The kaleidophone, intended to present visibly the movements of a sonorous body, consisted of a vibrating wire or rod carrying a silvered 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 series of papers on the eye, on the physiology of vision, on binocular vision, including the invention of one of the popular scientific instruments, the stereoscope (q.v.), and on colour. The polar clock, devised for use in place of a sun-dial, applies the fact that the plane of polarization of sky light is always 90° from the position of the sun; hence by measuring the azimuthal angle of the plane, even when the sun is below the horizon, correct apparent solar time may be obtained. In 1835, in a paper on "The Prismatic Decomposition of Electrical Light," he proved that 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 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 rate of 500 words a minute, 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 developed by Father A. Secchi and F. 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. Biographical notices of him will be found in his Proc. Inst. C.E., xlvii. 283, and Proc. Roy. Soc., xxiv. xvi. For his connexion with the growth of telegraphy, see Nature, xi. 510, and xii. 30 sq.
WHEATSTONE'S BRIDGE, an electrical instrument which consists of six conductors, joining four points, of such a character that when an electromotive force is applied in one branch the 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. Christie, in 1833[1].
The arrangement of the six conductors is diagrammatically
represented in fig. 1. In one of these branches is placed a battery B and
Fig. 1.
in another a 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 arm. The circuit in which the galvanometer is placed is the bridge circuit. Keys are inserted in the battery and galvanometer circuits to open or close them at pleasure. The resistance forming the four arms of the bridge can be so adjusted that if these resistances have values denoted by P, Q, R, and S, then when P:Q::R:S, the current in the galvanometer circuit will be zero when an electromotive force is applied in the battery circuit.
To prove this statement, let the conductors P, Q, R, S., be arranged in a lozenge shape, as in fig. I. Let E be the electromotive force in the battery circuit, and let be the current through the resistance P, the current through the resistance Q and that through B. Then by G. R. Kirchhoff's laws (see Electrokinetics) we have the current equations,
Rearranging the terms and solving for (the current through the galvanometer), we obtain
where Δ is a complex expression, involving the resistances P, Q, R, S, G, and B, which does not concern us. Hence when P:Q = R:S and the value of R can be determined in terms of P, Q and S.
Fig. 2.—Standard Wheatstone's Bridge.
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 of a galvanometer and a battery connected up as shown in the 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 key. In one form of Wheatstone's Bridge, known as the series pattern plug-resistance bridge, or Post Office pattern, the two ratio arms, P and Q, each consist of a series of coils of wire, viz. two 1-ohm coils, two 10-ohm coils, two 100-ohm coils and two 1000-ohm coils, which are joined up in series in the order, 1000, 100, 10, 1; 1, 10, 100, 1000, the junctions between each pair being connected to 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 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 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.
Fig. 3.—Diagram showing Connexions of a Dial and Plug pattern, Wheatstone's Bridge.
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 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 100-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 any outside block with the central block, the current can only pass from the zero 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 total resistance may be made anything from 1 to 9, 10 to 90, 100 to 900 ohms, &c. Three or four of the "dials" thus composed are arranged 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 advantage that the insertion or removal of a
- ↑ See Wheatstone's Scientific Papers, p. 129.
