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WHEATSTONE, SIR CHARLES (1802–1875)

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Originally appearing in Volume V28, Page 585 of the 1911 Encyclopedia Britannica.
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WHEATSTONE, See also:SIR See also:CHARLES (1802–1875) , See also:English physicist and the See also:practical founder of See also:modern telegraphy, was See also:born at See also:Gloucester in See also:February 1802, his See also:father being a See also:music-seller in that See also:city. In r8o6 the See also:family removed to See also:London. Wheatstone's See also:education was carried on in several private See also:schools, at which he appears to have displayed no remarkable attainments, being mainly characterized by a morbid shyness and sensitiveness that prevented him from making See also:friends. About 1816 he was sent to his See also:uncle, a musical See also:instrument maker in the Strand, to learn the See also:trade; but with his father's countenance he spent more See also:time in See also:reading books of all kinds than at See also:work. For some years he continued making experiments in See also:acoustics, following out his own ideas and devising many beautiful and ingenious arrangements. Of these the " acoucryptophone " was one of the most elegant—a See also:light See also:box, shaped like an See also:ancient See also:lyre and suspended by a metallic See also:wire from a piano in the See also:room above. When the instrument was played, the vibrations were transmitted silently, and became audible in the lyre, which thus appeared to See also:play of itself. On the See also:death of his uncle in 1823 Wheatstone and his See also:brother succeeded to the business; but he never seems to have taken a very active See also:part in it, and he virtually retired after six years, devoting himself to experimental See also:research, at first chiefly with regard to See also:sound. Although he occasionally read a See also:paper to scientific See also:societies when a See also:young See also:man, he never could become a lecturer on See also:account of his shyness. Hence many of his investigations were first described by See also:Faraday in his See also:Friday evening discourses at the Royal Institution. By 1834 his originality and resource in experiment were fully recognized, and he was appointed See also:professor of experimental See also:philosophy at See also:King's See also:College, London, in that See also:year. This See also:appointment was inaugurated by two events,—a course of eight lectures on sound, which proved no success and was not repeated, and the determination by means of a revolving See also:mirror of the See also:speed of electric See also:discharge in conductors, a piece of work leading to enormously important results.

The See also:

great velocity of See also:electrical transmission suggested the possibility of utilizing it for sending messages; and, after many experiments and the practical See also:advice and business-like co-operation of See also:William See also:Fothergill See also:Cooke (18o6-1879), a patent for an electric See also:telegraph was taken out in their See also:joint names in 1837. Wheatstone's See also:early training in making musical See also:instruments now See also:bore See also:rich See also:fruit in the continuous designing of new instruments and pieces of mechanism. His See also:life was uneventful except in so far as the variety of his work See also:lent it See also:colour. He became a See also:fellow of the Royal Society in 1837; in 1847 he married; and in 1868, after the completion of his See also:master-piece, the automatic telegraph, he was knighted. While in See also:Paris perfecting a receiving instrument for submarine cables, Sir Charles Wheatstone caught See also:cold, and died on the 19th of See also:October 1875. Wheatstone's See also:physical investigations are described in more than See also:thirty-six papers in various scientific See also:journals, the more important being in the Philosophical Transactions, the Proceedings of the Royal Society, the Comptes rendus and the See also:British Association Reports. They naturally See also:divide themselves into researches on sound, light and See also:electricity, but extend into other branches of physics as well. But his best work by far was in the invention of complicated and delicate mechanism for various purposes, in the construction of which he employed a See also:staff of workmen trained to the highest degree of excellence. For his insight into mechanism and his See also:power over it he was unequalled, except perhaps by Charles See also:Babbage. A cryptographic See also:machine, which changed the See also:cipher automatically and printed a See also:message, entirely unintelligible until translated by a duplicate instrument, was one of the most perfect examples of this. See also:Cryptography had a great See also:fascination for Wheatstone; he studied it deeply at one time, and deciphered many of the See also:MSS. in the British Museum which had defied all other interpreters. In acoustics his See also:principal work was a research on the transmission of sound through solids, the explanation of Chladni's figures of vibrating solids, various investigations of the principles of acoustics and the mechanism of See also:hearing, and the invention of new nusical instruments, e.g. the See also:concertina (q.v.).

