striations must be great enough to make the corpuscles ionize by collision and also to give enough energy to the ions to make them luminous when they recombine. The difference of potential between the bright parts of successive striations has been measured by Hohn (Phys. Zeit. 9, p.

558); it varies with the pressure and with the gas. The smallest value given by Hohn is about 15 volts. In some experiments made by J. J. Thomson, when the pressure of the gas was very low, the difference of potential between two adjacent dark spaces was as low as 3.75 volts. The Arc Discharge.—The discharges we have hitherto considered have been characterized by large potential differences and small currents. In the arc discharge we get very large currents with comparatively small potential differences. We may get the arc discharge by taking a battery of cells large enough to give a potential difference of 6o to 8o volts, and connecting the cells with two carbon terminals, which are put in contact, so that a current of electricity flows round the circuit. If the terminals, while the current is on, are drawn apart, a bright discharge, which may carry a current of many amperes, passes from one to the other. This arc discharge, as it is called, is characterized by intense heat and by the brilliant luminosity of the' terminals. This makes it a powerful source of light. The temperature of the positive terminal is much higher than that of the negative.

According to Violle (Comptes Rendus, 115, p. 1273) the temperature of the tip of the former is about 3500° C., and that of the latter 2700° C. The temperature of the arc itself he found to be higher than that of either of its terminals. As.the arc passes, the positive terminal gets hollowed out into a See also:

• CRATER

The See also:

• INTRINSIC (through Fr. intrinsique, from Lat. intrinsecus, inwardly; inter, within, secus, following, from root of sequi, to follow)

Zeit. 4, p. 150) gives for this connexion the relation V =m+nl, where m and n are constants. Mrs Ayrton (The Electric Arc, See also:

• CHAP

OP in A-inpa,03, Frohlich's equation have been determined by several experimenters. For carbon electrodes in air at atmospheric pressure m is about 39 volts, varying somewhat with the size and purity of the carbons; it is diminished by soaking the terminals in salt solution. The value of n given by different observers varies considerably, ranging from .76 to 2 volts when 1 is measured in millimetres; it depends upon the current, diminishing as the current increases. When metallic terminals are used instead of carbons, the value of m depends upon the nature of the metal, m in general being larger the higher the temperature at which the metal volatilizes. Thus v. See also:

• LANG, ANDREW (1844— )
• LANG, KARL HEINRICH, RITTER VON (1764-1835)

33, p. 609) gives Pt =28, Fe =20, Ag =8, while Arons (Wied. Ann. 31, p. 384) found for Hg the value 12.8; in this case the fall of potential along the arc itself was abnormally small. In comparing these values it is important to remember that Lecher (loc. cit.) has shown that with Fe or Pt terminals the arc discharge is intermittent. Arons has shown that this is also the case with Hg terminals, but no intermittence has been detected with terminals of C, Ag or Cu. The preceding measurements refer to mean potentials, and no conclusions as to the actual potential differences at any time can be drawn when the discharge is discontinuous, unless we know the law of discontinuity. The ease with which an arc is sustained depends greatly on the nature of the electrodes; when they are brass, zinc, cadmium, or magnesium it is exceedingly difficult to get the arc. The potential difference between the terminals is affected by the pressure of the gas. The most extensive series of experiments. on this point is that made by See also:

• DUNCAN
• DUNCAN, ADAM DUNCAN, 1ST VISCOUNT (1731-1804)
• DUNCAN, PETER MARTIN (1824-1891)
• DUNCAN, THOMAS (1807—1845)

6o), whose results are represented in fig. 20. We see from these curves that for very short arcs the potential difference increases continuously with the pressure, but for longer ones there is a critical pressure at which the potential difference is a minimum, and that this critical pressure seems to increase with the length of arc. Length of Arc. The nature of the gas also affects the potential difference. The magnitude of this effect may be gathered from the following values given by Arons (Ann. der Phys. 1, p. 700) for the potential difference required to produce an arc 1.5 mm. long, carrying a current of 4.5 amperes, between terminals of different metals in air and pure nitrogen. Terminal. Air. Nitrogen. Terminal.

