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RONTGEN RAYS, W
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See also:RONTGEN RAYS, W . K. Rontgen discovered in 1895 (Wied. See also:Ann. 64, p. I) that when the electric See also:discharge passes through a See also:tube exhausted so that the See also:glass of the tube is brightly phosphorescent, phosphorescent substances such as See also:potassium platinocyanide became luminous when brought near to the tube. He found that if a thick piece of See also:metal, a See also:coin for example, were placed between the tube and a See also:plate covered with the phosphorescent substance a See also:sharp See also:shadow of the metal was See also:cast upon the plate; pieces of See also:wood or thin plates of See also:aluminium cast, however, only partial shadows, thus showing that the See also:agent which produced the See also:phosphorescence could See also:traverse 'with considerable freedom bodies opaque to See also:ordinary See also:light. He found that as a See also:general See also:rule the greater the See also:density of the substance the greater its opacity to this agent. Thus while this effect could pass through the fleshit was stopped by the bones, so that if the See also:hand were held between the discharge tube and a phosphorescent See also:screen the outline of the bones was distinctly visible as a shadow cast upon the screen, or if a See also:purse containing coins were placed between the tube and the screen the purse itself cast but little shadow while the coins cast a very dark one. Rontgen showed that the cause of the phosphorescence, now called Rontgen rays, is propagated in straight lines starting from places where the See also:cathode rays strike against a solid obstacle, and the direction of See also:propagation is not See also:bent when the rays pass from one See also:medium to another, i.e. there is no See also:refraction of the rays. These rays, unlike cathode rays or Canalstrahlen, are not deflected by magnetic force; Rontgen could not detect any deflection with the strongest magnets at his disposal, and later experiments made with stronger magnetic See also:fields have failed to reveal any effect of the magnet on the rays. The rays affect a photographic plate as well as a phosphorescent screen, and shadow photographs can be readily taken. The See also:time of exposure required depends upon the intensity of the rays, and this depends upon the See also:state of the tube, and the electric current going through it, as well as upon the substances traversed by the rays on their See also:journey to the photographic plate. In some cases an exposure of a few seconds is sufficient, in others See also:hours may be required. The rays coming from different discharge tubes have very different See also:powers of penetration. If the pressure in the tube is fairly high, so that the potential difference between its electrodes is small, and the velocity of the cathode rays in consequence small, the Rontgen rays coming from the tube will be very easily absorbed; such rays are called " soft rays." If the exhaustion of the tube is carried further, so that there is a considerable increase in the potential See also:differences between the cathode and the anode in the tube and therefore in the velocity of the cathode rays, the Rontgen rays have much greater penetrating See also:power and See also:ale called "hard rays." With a highly exhausted tube and a powerful See also:induction coil it is possible to get appreciable effects from rays which have passed through sheets of See also:brass or See also:iron several millimetres thick. The penetrating power of the rays thus varies with the pressure in the tube; as this pressure gradually diminishes when the discharge is kept See also:running through the tube, the type of Rontgen See also:ray coming from the tube is continually changing. The lowering of pressure due to the current through the tube finally leads to such a high degree of exhaustion that the discharge has See also:great difficulty in passing, and the emission of the rays becomes very irregular. See also:Heating the walls of the tube causes some See also:gas to come off the sides, and by thus increasing the pressure creates a temporary improvement. A thin-walled See also:platinum tube is sometimes fused on to the discharge tube to remedy this defect; red-hot platinum allows See also:hydrogen to pass through it, so that if the platinum tube is heated, hydrogen from the See also:flame will pass into the discharge tube and increase the pressure. In this way hydrogen may be introduced into the tube when the pressure gets too See also:low. When liquid See also:air is available the pressure