The kaleidophone, intended to See also:

present visibly the movements of a sonorous See also:body, consisted of a vibrating wire or See also:rod carrying a silvered See also:bead reflecting a point of light, the motions of which, by persistence of the successive images on the retina, were thus represented in curves of light. In light there are a See also:series of papers on the See also:eye, on the See also:physiology of See also:vision, on See also:binocular vision, including the invention of one of the popular scientific instruments, the See also:stereoscope (q.v.), and on colour. The polar See also:clock, devised for use in See also:place ofPa See also:sun-See also:dial, applies the fact that the See also:plane of polarization of See also:sky light is always 90° from the position of the sun; hence by measuring the azimuthal See also:angle of the plane, even when the sun is below the See also:horizon, correct apparent See also:solar time may be obtained. In 1835, in a paper on " The Prismatic Decomposition of Electrical Light," he proved that See also:sparks from different metals give distinctive spectra, which afforded a ready means of discriminating between them. But it is by his electrical work that Wheatstone is best remembered. He not only guided the growth of scientific telegraphy on See also:land wires, but made the earliest experiments with submarine cables, foreseeing the practicability of this means of communication as early as 1840. He devised the " A, B, C " telegraph instrument, the automatic transmitter, by which messages may be sent at the See also:rate of 500 words a See also:minute, See also:printing telegraph receivers of various forms, electrical chronoscopes, and many forms of electrical recording apparatus,—amongst others two sets of registering meteorological instruments, of which the earlier, described in 1842, was afterwards See also:developed by Father A. See also:Secchi and F. See also:van Rysselberghe, but the later, put forward in 1867, included metallic thermometers and was less successful. Wheatstone's Scientific Papers were collected and published by the Physical Society of London in 1879. See also:Biographical notices of him will be found in his Proc. Inst. C.E., xlvii.

283, and Proc. See also:

Roy. See also:Soc., See also:xxiv. xvi. For his connexion with the growth of telegraphy, see Nature, xi. 51o, and xii. 30 sq. WHEATSTONE'S See also:BRIDGE, an electrical instrument which consists of six conductors, joining four points, of such a See also:character that when an electromotive force is applied in one See also:branch the See also:absence of a current in another branch (called the conjugate branch) establishes a relation between the resistance of the four others by which we can determine the value of the resistance in one of these, that of the others being assumed to be known. This arrangement was not invented by Sir Charles Wheatstone—although it bears his name and is commonly attributed to him, and was employed by him in some of his electrical researches---but by S. H. See also:Christie, in 1833.1 The arrangement of the six conductors is diagrammatically repre- sented in fig. I. In one of these branches is placed a See also:battery B and in another a See also:galvanometer G; the four other resistances are denoted by the letters P, Q, R, S.

The circuits in which the battery and galvanometer are placed are called conjugate circuits, and the circuits P, Q, R, and S are called the arms of the bridge, the branches P and Q being called the ratio arms and S the measuring See also:

arm. The See also:circuit in which the galvanometer is placed is the bridge circuit. Keys are inserted in the battery and galvanometer circuits to open or See also:close them at See also:pleasure. The resistance forming the four arms of B the bridge can be so adjusted that if To prove this statement, let the conductors P, Q. R, S., be arranged in a See also:lozenge shape, as in fig. 1. Let E be the electromotive force in the battery circuit, and let (x+y) be the current through the resistance P, y the current through the resistance Q and z that through B. Then by G. R. See also:Kirchhoff's See also:laws (see See also:ELECTROKINETICS) we have the current equations, (P+G+R) (x+y)—Gy—Rz=O (Q+G+S)y—G (x+y)—Sz=O (R+S+B)z—R(x +y)—Sy=E Rearranging the terms and solving for x (the current through the galvanometer), we obtain x = (PS—RQ)E/0. where A is a complex expression, involving the resistances P, Q, R, S, G, and B, which does not concern us. Hence when x = o, P : Q = R :S and the value of R can be determined in terms of P, Q and S.

