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FIRST PERIOD
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FIRST See also:PERIOD .—Gilbert was probably led to study the phenomena of the attraction of See also:iron by the lodestone in consequence of his See also:conversion to the Copernican theory of the See also:earth's See also:motion, and thence proceeded to study the attractions produced by See also:amber. An See also:account of his See also:electrical discoveries is given in the De magnete, See also:lib. ii. cap. 2.2 He invented the versorium or
1 See also: See also:Francis Hawksbee (d. 1713) published in his book Physico-See also:Mechanical Experiments (1709), and in several See also:Memoirs in the Phil. Trans. about 1707, the results of his electrical inquiries. He showed that light was produced when See also:mercury was shaken up in a glass See also:tube exhausted of its See also:air. Dr See also:Wall observed the spark and crackling See also:sound when warm amber was rubbed, and compared them with See also:thunder and See also:lightning (Phil. Trans., 1708, 26, p. 69). See also:Stephen See also: 16, 166 and 400). See also:Jean See also:Theophile Desaguliers (1683–1744) announced soon after that electrics were non-conductors, and conductors were nonelectrics. C. F. de C. du See also:Fay (1699–1739) made the great discovery that electricity is of two kinds, vitreous and resinous (Phil. Trans., 1733, 38, p. 263), the first being produced when glass, crystal, &c. are rubbed with silk, and the second when See also:resin, amber, silk or paper, &c. are excited by friction with See also:flannel. He also discovered that a body charged with See also:positive or negative electricity repels a body See also:free to move when the latter is charged with electricity of like sign, but attracts it if it is charged with electricity of opposite sign, i.e. positive repels positive and negative repels negative, but positive attracts negative. It is to du Fay also that we owe the abolition of the distinction between electrics and non-electrics. He showed that all substances could be electrified by friction, but that to electrify conductors they must be insulated or supported on non-conductors. Various improvements were made in the electrical machine, and thereby experimentalists were provided with the means of generating strong electrification; C. F. Ludolff (1707–1763) of See also:Berlin in 1744 succeeded in igniting See also:ether with the electric spark (Phil. Trans., 1744, 43, p. 167).
For a very full See also:list of the papers and See also:works of these See also:early electrical philosophers, the reader is referred to the bibliography on Electricity in Dr See also: Hence the cause of the See also:shock and spark when the jar is discharged, or when the superabundant or plus electricity of the inside is transferred by a conducting body to the defective or minus electricity of the outside. This theory of the Leyden phial Franklin supported very ingeniously by showing that the outside and the inside coating possessed electricities of opposite sign, and that, in charging it, exactly as much electricity is added on one See also:side as is subtracted from the other. The abundant discharge of electricity by points was observed by Franklin is his earliest experiments, and also the See also:power of points to conduct it copiously from an electrified body. Hence he was furnished with a See also:simple method of See also:collecting electricity from other bodies, and he was enabled to perform those remarkable experiments which are chiefly connected with his name. Hawksbee, Wall and J. A. See also:Nollet (1700-1770) had successively suggested the identity of lightning and the electric spark, and of thunder and the snap of the spark. Previously to the See also:year 1750, Franklin See also:drew up a statement, in which he showed that all the See also:general phenomena and effects which were produced by electricity had their See also:counter-parts in lightning. After waiting some See also:time for the erection of a See also:spire at See also:Philadelphia, by means of which he hoped to bring down the electricity of a thunderstorm, he conceived the See also:idea of sending up a See also:kite among thunder-clouds. With this view he made a small See also:cross of two small light strips of See also:cedar, the arms being sufficiently See also:long to reach to the four corners of a large thin silk handkerchief when extended. The corners of the handkerchief were tied to the extremities of the cross, and when the body of the kite was thus formed, a tail, See also:loop and string were added to it. The body was made of silk to enable it to See also:bear the violence and wet of a thunderstorm. A very See also:sharp pointed See also:wire was fixed at the See also:top of the upright stick of the cross, so as to rise a See also:foot or more above the See also:wood. A silk ribbon was tied to the end of the twine next the See also:hand, and a See also: About the same time that Franklin was making his kite 1 See Sir See also:Oliver See also:Lodge, " Lightning, Lightning Conductors and Lightning Protectors," Journ. Inst. Elec. Eng. (1889), 18, p. 386, and the discussion on the subject in the same See also:volume; also the book by the same author on Lightning Conductors and Lightning See also:Guards (London, 1892).experiment in See also:America, T. F. Dalibard (1703-1779) and others in See also:France had erected a long iron See also:rod at Marli, and obtained results agreeing with those of Franklin. Similar investigations were pursued by many others, among whom See also:Father G. B. See also:Beccaria (1716-1781) deserves especial mention. John See also:Canton (1718-1772) made the important contribution to knowledge that electricity of either sign could be produced on nearly any body by friction with appropriate substances, and that a rod of glass roughened on one See also:half was excited negatively in the rough See also:part and positively in the smooth part by friction with the same rubber. Canton first suggested the use of an See also:amalgam of mercury and See also:tin for use with glass See also:cylinder electrical See also:machines to improve their See also:action. His most important discovery, however, was that of electrostatic See also:induction, the fact that one electrified body can produce charges of electricity upon another insulated body, and that when this last is touched it is See also:left electrified with a See also:charge of opposite sign to that of the inducing charge (Phil. Trans., 1753-1754). We shall make mention See also:lower down of Canton's contributions to electrical theory. Robert Symmer (d. 1763) showed that quite small See also:differences determined the sign of the electrification that was generated by the friction of two bodies one against the other. Thus wearing a See also:black and a See also: Abhandl., 1762, 24, p. 213).
