88 E L E T K I C I T Y [KLECTKOMOTIVK KORCE. tion. motive force under that of a cell of Daniell. Helmholtz has given the name of electrolytic convection to this species of electrolytic conduction. He has shown that the phenomenon comes to an end when the liquid is perfectly freed from gas. The absorption of the gases by the electrodes plays a great part here, and gives rise in gas-free liquids to a phenomenon analogous to the residual discharge. When the battery is first turned on, a rush of electricity takes place, then there is a small current which gradually dies away. The first rush is like the instantaneous charge of a condenser; the small current which arises from the slow penetration of the ions into the platinum corresponds to the formation of latent charge. "When the voltameter is disconnected from the battery and discharged through a galvanometer, we have a first rush due to the part of the ions accumulated on the surface of the platinum, and then a gradually decreasing current due to the emergence of the gas which had penetrated into the metal. When the electromotive force which presses the gas into the electrode is removed, the absorbed gas will move very nearly in accordance with the ordinary law of diffusion, and the rate of its reappearance will depend very little 1 on the electromotive force at the surface of the electrode. Consequently . the residual current furnished by such an apparatus will not depend on its external resistance. A sudden increase of the external re sistance will simply cause the current to diminish until sufficient surface density has been attained to raise the electromotive force to the value required to keep up the same current as before through the increased resistance ; and the converse will happen if the external resistance be decreased. This passage of the gas into the substance of the electrode has, at the instance of Helmholtz, lately been investigated by Root. 2 He finds that in certain cases, when only one side of a platinum foil is exposed to electrolysis, the deposited gas, whether H or 0, actually passes through and produces the corresponding polarization on the other side of the foil. Maxi- It might at first sight be expected that in all cases where mum of the electromotive force in the circuit is sufficient to pro polariza- d uce continual evolution of the ions the polarization would be the same. This is not by any means the case, however, owing to the fact that the final state of the liberated ions varies with the strength of the current, or, more correctly speaking, with the current density, i.e., the amount of electricity which crosses unit section of the electrode in unit time. When H 2 and O are being liberated from dilute H 2 S0 4 , this depends mainly ou the formation of vari able quantities of ozone and H 2 O 2 . This variation of the physical state and intrinsic energy of the liberated ions, is a fact of the greatest importance in the art of electro metallurgy. A better instance could not be given than Gore s electrolytic modification of antimony, whose intrinsic energy is strikingly manifested by its explosive properties. The effect of enlarging the surface of the electrode in diminishing the polarization in the case where the maxi mum polarization is not reached was noticed above (p. 48). It has also the effect of diminishing the nuiximum of polarization in the case where the ions are completely set free. Platinizing has the same effect. Thus Foggendorff 3 found for the maximum polarization 2 12 to2 32 4 for bright platinum plates, while it was only 1 83 to 1 85 for platinized plates. The effect of platinizing on the hydrogen and oxygen polarization was about equal for strong currents, but greater on the hydrogen polarization when the current was weaker. On the other hand, by using small platinum points to decompose water in Wollaston s manner, Buff 5 found the polarization as high as 3 31. Poggendorff 6 and Crova 7 have investigated the depen dence of the maximum of polarization on the current density. It follows from their researches that it can be represented by A - B~" , I being the current density. It would appear that the maximum of polarization is decreased by increasing the temperature of the cell. The 1 Within certain limits, of course. 2 Pogg. Ann., 1876. 3 Wiedemann, Bd. i. 480 ; Pogg. Ann., 1847.
- Unless otherwise stated, our unit of the electromotive force is for
tlia present the electromotive force of a Daniell s cell. 5 Wiedemann, Bd. i. 473 ; Pogg. Ann., 1867. 6 Pogg, Ann., 1864. 7 Ann. de Chim. el dc Phys., 1863. effect, however, in some cases at least which have been adduced to prove this, might be explained by the decrease of the internal resistance of the cell. Agitating the electrodes, stirring the liquid in their neigh bourhood, or any other process which tends to dissipate the deposit on the electrode, leads, as might be expected, to a diminution of the polarization. The effect of increased pressure in retarding or helping electrolysis might be appreciable in certain cases. Suppose that an electro chemical equivalent of the ions, during liberation under a pressure p, increases in volume by v, then during the pass age of a unit of electricity work to the extent pv is done; the electromotive force required to free the ions must there fore have a part equal to pv which may increase or de crease as the process goes on. If the ions be gases which obey Boyle s law very nearly, then pv is constant, so long as the temperature remains the same; so that we cannot expect, within reasonable limits, to check the electrolysis of dilute sulphuric acid by conducting it in a closed vessel. 8 We have repeatedly drawn attention to the rapidity with Rapidi which the polarization decays in the first few instants after of de- the plates are connected through a circuit of moderate resist- P olarizi ance. Direct proofs of this have been given by Beetz - 1 and Edlund 10 . The former shows that the oxygen polarization decays much more rapidly than the hydrogen polarization, which is not to be wondered at, considering the greater readiness of platinum to absorb hydrogen; with palladium electrodes the difference would doubtless be still more marked. The reader may also consult an interesting paper on this subject by Bernstein 11 who concludes that in a certain case the polarization diminished by ^ to T ^ of its value in about ^^ of a second. There seems to be little reason to doubt the substantial accuracy of the facts just mentioned ; and the reader will not fail to see the application to the theory and practice of the measurement of the electromotive forces of inconstant electromotors, a category under which, unfortunately for the electrician, all known voltaic batteries must be classed. The remark applies with double force to the measurement of the electromotive force of polarization. Many measure ments of the latter have been made. We quote a few, to give the reader a general idea of the magnitudes involved; into a discussion of the methods we cannot enter here. Hydrogen and Oxycjcn Polarization of bright Platinum 1 latcs. - Numer: cal re sults. Whole Polarization. II Polarization. Polarization. Obsci vi .r. 2 33 Wheatstone. 2-56 Buff. 2-31 115 1-16 Svanberg. 2-33 1-16 1-16 Poggcndorff . 1-15 ... Beetz. Polarization of Platinum Plains with different Gases compared with tlie Electromotive Force of Platinum Plates with the same Gas<x against a fresh Platinum Plate in Grove s Gas Battery. Gas Polarization. Pt.GiPt. 1 171 161 Br 329 323 Cl 505 466 H 910 814 Cl and H 1-375 1-336 8 Maxwell, vol. i. 263. Other matters of great interest are stated there. See also the instructive analysis of the phenomena of pulari/a- tion in 294-271. 9 Poyg. Ann., 1850. 10 Pogg. Ann., 1852; see also Wiedemann, Bd. i. 495 ; &c. 11 Pogg. Ann., 1875. ls From Wiedemann, Bd. i. 478.
13 Beetz, q.uoted in Wiedemann; Pogg. Ann., 1853.