In the practical instrument the three arms of the bridge P, Q, and S are generally composed of coils of wire contained in a box, whilst R is the resistance the value of which is to be determined. This last resistance is connected to the other three with the addition 1 Sec Wheatstone's Scientific Papers, p. 129. of a galvanometer and a battery connected up as shown in the See also:

diagram. The operation of determining the value of the resistance R therefore consists in altering the ratio of the three resistances P, Q, and S, until the galvanometer indicates no current through it when the battery circuit is completed or closed by the See also:key. In one See also:form of Wheatstone's Bridge, known as the series See also:pattern plug-resistance bridge, or See also:Post See also:Office pattern, the two ratio arms, P and Q, each consist of a series of coils of wire, viz. two 1-See also:ohm coils, two to-ohm coils, two too-ohm coils and two moo-ohm coils, which are joined up in series in the See also:order, moo, too, 10, I ; 1, 10, too, moo, the junctions between each pair being connected to See also:brass blocks, a series of which are mounted upon an ebonite slab that forms the lid of the box. The blocks are bored out with a hole partly in one See also:block and partly in the other (see fig. 2) so that they can be connected by accurately fitting conical plugs. When the blocks are interconnected by the plugs all the coils are See also:short-circuited; but if the plug or plugs are taken out, then a current flowing from one end of the series to the other is compelled to pass through the corresponding coils. In series with this set of coils is another set, S, which forms a measuring arm, the resistances of which are generally 1, 2, 3, 4,'10, 20, 30, 40, 100, 200, 300, 400, 1000, 2000, 3000, 4000 ohms. The junction between each pair of coils is connected as above described to a block, the blocks being interconnected by plugs all of which are made interchangeable. Another form of Wheatstone's Bridge, shown in fig.

2, is known as the dial pattern. Ten brass blocks are arranged parallel to or around another brass block, and by means of a plug which fits into holes bored partly out of the See also:

common block and partly out of the surrounding blocks, any one of the latter can be connected with the common one. A series of nine equal resistances, say 1-ohm coils, or nine too-ohm coils, are joined in between these circumferential blocks (fig. 3). It will be seen that if a plug is placed so as to connect 1111-! to Wheatstone's Bridge. any outside block with the central block, the current can only pass from the zero See also:outer block to the central block by passing through a certain number of the resistance coils. Hence according to the magnitude of each coil the See also:total resistance may be made anything from 1 to 9, 10 to 90, or See also:loo to 900 ohms, &c. Three or four of the " dials " thus composed are arranged See also:side by side, the brass blocks being mounted on a slab of ebonite and the coils contained in the box underneath, and they are so joined up that the central block of one dial is connected to the outside block of the next marked O. This arrangement forms the measuring arm of the bridge, the ratio arms being constructed on the series plug pattern just described. A bridge of this pattern has the See also:advantage that the insertion or removal of a plug in the measuring arm does not tend to tighten or loosen all the See also:rest of the plugs; moreover, there are fewer plugs to manipulate, and each plug is occupied. The resistance coils themselves are generally See also:wound on brass or See also:copper bobbins, with See also:silk-covered manganin wire, which should first be aged by See also:heating for about ten See also:hours to a temperature of 1400 C., to remove the slight tendency to See also:change in resistivity which would otherwise present itself.