Pyro-electricity.—The subject of pyro-electricity, or the power possessed by some minerals of becoming electrified when merely heated, and of exhibiting positive and negative electricity, now began to attract See also:notice. It is, possible that the lyncurium of the ancients, which according to See also:Theophrastus attracted light bodies, was See also:tourmaline, a See also:mineral found in See also:Ceylon, which had been christened by the Dutch with the name of aschcntrikker, or the attractor of ashes. In 1717 See also: The subject, however, had already engaged the See also:attention of the See also:German philosopher, F. U. T. See also:Aepinus, who published an account of them in 1756. Hitherto nothing had been said respecting the See also:necessity of See also:heat to excite the tourmaline; but it was shown by Aepinus that a temperature between 99zo and 212° Fahr. was requisite for the development of its attractive See also:powers. Benjamin See also: C. P. Brard (1788–1838) discovered that pyro-electricity was a property of See also:axinite; and it was afterwards detected in other minerals. In repeating and extending the experiments of Hauy much later, Sir See also:David See also:Brewster discovered that various artificial salts were pyro-electric, and he mentions the tartrates of potash and soda and tartaric See also:acid as exhibiting this property in a very strong degree. He also made many experiments with the tourmaline when cut into thin slices, and reduced to the finest See also:powder, in which state each particle preserved its pyro-electricity; and he showed that scolezite and mesolite, even when deprived of their See also:water of See also:crystallization and reduced to powder, retain their property of becoming electrical by heat. When this white powder is heated and stirred about by any substance whatever, it collects in masses like new-fallen See also:snow, and adheres to the body with which it is stirred. For Sir David Brewster's work on pyro-electricity, see Trans. See also:Roy. See also:Soc. Edin., 1845, also Phil. Mag., Dec. 1847. The reader will also find a full discussion on the subject in the See also:Treatise on Electricity, by A. de la Rive, translated by C. V. See also: Chim., 1805), and See also:Etienne See also:Geoffroy See also:Saint-Hilaire (Gilb. Ann., 1803) pursued the subject with success; and See also: Humboldt gives a very graphic account of the combats which are carried on in See also:South America between the gymnoti and the See also:wild horses in the vicinity of See also:Calabozo. Cavendish's Researches.—The work of Henry Cavendish (1731–181o) entitles him to a high See also:place in the list of electrical investigators. A considerable part of Cavendish's work was rescued from oblivion in 1879 and placed in an easily accessible form by See also:Professor Clerk See also:Maxwell, who edited the original See also:manuscripts in the See also:possession of the See also:duke of See also:Devonshire.' Amongst Cavendish's important contributions were his exact measurements of electrical capacity. The leading idea which distinguishes his work from that of his predecessors was his use of the phrase " degree of electrification " with a clear scientific See also:definition which shows it to be See also:equivalent in meaning to the modern See also:term " electric potential." Cavendish compared the capacity of different bodies with those of conducting See also:spheres of known See also:diameter and states these capacities in " globular inches," a globular See also:inch being the capacity of a See also:sphere x in.'in diameter. Hence his measurements are all directly comparable with modern electrostatic measurements in which the unit of capacity is that of a sphere 1 centimetre in See also:radius. Cavendish measured the capacity of disks and condensers of various forms, and proved that the capacity of a Leyden See also:pane is proportional to the surface of the tinfoil and inversely as the thickness of the glass. In connexion with this subject he anticipated one of Faraday's 1 The Electrical Researches of the Hon. Henry Cavendish 1771-!q81, edited from the original manuscripts by J. Clerk Maxwell, F.R.S. (See also:Cambridge, 1879).greatest discoveries, namely, the effect of the See also:dielectric or insulator upon the capacity of a condenser formed with it, in other words, made the discovery of specific inductive capacity (see Electrical Researches, p. 183). He made many measurements of the electric conductivity of different solids and liquids, by comparing the intensity of the electric shock taken through his body and various conductors. He seems in this way to have educated in himself a, very precise " electrical sense," making use of his own See also:nervous See also:system as a See also:kind of physiological See also:galvanometer. One of the most important investigations See also:hay made in this way was to find out, as he expressed it, " what power of the velocity the resistance is proportional to." Cavendish meant by the term velocity " what we now See also:call the current, and by " resistance," the electromotive force which maintains the current. By various experiments with liquids in tubes he found this