in the tube may be kept See also:constant by fusing on to the discharge tube a tube containing See also:charcoal; this dips into a See also:vessel containing liquid air, and the charcoal is saturated with air at the pressure which it is desired to maintain in the tube. Not only do bulbs emit different types of rays at different times, but the same bulb emits at the same time rays of different kinds. The See also:property by which it is most convenient to identify a ray is the absorption it suffers when it passes through a given thickness of aluminium or See also:tin-See also:foil. Experiments made by McClelland and See also:Sir J. J. See also:Thomson on the absorption of the rays produced by sheets of tin-foil showed that the absorption by the first sheets of tin-foil traversed by the rays was much greater than that by the same number of sheets when the rays had already passed through several sheets of the foil. The effect is just what would occur if some of the rays were much more readily absorbed by the tin-foil than others, for the first few layers would stop all the easily absorbable rays while the ones See also:left would be those that were but little absorbed by tin-foil. The fact that the rays when they pass through a gas ionize it and make it a conductor of See also:electricity furnishes the best means of measuring their intensity, as the measurement of the amount of conductivity they produce in a gas is both more accurate and more convenient than measurements of photographic or phosphorescent effects. Rontgen rays when they pass through See also:matter produce—as Perrin (Comptes rendus, 124, p. 455), Sagnac (Jour. de Phys., 1899, (3), 8, and J. Townsend (Proc. Camb. Phil. See also:Soc., 1899, 10, p. 217, have shown—secondary Rontgen rays as well as cathodic rays. A very See also:complete investigation of this subject has been made by Barkla and See also:Sadler (Barkla, Phil. Meg., See also:June 1906, pp. 812–828; Barkla and Sadler, Phil. Mag., See also:October 19o8, pp. 550—584; Sadler, Phil. Mag., See also:July 1909, p. 107; Sadler, Phil. Mag., See also: This is shown in the table, which gives for the different elements the reciprocal of the distance, measured in centimetres, through which the rays from the element can pass through aluminium before their See also:energy sinks to 172.7 of the value it had when entering the aluminium; this quantity is denoted in the table by A. Element. See also:Chromium Iron . . See also:Cobalt See also:Nickel . See also:Copper See also:Zinc . . See also:Arsenic See also:Selenium . See also:Strontium See also:Molybdenum See also:Rhodium See also:Silver . Tin . The radiation from chromium cannot pass through more than a few centimetres of air without being absorbed, while that from tin is as penetrating as that given out by a fairly efficient Rontgen tube. Barkla and Sadler found that the radiation characteristic of the metal is not excited unless the primary radiation is more penetrating than the characteristic radiation. Thus the characteristic radiation from silver can excite the characteristic radiation from iron, but the characteristic radiation from iron cannot excite that from silver. We may compare this result with See also:Stokes's rule for phosphorescence, that the phosphorescent light is of longer See also:wave-length than the light which excites it. The See also:discovery that each element gives out a characteristic radiation (or, as still more See also:recent See also:work indicates, a See also:line spectrum of characteristic radiation) is one of the utmost importance. It gives us, for example, the means of getting homogeneous Rontgen radiation of a perfectly definite type: it is also of fundamental importance in connexion with any theory of the Rontgen rays. We have seen that there is no See also:evidence of refraction of the Rontgen rays; it would be interesting to try if this were the See also:case when the rays passing through the refracting substance are those characteristic of the substance. Secondary Cathodic Rays.