For the accurate comparison of resistance coils it is usual to make use of the 1blatthiessen and Hockin bridge, and to employ the method of See also:

differential comparison due to G. See also:Carey See also:Foster.' On a See also:board is stretched a See also:uniform metallic wire a b, generally of See also:platinum See also:silver. The ends of this wire are connected to copper blocks, which them-selves are connected to a series of four resistance coils, A, B, and P, Q (fig. 4). A and B are the coils to be compared, P and Q are two other b-x FIG. 4. coils of convenient value. Over the stretched wire moves a contact maker S, which makes contact with it at any desired point, the position of which can be ascertained by means of an underlying See also:scale. A battery C of two or three cells is connected to the extremities of the slide wire, and the sensitive galvanometer G is connected in between the contact-maker and the junction between the coils P and Q. The observer begins by moving the slider until the galvanometer shows no current. The position of the coils A and B is then interchanged, and a fresh See also:balance in position on the bridge is obtained. It is then easily shown that the difference between the resistance of the coils A and B is equal to the resistance of the length of the slide wire intercepted between the two places at which the balance was found in the two observations.

Let the balance be supposed to be attained, and let x be the position of the slider on the wire, so that x and l—x are the-two sections of the slide wire, then the relation between the resistance is (A+x) / (B+l—x)=P/Q• Next, let the position of A and B be interchanged, and the slide-wire reading be x'; then (B+x') / (A+l—x') = P/Q. Hence it follows that A—B=x'—x, or the difference of the resistances of the coils A and B is equal to the resistance of that length of the slide wire between the two points where balance is obtained. Various plans have been suggested for effecting the rapid inter-change of the two coils A and B; one of the most convenient was designed by J. A. See also:

Fleming in 188o, and has been since used by the British Association See also:Committee on Electrical See also:Units for making comparison between See also:standard coils with great accuracy (see Phil. Mag., 188o, and Proc. Phys. Soc., 1879). In all very exact resistance measurements the See also:chief difficulty, however, is not to determine the resistance of a coil, but to determine the temperature of the coil at the time when the resistance measurement is made. The difficulty is ' " On a Modified Form of Wheatstone's Bridge, and Methods of Measuring Small Resistances," by Professor G. Carey Foster, Proc. Soc.

Tel. Eng. (1872), 1.caused by the fact that the coil is heated by the current used to measure its resistance, which thus alters in value. In accurate comparisons, therefore, it is necessary that the coils to be compared should be immersed in melting See also:

ice, and that sufficient time should be allowed to elapse between the measurements for the See also:heat generated in the coil to be removed. The standard resistance coil employed as a means of comparison by which to regulate and check other coils consists of a wire, generally of manganin or platinum silver, insulated with silk and wound on a brass See also:cylinder (fig. 5). This is soldered to two thick terminal rods of copper, and the coil is enclosed in a See also:water-tight brass cylinder so that it can be placed in water, or preferably in See also:paraffin oil, and brought to any required temperature. In the form of standard coil recommended by the See also:Berlin Reichsanstalt the coil is immersed in an insulating oil which is kept stirred by means of a small electric motor during the time of making the measurement. The temperature of the oil can best be ascertained by means of a platinum resistance thermometer. For the measurement of See also:low resistances a ,modification of the Wheatstone's bridge devised by See also:Lord See also:Kelvin is employed. The Kelvin bridge consists of nine conductors joining six points, and in one practical form is known as a Kelvin and See also:Varley slide. Modifications of the See also:ordinary Wheatstone's bridge for very accurate measurements have been devised by H.

L. Callendar and by Callendar and E. H. Griffiths (see G. M. See also:

Clark, the Electrician, 38, p. 747). A useful bridge method for measurement of low resistances has been given by R. T. See also:Housman (the Electrician, 40, p. 300, 1897). These and numerous modifications of the Wheatstone's bridge will be found described in J.

A. Fleming's Handbook for the Electrical Laboratory and Testing-' Room, vol. i. (1903).

End of Article: WHEATSTONE, SIR CHARLES (1802–1875)

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