Air. Nitrogen. Ag . . 21 ? Pt 36 3o Zn . . 23 21 Al 39 27 Cd . . 25 21 Pb .. 18 Cu . . 27 30 Mg .. 22 Fe . . 29 20 Thus, with the discharge for an arc of given length and current, the nature of the terminals is the most important factor in deter-See also:

• MINING

The potential gradient in the arc is very far from being uniform. With carbon terminals Luggin (Wien. Ber. 98, p. 1192) found that, with a current of 15 amperes, there was a fall of potential of 337 close to the anode, and one 8.7 close to the cathode, so that the curve representing the distribution of potential between the terminals would be somewhat like that shown in fig. 21. We have seen that a somewhat analogous distribution of potential holds in the case of conduction through flames, though in that case the greatest drop of potential is in general at the cathode and not at the anode. The difference between the changes of potential at the anode and cathode is not so large with Fe and Cu terminals as with carbon ones; with mercury terminals, Arons (Wied. Ann. 58, p. 73) found the anode fall to be 7.4 volts, the cathode fall 5.4 volts. The case of the arc when the cathode is a See also:

• POOL

447), See also:

• WILLS, WILLIAM GORMAN (1828-1891)

If, however, there is ionization either in the gas or at the anode, the positive ions will diffuse into the region of the discharge until they are sensibly equal in number to the negative ions. When this is the case the back electromotive force is destroyed and the same potential difference will carry a much larger current. The arc discharge may be regarded as analogous to the discharge between incandescent terminals, the only difference being that in the arc the terminals are maintained in the state of incandescence by the current and not by external means. On this view the cathode is bombarded by positive ions which heat it to such a temperature that negative corpuscles sufficient to carry the current are emitted by it. These corpuscles See also:

• BOMBARD (derived through Med. Lat. and Fr. forms from Gr. (3oµ(3eiv, to make a humming noise)

285), and found to agree with that of the ions produced by RSntgen or uranium radiation, while Townsend (Phil. Trans. 195, p. 2S9) has shown that the charge on these ions is the same as that on the ions streaming from the point. If the pointed electrode be placed at right angles to a metal plane serving as the other electrode, the discharge takes place when, for a given distance of the point from the plane, the potential difference between the electrodes exceeds a definite value depending upon the pressure and nature of the gas through which the discharge passes; its value also depends upon whether, beginning with a small potential difference, we gradually increase it until discharge commences, or, beginning with a large potential difference, we decrease it until the discharge stops. The value found by the latter method is less than that by the former. According to Chattock's measurements the potential difference V for discharge between the point and the plate is given by the linear relation V a+bl, where l is the distance of the point from the plate and a and b are constants. From v. Obermayer's (Wien. Ber. Too, 2, p. 127) experiments, in which the distance l was greater than in Chattock's, it would seem that the potential for larger distances does not increase quite so rapidly with 1 as is indicated by Chattock's relation.

