jaKCTKOMOTlVK FOECE.J From these three results we get A-B L E <J T it i <J I T 85 It is not necessary to quote Hankel s results here. The reader may refer to Wiedemauu s Galvanism us, or to Hankel s paper. Sir William Thomson has given a new proof of the s de- existence of Volta s contact force as follows. 1 A ring is istra- formed, one-half of which is copper the other half zinc. 1 of This ring is placed horizontally, and a needle made of thin e ai sheet metal is so balanced as to form a radius of the ring. It when the needle is unelectrified it be adjusted so as to be over the junction of the two metals, then, when it is positively electrified, it will deviate towards the copper, and when negatively electrified, towards the zinc. Again, if a whole, instead of a half needle as above, be suspended over a disc made of alternate quadrants of zinc and copper, or, better still, inside a flat cylindrical box constructed in a similar way, so that when the needle is unelectrified its axis coincides with one of the diameters in which the disc is divided, then when the needle is positively electrified it will take up a position such that its axis bisects the copper quadrants ; if it be negatively electrified, its axis will bisect the zinc quadrants. Thomson has also given an elegant demonstration of the contact force between copper and zinc by means of an apparatus which is a modification of his water-dropping apparatus. 2 A copper funnel is placed in a cylinder of zinc, and drops copper filings at a point near the centre of the cylinder. The filings are charged negatively by induction as they fall, owing to the excess of the potential of the zinc cylinder over that of the copper. If therefore the filings be caught in an insulated metal can, they will communicate to it a continually increasing negative charge, while the zinc cylinder and the copper funnel will become charged more and more positively. Thomson finds, in agreement with Kohlrausch, that, when the copper and zinc are bright, the electromotive force of contact is about half that of a Daniell s cell. When the cupper is oxidi/ed by heating in air, the contact force is equal to a Danioll or more. He has gone a step farther, and shown that when two bright pieces of copper and zinc are connected by a drop of distilled water, their potentials are as nearly as can be observed the same. 3 li. ti. The subject of contact electricity has been taken up quite recently by Clifton. 4 He experiments on the contact force between a metal and a liquid by a method which is a .simplification o*" Hankel s. Two horizontal plates are used of the metal M ; the liquid L is placed in a glass vessel on the lower plate and connected with the lower plate by a strip of the same metal which dips into it. The upper plate is lowered to a distance of O l or 0-2 mm. from the surface of the liquid, which acts as the lower surface of the condenser, and the upper plate and lower plate are connected by a copper wire. The difference of potential between the two surfaces <>f the condenser is therefore M|L. The copper wire is then removed, the upper plate raised, and its potential measured with a Thomson s electrometer. In this way a contact force equal to the thousandth part of ZnjCu can be detected. Clifton finds zinc and copper to be both positive to water to about the same degree, and both very slightly negative to dilute sulphuric acid. He concludes therefore that zinc and copper dipping in water will be at the same potential. This he verifies directly, finding that any difference of | Proc. Lit. and Phil. Soc. of Manchester, 186 2,or Reprint, p. 319. - Reprint, p. 324. 3 .Tenkin, KUr.tr. and Mag. 22
- Proa. R. S^ June 1877.
potential, if it exist, must be less than "00079 of the elec tromotive force of a Darnell. The result of Sir William Thomson is therefore confirmed. There are many other points of interest in Clifton s paper, but, as the results are given in most instances as preliminary, we need not discuss the matter farther. Before leaving this subject, it may be well to notice that Source of there is one point which is not touched by all these ex- periments, viz., the question whether there is or is not a contact force between metals or even liquids and air. It has not yet been shown that the results of the experiments which are supposed to demonstrate that ZniCu is about half the electromotive force of a Daniell could not be equally well explained by supposing the difference of potential to be 5 Cu|A + AjZu + CujZn, whence Cu|Zn is very small compared with CuJA and A|Zn. This supposition would not invalidate Volta s law ; nor would it contradict Clifton s results, for we have, in accordance with his experi ments, on the new hypothesis, Aql A -i- Cu| Aq + AqjCu = Aq! A + Zii|Aq -f A Zn, ! therefore, transposing, Zn|Aq + AqlCu + Cu!A + A;Zn = , which, according to the new hypothesis, means that copper and zinc immersed in water are at the same potential. In this view, the important part of the contact force usually observed between zinc and copper would be Cu|A + A^n,* 5 which must therefore, by Sir Win. Thomson s result, be the same as Cu|Aq + Aq|Zn. It is not very easy to see how this point is to be settled by direct measurements of electromotive force. Supposing, however, that it were settled, and that the contact force between two given metals A and B, and between each of them and a given liquid L, were known, then the differ ence of potentials between the two metals when immersed in any liquid could be predicted in all cases, and also the initial electromotive force tending to send a current through a circuit made by connecting the metals together and dip ping them into the liquid. A number of cases of this kind have been investigated by Gerlaud ;" but satisfactory agreement between theory and observation has been attained in but a few cases. Researches of this kind are beset with a double array of almost insuperable difficulties. The direction of the initial resultant electromotive force in any circuit of two metals and one fluid may be found by observing the first swing of a galvanometer through which the circuit is suddenly connected. Considerable precau tions must be taken to obtain consistent results, and when all care has been taken, it is not found that the results of one observer always agree with those of another. This is scarcely to be wondered at, when we consider the difficulty of making sure that in two different experiments we are operating with absolutely the same metals and fluid in absolutely the same conditions as to surface. When the current tends to pass from one metal to an other through the liquid in which they are immersed, the former metal is said to be positive to the latter. If the whole electromotive force in the circuit be the sum of all the contact forces at the various junctions, then it follows easily that we ought to be able to arrange the metals in a series, such that any one in the series is positive to any following one and negative to any preceding when both are dipped in the same liquid. It does not follow that the order of the series is the same for different liquids ; this would be so if the metallic contacts alone were operative. Many electromotive series of this kind have been given by different experimenters; but they have last much of their 8 A stands for air. e See Maxwell i Klectricity, vol. i. 241). 1 See Viedemann, Bd. i. J 8G. Ger- l ;1Ilcl s
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