power was nearly unity. This result thus obtained by Cavendish in See also:January 1781, that the current varies in See also:direct proportion to the electromotive force, was really an anticipation of the fundamental See also:law of electric flow, discovered independently by G. S. See also:Ohm in 1827, and since known as Ohm's Law. Cavendish also enunciated in 1776 all the See also:laws of See also:division of electric current between circuits in parallel, although they are generally supposed to have been first given by Sir C. See also:Wheatstone. Another of his great investigations was the determination of the law according to which electric force varies with the distance. Starting from the fact that if an electrified globe, placed within two hemispheres which See also:fit over it without touching, is brought in contact with these hemispheres, it gives up the whole of its charge to them—in other words, that the charge on an electrified body is wholly on the surface—he was able to deduce by most ingenious reasoning the law that electric force varies inversely as the square of the distance. The accuracy of his measurement, by which he established within 2% the above law, was only limited by the sensibility, or rather insensibility, of the See also:pith ball See also:electrometer, which was his only means of detecting the electric charge .2 In the accuracy of his quantitative measurements and the range of his researches and his See also:combination of mathematical and See also:physical knowledge, Cavendish may not inaptly be described as the See also:Kelvin of the 18th See also:century. Nothing but his curious in-difference to the publication of his work prevented him from securing earlier recognition for it. See also:Coulomb's Work.—Contemporary with Cavendish was C. A. Coulomb (1736–18o6),,,who in France addressed himself to the same kind of exact quantitative work as Cavendish in England. Coulomb has made his name for ever famous by his invention and application of his torsion See also:balance to the experimental verification of the fundamental law of electric attraction, in which, however, he was anticipated by Cavendish, namely, that the force of attraction betweemtwo small electrified spherical bodies varies as the product of their charges and inversely as the square of the distance of their centres. Coulomb's work received better publication than Cavendish's at the time of its accomplishment, and provided a basis on which mathematicians could operate. Accordingly the See also:close of the 18th century drew into the See also:arena of electrical investigation on its mathematical side P. S. See also:Laplace, J. B. See also:Biot, and above all, S. D. See also:Poisson. Adopting the See also:hypothesis of two fluids, Coulomb investigated experimentally and theoretically the See also:distribution of electricity on the surface of bodies by means of his See also:proof See also:plane. He determined the law of distribution between two conducting bodies in contact; and measured with his proof plane the See also:density of the electricity at different points of two spheres in contact, and enunciated an important law. He ascertained the distribution of electricity among several spheres (whether equal or unequal) placed in contact in a straight See also:line; and he measured the distribution of 2 In 1878 Clerk Maxwell repeated Cavendish's experiments with improved apparatus and the employment of a Kelvin quadrant electrometer as a means of detecting the See also:absence of charge on the inner conductor after it had been connected to the outer case, and was thus able to show that if the law of electric attraction varies inversely as the nth power of the distance, then the exponent n must have a value of 2 E 1 o o. See Cavendish's Electrical Researches, P. 419. electricity on the surface of a cylinder, and its distribution between a sphere and cylinder of different lengths but of the same diameter. His experiments on the dissipation of electricity possess also a high value. He found that the momentary dissipation was proportional to the degree of electrification at the time, and that, when the charge was moderate, its dissipation was not altered in bodies of different kinds or shapes. The temperature and pressure of the See also:atmosphere did not produce any sensible See also:change; but he concluded that the dissipation was nearly proportional to the See also:cube 'of the quantity of moisture in the air.' In examining the dissipation which takes place along imperfectly insulating substances, he found that a See also:thread of See also:gum-See also:lac was the most perfect of all insulators; that it insulated ten times as well as a dry silk thread; and that a silk thread covered with See also:fine sealing-See also:wax insulated as powerfully as gum-lac when it had four times its length. He found also that the dissipation of electricity along insulators was chiefly owing to adhering moisture, but in some measure also to a slight conducting power. For his memoirs see Mem. de math. et phys. de l'acad. de sc., 1785, &c. Additional information and CommentsThere are no comments yet for this article.
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