Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/58

48 E L E T K I C I T Y [ELECTRIC CURRENT. 5S tro * wi Hors ford. lation of the ions has generated an opposing electromotive force, equal to that of the battery, when of course the cur rent must stop. We cannot justify this position very easily by direct experiment; yet there are many facts to support it, and so long as it is tenable it seems to afford the most philosophical view of the matter. Having explained the phenomena of polarization so far as is necessary for our immediate purpose, we now proceed to inquire how far experience justifies the application of Ohm s law to electrolytes, or, which is much the same thing, to examine how far the methods of different physicists for measuring electrolytic resistance have led to concordant results. Mea- One of the earliest methods, in which polarization was eliminated, was that of Horsford. 1 He filled a rectangular electrolyte, and inserted in the trough two lytic electrodes very nearly fitting the cross section. These resist- electrodes could be set at different measured distances ance. apart. They were coated on the further side with non conducting substance, so that the current could flow between the opposed sides only. In this way he secured that the stream lines in the neighbourhood of the electrodes should depend as little as possible on the distance between them. This trough was inserted in the battery circuit along with a tangent galvanometer; then the distance between the plates was decreased, and a metallic resistance R inserted ia the circuit, so as to bring the current to the same strength as before. The current being the same in both cases, it is assumed that the polarization in both is the same, in which case the resistance of a length of the electrolyte equal to the difference of the distances between the elec trodes in the two cases is equal to R. Knowing the section of the trough, we might calculate from E, the specific resis tance of the electrolyte. If the values arrived at be the same when deduced from different lengths of the electrolyte, and for different strengths of current, it may be concluded that Ohm s law applies. The application of this method requires the passage of a permanent current, in consequence of which the ions appear at the electrodes, and the solution in the neighbourhood becomes altered; so that it is difficult to make certain that the polarization is exactly the same in the two cases, and that no resistance of transition is gene rated. Matters may be mended a little by passing the current for the same time in both cases ; but this is scarcely a satisfactory remedy. Still valuable results were obtained with this method by Horsford and Wiedemann; the latter, in applying it to silver and copper solutions used electrodes of silver and copper respectively, whereby the polarization to be eliminated was very much reduced. Taking advantage of the discovery of Matteucci and Du Bois Reymond, 2 that carefully amalgamated zinc electrodes in a neutral 3 solution of zinc sulphate are not polarizable, Beetz. Beetz 4 determined, by means of Wheatstone s bridge, the resistance of various solutions of this electrolyte. The liquid was inclosed in a cylindrical tube, 29 "7 cm. long, with a mean section of 1 4051 sq. cm. Amalgamated zinc plates were applied to the ends of the tube, and fastened on by india-rubber collars. The ends were then inserted tightly into openings in the sides of two bottles which were filled with the solution (the same as that contained in the tube). The thick electrodes leading to the discs, and the backs of the zinc discs themselves, were lacquered, to insulate them from the liquid in the bottles. The whole apparatus was immersed in a trough of water, which could be heated to any desired temperature. In the course of his experiments Beetz demonstrated the absence of polarization when amalgamated zinc electrodes are used, and eliminated the transition resistance by boiling the electrodes in zinc sulphate, and transferring them to the ends of the tube without exposure to the air. Beetz farther proposed to find the specific conductivity of other electrolytes in terms of that ef zinc sulphate, by experimenting on 1 Pogg. Ann., 1847. 8 Monatsber. der Lerl. Akad., 1859. 8 Patry, Pogg. Ann., cxxxvi., 1869. 4 Pogg. Ann., cxvii., 1862. closed circuits consisting citiirely of the electrolyte to be examined. He tried damping experiments for this purpose, but the efl ects to lw observed turned out too small for accurate observation. Paalzow 5 inclosed the electrolyte to be examined in a siphon, the two ends of which dipped into vessels of porous clay also filled with the electrolyte. The clay vessels were immersed in beakers filled with zinc sulphate, at the bottoms of which were placed large amalgamated zinc discs, which formed the electrodes. The only polarization or transition resistance to be feared is that at the boundary of the two liquids, and this is very small. What little remained was eliminated, as in Horsford s method, by taking differences. The resistance of the whole arrangement was measured by means of Wheatstone s bridge, and then the siphon was replaced by a shorter one filled with the same liquid. If R u E 2 be the resistances found in the two cases, Rj-Rj is obviously the resistance of a length of the electrolyte equal to the difference between the lengths of the siphons. If R/, R 2 be similar values obtained when the elec trolyte is replaced by mercury, then the specific resistance of the T> _ T> electrolyte is I ? 7 , that of mercury being taken as unity. K 1 - K, The most important of all the recent researches on the application of Ohm s law to electrolytes are those of F. Kohlrausch and Nippoldt. In order to avoid the effects of polarization, they used the alternating currents of an electromagnetic machine. These currents varied very nearly as the sine of the angle of rotation, and could be sent in rapid succession through the electrolyte. The whole quantity of electricity that passes in the first part of any alternation is exactly equal and opposite to that which passes in the second ; hence equal quantities of the two ions (say H and 0) will be separated at each elec trode. If the H 2 and O combine to form water, it is obvious that, on the whole, there will be no resultant elec tromotive force of polarization either way; and if they coexist side by side without combining, there will still be no res^dtant electromotive force, provided the electrodes be exactly similar. There are two advantages in this method. There is no evolution of gas or other ion, and consequently no alteration of the solution and electrode, such as goes on with a constant current. We have, besides, another great advantage, which is denied us with constant currents, viz., that by increasing the size of the electrode, we can diminish the effects of polarization. The whole amount of electricity which passes in each induction current is the same, and consequently the whole amount of ion deposited on the electrode is the same ; hence, if we increase the surface of the electrode, the density of the deposit is decreased in an inverse ratio. Now, the researches of Kohlrausch and Nippoldt have shown 7 that, within certain limits, the electromotive force is proportional to the surface density of the deposit. Hence, by sufficiently increasing the surface of the electrodes, the polarization may be made as small as we please. In the earlier experiments platinum electrodes, having a surface of 1 - 08 cm. were used, and it was found that each induction cur rent of the magneto-electric machine deposited on each square millimetre of the positive electrode only ^^Q^QQ c.cm. of oxygen. It was therefore expected that the polarization would be insensible, and that the electrolyte would behave like a metallic resistance. The magneto-electric machine and the electrolyte were connected up with an electrodynamometer, and it was found that the deflec tion of the suspended coil of the electrodynamometer was scarcely sensible when the machine made 10 revolutions per second, although it was 15 scale divisions when the electrolyte was replaced by 70 Siemens units. On the other hand, when the velocity reached 77 revolutions per second, the deflection was much greater with the electrolyte than with 70 Siemens units. It was found, however, that when the surface of the electrodes was increased to 29 cm. a metallic resistance could be found, which gave the same deflection (within errors of observation) as the electrolyte for speeds varying from 4 8 to 76 - 9 revolutions per second. Paah Kohl ranse and 1 poldi Sine ductc 5 Pogg. Ann., cxxxvi., 1869. 6 The advantage gained even with constant currents by increasing the size of the electrodes is, however, appreciable (see below, p. 88).

7 Pogg. Ann., 1873, and "Jubelbd.," 1874.