—The incidence of Rontgen rays on matter causes the matter to emit cathodic rays. The velocity of these rays is independent of the intensity of the primary Rontgen rays, but depends upon the " hardness" of the rays; it seems also to be independent of the nature of the matter exposed to the primary rays. The velocity of the cathodic rays increases as the hardness of the primary Rontgen rays increases. Innes (Proc. See also:Roy. Soc. 79, p. 442) measured the velocity of the cathodic radiation excited by the rays from Rontgen tubes, and found velocities varying from 6.2 X 1o9cm./sec. to 8-3X 1o9 em./sec. according to the hardness of the rays given out by the tube. The cathodic rays given out under the See also:action of the homogeneous secondary Rontgen radiation characteristic of the different elements have been studied by Sadler (Phil. Mag., March 1910) and Beatty (Phil. Mag., See also:August 191o). The following table giving the properties of the cathode rays excited by the radiation from various elements is taken from Beatty's See also:paper; lL is the thickness of air at atmospheric pressure andtemperature required to absorb one-See also:half of the energy of the cathode particles, t2 is the corresponding quantity for hydrogen. Radiator. tl t2 Iron .00804 .0410 Copper. •0135 '0733 Zinc .0164 •o909 Arsenic . .0255 Tin . .1672 1'37 The properties of the cathode rays excited by the radiation from tin correspond very closely with those produced in a discharge tube when the potential difference between the anode and cathode is about 30,000 volts. When Rontgen rays pass through a thin plate the cathodic radiation on the See also:side the rays emerge is more intense than on the side they enter. See also:Kaye (Phil. Trans. 209, p. 123) has shown that when cathode rays fall upon a metal two kinds of Rontgen rays are excited, one being the characteristic radiation of the metal and the other a kind independent of the nature of the metal and dependent only upon the velocity of the cathode rays. The faster the cathode rays the harder the Rontgen rays they See also:pro-duce. It would be interesting to see if there is any connexion between the velocity of the cathode rays required to excite Rontgen rays as hard as those given out say by tin and the velocity of the cathode rays which the radiation from tin produces when it falls upon any metal. Sadler has shown that metals can give off cathodic radiation even when the incident Rontgen rays are too soft to excite the characteristic Rontgen radiation of the metal, but that there is a large increase in the cathodic radiation as soon as the characteristic Rontgen radiation is excited. It is possible that the See also:shock produced by the emission of these cathode particles starts the vibrations which give rise to the characteristic rays; the cathode particles emitted when the incident rays are too soft to excite the characteristic radiation coming from a different source from those tapped by the hard rays. Absorption of Rontgen Rays.—The wide See also:variations in the penetrating power of Rontgen rays from different See also:sources is shown by the above table of the penetrating power of the characteristic rays of the different elements. Many experiments have been made on the penetration of the same rays for different substances. It is a rule to which there is no well-established exception that the greater the density of the sub-stance the greater is its power of absorbing the rays. The connexion, however, between the absorption and the density of the substance is not in general a See also:simple one, though there is evidence that for exceedingly hard rays the absorption is proportional to the density. The power of any material to absorb rays is usually measured by a coefficient A, the See also:definition of which is that a plate 1/A centimetres thick reduces the energy of the rays when they pass through it normally to Ile of their See also:original value, where e is the See also:base of the Napierian logarithms and equal to 2.7128. It has been shown that however the See also:physical state of a substance may alter if, for example, it changes from the liquid to the gaseous,—A/D, where D is the density of the substance, remains constant. It has also been shown that if we have a See also:mass M made up of masses ML, M2, M.. . of substances having coefficients of absorption AL, See also:A2, A3, . and densities Di, D2, Da, . . . then if AID for the mixture is given by the See also:equation MA/D = M1AL/DI+M2A2/D2+M,A3/D3+ this equation is true whether the substances are chemically combined or chemically mixed. From this equation, when we know AID for a binary See also:compound and for one of its constituents, we can find the value of AID for the other constituent. By the use of this principle we can find the value of AID for the elements which cannot be obtained in a See also:free state. Benoist (Jour. de Phys. (7), 28, p. 289) has shown that if the values of AID are plotted against the atomic weight we get a smooth See also:curve; if we draw this curve it is evident that we have the means of determining the atomic weight of an element by measuring its transparency to Rontgen rays when in combinatiogwith elements whose transparency is known. Benoist has applied this method to determine the atomic weight of See also:indium. The value of A/D for any one substance depends upon the