86 ELECTRICITY [ELECTROMOTIVE FOECE. Electro motive series. Two metals and one liquid. Contact of two liquids. interest now that the theory of metallic contact, pure and simple, is given up. The following set is given by Fara day : : HN0 3 (dil.) (d 2 il.) 4 HCl. HN0 3 (strong). KHO. KIIS. K 2 S 5 Ag Ao- Sb Ni Ag Fe Fe Cu Cu Ag Ag Ni Ni Ni Sb Sb Ni Sb Cu Bi Bi Bi Bi Bi Cu Fe Pb Sb Ni Ni Cu Bi Bi Ag Pb Fe Fe Fe Fe Pb Sb Ag Sn Pb Pb Sn Sb Sn Sn Pb Sn Sn Pb Cd Cu Cd Cd Cd Cd Zn Sn Zn Cu Zn Zn Zu Cd Zn Cd Zn It will be seen that the order of the metals is not the same for different liquids. Just as between different metals and between metals and liquids there is a contact force, so there is a contact force between different liquids. The direct observations of this contact force are very few and uncertain. One thing, however, is settled, viz., that the contact forces between liquids do not universally at least obey the law of Volta, or, at all events, do not form a consistent series with the metals ; for a great variety of circuits of one metal arid two solutions have been discovered in which the resultant initial electromotive force is not zero. Faraday 2 has even found cases of this kind with one metal and two different strengths of the same solution. The cell of Becquerel is a favourite illustration of such a circuit. It consists of a porous vessel filled with a solution of potash and immersed in a beaker containing nitric acid ; two strips of platinum immersed in the potash arid nitric acid respectively form the plates. The current goes in the cell from the potash to the nitric acid. The following additional examples are taken from Wiedemann. 3 The current flows from the metil through the liquids in the order named to the same metal again. For brevity, i named only once. Metal. 1st Fluid. 2(1 Fluid. Pt KHO Acids Pt CuS0 4 Dil. H 2 S0 4 Pt NaCl ZnCl, Pt NH 3 CuS0 4 L CaCl, Dil. HN0 3 M Conc.H s S0 4 HN0 3 R KCy HN0 3 L stands for Zn, Cu, or Pt. M Cu, Fe, Pb, Sn, or Ag. K Ni, Bi, Pt, Hg, Pd, Sb, Fe, C, Ag, Zn, Cu, Cd, or Sn. Two A great variety of active voltaic circuits have been formed metals with two liquids and two metals. The best known class of and two cases { s that in which the metals are in contact, as in the quu s> two-fluid batteries of Daniell, Grove, and Bunsen. But Faraday 4 gives a list of some thirty cases in which the fluids and metals are placed alternately, so that there is no metallic contact. He marks the following combinations as powerful : Iron Diluted nitric acid. Platinum Green nitrous acid. Do. Hydrochloric acid. Do. Do. do. Do. Solution of com. salt. Do. Do. do. Copper ! Potassium sulphide. Iron Dil. nitric acid. Do. j Strong nitric acid. Do. Do do. It must be carefully noticed that the galvanometer indi- 1 Exp Res, 2012. 8 Oulvanismzis, Bd. i. 63. Exp. Res., 1975. Exp. Res., 2020. cation in the first instant only is to be considered. After the first rush of electricity the direction even of the current may alter. Above all, no conclusion concerning the value of the initial electromotive force is to be drawn from measurements of the subsequent current. Quantitative Mea determinations of the electromotive force in many of the men above cases have been made by various methods, of which eled an account will be found in Wiedemann s Gahanismus, m Bd. i. 230. The most convenient plan is to use Thomson s quadrant electrometer, Lippmann s capillary electrometer, or some other instrument which allows us to measure the electromotive force while no current is passing through the cell. The galvanometer may also be used as in Latimer Clark s modification of Poggendorff s compensation method. The apparatus may be arranged according to the scheme Metl in fig. 53. ABC denotes part of the resistance in the of P circuit of the battery K ; the circuits A^ELB, A^/FMC each contain a gal vanometer, a cell, and a key. The cells E and F are so arranged as to tend to send currents in the same directions as K, but the re sistances AB, AC are so adjusted that when the key L or the key M is de pressed, no current is indicated by p or q. When this is so, we must obviously have E = VA - V n , F = V A - V c , <fec., VA, V B , V c denoting the potentials at A, B, and C. Hence, if P, Q, R denote the resistances in AB, AC, and in the whole circuit of K, P K, F=K. E = R R If K were a constant battery, and its internal resistance were either known or else so small as to form only an in appreciable fraction of 11, then each of the equations just written might be used singly, and we might operate with one cell and one galvanometer, comparing the electromotive force of the cell with K. In general, however, this will not be possible, and then we have, eliminating K and K, E _ P F"-Q from which we get the ratio of E to F independent of the variation of K and R. We can by this method therefore find the ratio of the electromotive force of a given com bination to that of a standard cell, when no current is pass ing through either. The process would be perfect in practice if a standard cell could be constructed whose abso lute constancy could be relied on. Contact force from polarization. The flow of electri city through the cell is accompanied by a deposition of the products of chemical decomposition on the plates, which alters the surface contact forces. This constitutes the phenomenon of polarization, which we have already par tially studied. It will be useful to consider a little more in detail some of the forms in which it is met with. The products of electrolysis which accumulate at the Van electrodes may be simply held in solution or precipitated, ? f P or they may combine chemically with the solution ; they may be deposited as a crust on the electrode, or they may enter into more or less intimate connection with it. Several of these different effects may occur together; but in almost all cases the result is the same, vi?.. a great weakening of the current after the first instant or so. This weakening of the current might be due either to a transition resistance caused by the alterations at the electrodes, or to an op-
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