The potential required to produce this discharge is much less than that required to produce a spark of length 1 between parallel plates; thus from Chattock's experiments to produce the point discharge when 1=.5 cm. in air at atmospheric pressure requires a potential difference of about 3800 volts when the pointed electrode is positive, while to produce a spark at the same distance between plane electrodes would require a potential difference of about 15,000 volts. Chattock showed that with the same pointed electrode the value of the electric intensity at the point was the same whatever the distance of the point from the plane. The value of the electric intensity depended upon the sharpness of the point. When the end of the pointed electrode is a hemisphere of radius a, Chattock showed that for the same gas at the same pressure the electric intensity f when discharge takes place is roughly proportioned to a-0'8. The value of the electric intensity at the pointed electrode is much greater than its value at a plane electrode for long sparks; but we must remember that at a distance from a • pointed electrode equal to a small multiple of the radius of curvature of its extremity the electric intensity falls very far I Anode C,Mede below that required to produce discharge in a uniform field, so that the discharge from a pointed electrode ought to be compared with a spark whose length is comparable with the radius of curvature of the point. For such short sparks the electric intensity is very high. The electric intensity required to produce the discharge from a gas diminishes as the pressure of the gas diminishes, but not nearly so rapidly as the electric intensity for long sparks. Here again the discharge from a point is comparable with short sparks, which, as we have seen, are much less sensitive to pressure changes than longer ones. The minimum potential at which the electricity streams from the point does not depend upon the material of which the point is made; it varies, however, considerably with the nature of the gas. The following are the results of some experiments on this point. Those in the first two columns are due to Rontgen, those in the third and See also:

• FOURTH

Point +. Pressure 76o. Pressure 205. Pressure 1io. Point +. Point —. Volts. Volts. Volts. Volts. Hz 1296 1174 2125 1550 Oz. 2402 1975 2800 2350 CO .

2634 2100 9 3 253 NO . 3 3 2543 CO, . 3287 2655 3475 2100 N2 . .. .. 2600 2000 Air . .. .. 2750 2050 We see from this table that in the case of the discharge from a positively electrified point the greater the molecular weight of the gas the greater the potential required for discharge. Rontgen concluded from his experiments that the discharging potential from a positive point in different gases at the same pressure varies inversely as the mean free path of the molecules of the gas. In the same gas, however, at different pressures the discharging potential does not vary so quickly with the pressure as does the mean free path. In Precht's experiments, in which different gases were used, the variations in the discharging potential are not so great as the variations in the mean free path of the gases.

The current of electrified air flowing from the point when the electricity is escaping—the well-known " electrical See also:

• WIND
• WIND (a common Teut. word, cognate with Skt. vats, Lat. ventus, cf. " weather," to be of course distinguished from to " wind," to coil or twist, O.Eng. windan, cf. "wander," "wend," &c.)

This also agrees with Arrhenius' results for the discharge from a positive point. The velocity of the negative ion is greater than that of a positive one under the same potential gradient, so that the reaction for the negative point should be less than that for a positive one, but the excess of the positive reaction over the negative is much greater than that of the velocity of the negativeion over the velocity of the positive. There is, however, reason to believe that a considerable condensation takes place around the negative ion as a nucleus after it is formed, so that the velocity of the negative ion under a given potential gradient will be greater immediately after the ion is formed than when it has existed for some time. The measurements which have been made of the velocities of the ions relate to those which have been some time in existence, but a large part of the reaction will be due to the newly-formed ions moving with a greater velocity, and thus giving a smaller reaction than that calculated from the observed velocity. With a given potential difference between the point and the neighbouring conductor the current issuing from the point is greater when the point is negative than when it is positive, except in oxygen, when it is less. Warburg (Sitz. Akad. d. Wissensch. zu Berlin, 1849, 50, p. 770) has shown that the addition of a small quantity of oxygen to nitrogen produces a great diminution in the current from a negative point, but has very little effect on the discharge from a positive point. Thus the removal of a trace of oxygen made a leak from a negative point 50 times what it was before. Experiments with hydrogen and helium showed that impurities in these gases had a great effect on the current when the point was negative, and but little when it was positive. This suggests that the impurities, by condensing round the negative ions as nuclei, seriously diminish their velocity.

If a point is charged up to a high and rapidly alternating potential, such as can be produced by the electric oscillations started when a Leyden See also:

• JAR

For let Eo be the electromotive force of the battery, R the resistance of the leads, i the current, the potential difference between the terms in the gas will be Eo—Ri. Let See also:

• ABC

11, p. 158). The geometrical interpretation of this condition is that the straight line LM must, at the point where it cuts the characteristic curve, be steeper than the tangent to characteristic curve.