type of ray used, and the ratio of the values of AID for two substances may vary very greatly with the type of ray; this is especially the case when one of the substances is hydrogen. Thus See also:Crowther (Proc. Roy. Soc., March 1909) has shown that the ratio of A for air to A for hydrogen varied from See also:loo for rays given out by a Rontgen tube at a comparatively high pressure when the rays were very soft to 5.56 when the pressure in the bulb was very low and the rays very hard. Beatty (Phil. Mag., August 1910) found that this ratio was as large as 175 for the characteristic rays given out by iron, copper, zinc and arsenic, but See also:fell to 25.0 for the rays from tin. Polarization of Rontgen Rays.—A great See also:deal of See also:attention has been paid to a phenomenon called the polarization of the Atomic weight. A. 52 367 55.9 239 59O I • 93'2 58.7? (61.3) 159.5 63.6 128.9 65.4 106.3 75.0 60.7 79.2 51.o 87.6 35.2 96.o 12.7 103.0 8.44 107.9 6.75 119.0 4'33 Rontgen rays. The nature of this effect may be illustrated by fig. 1. Suppose that AB is a stream of cathode rays striking against a solid obstacle B and P giving rise to Rontgen rays, let these rays impinge on a small See also:body P, P under these condi- tions will emit secondary rays in all directions. Barkla (Phil. Trans., 1905, A, 204, p. 467; Proc. Roy. Soc. 77, p. 247) A found that the intensity of the secondary rays, tested by in air, was less intense in the See also:plane ABP than in a plane through PB at right angles to this plane, the distances from P being the same in the two cases; the difference in the intensities amounting to about 15%. Haga (Ann. d. Phys. 28, p. 43g), who tried a similar experiment but used a photographic method to measure the intensity of the secondary rays, could not detect any difference of intensity in the two planes, but experiments by Bassler (Ann. der Phys. 28, p. 8o8) and Vegard (Proc. Roy. Soc. 83, p. 379) have confirmed Barkla's original observations.
The " polarization " is much more marked if instead of exciting the secondary radiation in P by the Rontgen rays from a discharge tube we do so by means of secondary rays. If, for example, in the case illustrated by fig. 1 we allow a See also:beam of Rontgen rays to fall upon B instead of the cathode rays, the difference between the intensities in the plane ABP and in the plane at right angles to it are very much increased. It is only the scattered secondary radiation which shows this " polarization "; the characteristic secondary radiation emitted by the body at P is quite unpolarized. The existence of this effect has a very important bearing on the nature of Rontgen rays. Whether Rontgen rays are or are not a See also:form of light, i.e. are some form of electromagnetic disturbance propagated through the See also:aether, is a question on which See also:opinion is not unanimous. They resemble light in their rectilinear propagation; they affect a photographic plate and, See also:Brandes and Dorn have shown, they produce an effect, though a small one, on the retina, giving rise to a very faint See also:illumination of the whole See also: Rontgen has never succeeded in observing effects which prove the existence of diffraction. Fomm (Wied. Ann. 59, p. 50) observed in the photograph of a narrow slit light and dark bands which looked like diffraction bands; but observation with slits of different sizes showed that they were not of this nature, and Haga and See also:Wind (I4'ied. Ann. 68, p. 884) have explained them as contrast effects. These observers, however, noticed with a very narrow See also:wedge-shaped slit a broadening of the See also:image of the narrow See also:part which they are satisfied could not be explained by the causes. See also:Walter and Pohl (Ann. der Phys. 29, p. 331) could not observe any diffraction effects, though their arrangement would have enabled them to do so if the wave-length had not been smaller than 1.5 X 10–9 cm. Sir See also:George Stokes (Prot. See also:Manchester Lit. and Phil. Soc., 1898) put forward the view that the disturbances which constitute the rays are not regular periodic undulations but very thin pulses. Thomson (Phil. Mag. 45, p. 172) has shown that when charged particles are suddenly stopped, pulses of very intense electric and magnetic disturbances are started. As the cathode rays consist of negatively electrified particles, the impact of these on a solid would give rise to these intense pulses. The electromagnetic theory therefore shows that effects resembling light, inasmuch as they are electromagnetic disturbances propagated through the aether, must be produced when the cathode rays strike against an obstacle. Since under these circumstances Rontgen rays are produced, itseems natural, unless See also:direct evidence to the contrary is obtained, to connect the Rontgen rays with these pulses. This view explains very simply the " polarization " of the rays; for, suppose the cathode particle moving from A to B were stopped at its first impact with the plate B (fig. 1), the electric force transmitted along BP would be in the plane ABP at right angles to BP. When this electric force reached the body at P it would accelerate any electrified particles in that body, the See also:acceleration being parallel to AB. Each of these accelerated particles would start electric waves. The theory of such waves shows that their intensity vanishes along a line through the particle parallel to the direction of acceleration, while it is a maximum at right angles to this line; thus the intensity of the rays along a See also:horizontal line through P would vanish, while it would be a maximum in the plane at right angles to this line. In this case there would be complete polarization. In reality the cathode particle is not stopped at its first encounter, but makes many collisions, changing its direction between each; and these collisions will send out electric disturbances which when they fall on P are able to excite waves which send some energy along PC. The polarization will therefore be only partial and will be of the kind found by Barkla. The velocity with which the waves travel has not yet been definitely settled. See also:Marx (Ann. der Phys. 20, p. 677) by an ingenious but elaborate method came to the conclusion that they travelled with the velocity of light ; his See also:interpretation of his experiments has, however, been criticized by See also:Franck and Pohl (Verh. d. D. Physik Ges. ro, p. 489). Another view of the nature of Rontgen rays has been advocated by See also:Bragg (Phil. Mag. 14, p. 429); he regards them as neutral electric doublets consisting of a negative and a See also:positive See also:charge of electricity which are usually held together by the attraction between them, but which may be knocked asunder when the rays strike against matter and turned into cathodic rays. On this view when the rays pass' through a gas only a few of the molecules of the gas are struck by the rays and so we can easily understand why so few of the molecules are ionized. On the ordinary view of an electric wave all the molecules would be affected by the wave when it passed through a gas, and to explain the small fraction ionized we must either suppose that systems sensitive to the Rontgen rays are at any time See also:present only in a very small fraction of the See also:molecule or else that the front of an electric or light wave is not continuous but that the energy is concentrated in patches which only occupy a fraction of the wave front. Apparatus for producing Rontgen Rays.—The tube now used most frequently for producing Rontgen rays is of the kind introduced by See also:Porter and known as a See also:focus tube (fig. 2). The cathode is a portion of a hollow See also:sphere, and the cathode rays come to a point on or near a metal plate A, called the See also:anti-cathode, connected with the anode; this plate is the source of the rays. This ought to be made of a very unfusible metal such as platinum or, still better, See also:tantalum, and kept cool by a See also:water-cooling arrangement. The anti-cathode is generally set at an See also:angle of 45° to the rays; it is probable that the action of the tube would be improved by putting the anti-cathode at right angles to the cathode rays. The walls of the tube get strongly electrified. This electrification affects the working of the tube, and the See also:production of rays can often be improved by having an See also:earth-connected piece of tin-foil on the outside of the bulb, and moving it about until the best position is attained. To produce the discharge an induction coil is generally employed with a See also:mercury interrupter. Excellent results have been obtained by using an electrostatic induction See also:machine to produce the current, the emission of rays is more See also:uniform than when an induction coil is used. The rays are emitted See also:pretty uniformly in all directions until the plane of the anti-cathode is approached; in the neighbourhood of this plane there is a rapid falling off in the intensity of the rays. After See also:long use the glass of the bulb often becomes distinctly See also:purple. This is believed to be due to the presence of See also:manganese compounds in the glass. (J. J. Additional information and CommentsThere are no comments yet for this article.
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