THE ELECTRIC CURRENT

120. If the Daniell’s cell is taken out of the circuit, and the circuit again closed, the galvanometer indicates a current passing through the electrolytic cell in the opposite direction to the original current. This current rapidly diminishes in strength and soon vanishes, so that the whole quantity of elec- tricity which is transmitted by it is never greater than that of the primitive current. This reverse current indicates that the platinum plates have acquired a difference of properties by being used as electrodes. They are said to be polarized. The cathode is polarized positively and the anode negatively, so that an electromotive force is exerted in the circuit opposite to that of the Daniell’s cell. This electromotive force, which is called the electromotive force of polarization, is the cause of the rapid diminution in the strength of the original current, and of its final cessation.

A chemical examination of the platinum plates shews that a certain quantity of hydrogen has been deposited on the cathode. This hydrogen is not in the ordinary gaseous form, but adheres to the surface of the platinum so firmly that it is not easy to remove the last traces of it.

121.] Faraday’s law, that conduction takes place in electrolytes only by electrolysis, was long supposed not to be strictly true. In the experiment in which a single Daniell’s cell furnishes the electromotive force in a circuit containing an electrolyte and a galvanometer, it is found that the current soon becomes very feeble but never entirely vanishes, so that if the electromotive force is maintained long enough, a very considerable quantity of electricity may be passed through the electrolyte without any visible decomposition. Hence it was argued that electrolytes conduct electricity in two different ways, by electrolysis in a very conspicuous manner and also, but in a very slight degree, in the manner of metals, without decomposition. Helmholtz has recently∗ shewn that the feeble permanent current can be explained in a

Ueber galvanische Polarisation in gusfreien Flüssigkeiten. Monatsbericht d. K. Akad. d.

ELECTROLYTIC CONVECTION.

different manner, and that we have no evidence that an electrolyte can conduct electricity without electrolysis.

122.] In the case of platinum plates immersed in dilute sulphuric acid, if the liquid is carefully freed from all trace of oxygen or of hydrogen in solu- tion, and if the surfaces of the platinum plates are also freed from adhering oxygen or hydrogen, the current continues only till the platinum plates have become polarized and no permanent current can be detected, even by means of a sensitive galvanometer. When the experiment is made without these pre- cautions, there is generally a certain amount of oxygen or of hydrogen in solution in the liquid, and this, when it comes in contact with the hydro- gen or the oxygen adhering to the platinum surface, combines slowly with it, as even the free gases do in presence of platinum. The polarization is thus diminished, and the electromotive force is consequently enabled to keep up a permanent current, by what Helmholtz has called electrolytic convection. Besides this, it is probable that the molecular motion of the liquid may be able occasionally to dislodge molecules of oxygen or of hydrogen adhering to the platinum plates. These molecules when thus absorbed into the liquid will travel according to the ordinary laws of diffusion, for it is only when in chemical combination that their motions are governed by the electromo- tive force. They will therefore tend to diffuse themselves uniformly through the liquid, and will thus in time reach the opposite electrode, where, in con- tact with a platinum surface, they combine with and neutralize part of the other constituent adhering to that surface. In this way a constant circulation is kept up, each of the constituents travelling in one direction by electrolysis, and back again by diffusion, so that a permanent current may exist without any visible accumulation of the products of decomposition. We may there- fore conclude that the supposed inaccuracy of Faraday’s law has not yet been confirmed by experiment.

123.] The verification of Ohm’s law as applied to electrolytic conduction is attended with considerable difficulty, because the varying polarization of the electrodes introduces a variable electromotive force, and renders it difficult to ascertain the true electromotive force at any instant. By using electrodes Berlin, July 1873, p. 587.ELECTROLYSIS.

in the form of plates, having an area large compared with the section of the electrolyte, and employing currents alternately in opposite directions, the ef- fect of polarization may be diminished relatively to that of true resistance. It appears from experiments conducted in this way that Ohm’s law is true for electrolytes as well as for metals, that is to say, that the current is always proportional to the electromotive force, whatever be the amount of that force. The reason that the external resistance of an electrolyte appears greater for small than for large electromotive forces is that the external electromotive force between the metallic electrodes is not the true electromotive force act- ing on the electrolyte. There is, in general, a force of polarization acting in the opposite direction to the external electromotive force, and it is only the excess of the external force above the force of polarization that really acts on the electrolyte.

It appears, therefore, that the very smallest electromotive force, if it really acts on the electrolyte, is able to produce conduction by electrolysis. How, then, is this to be reconciled with the fact that in order to produce complete decomposition a very considerable electromotive force is required?

124.] Clausius∗ has pointed out that on the old theory of electrolysis, ac- cording to which the electromotive force was supposed to be the sole agent in tearing asunder the components of the molecules of the electrolyte, there ought to be no decomposition and no current as long as the electromotive force is below a certain value, but that as soon as it has reached this value a vigorous decomposition ought to commence, accompanied by a strong cur- rent. This, however, is by no means the case, for the current is strictly pro- portional to the electromotive force for all values of that force. Clausius explains this in the following way:—

According to the theory of molecular motion of which he has himself been the chief founder, every molecule of the fluid is moving in an exceedingly irregular manner, being driven first one way and then another by the impacts of other molecules which are also in a state of agitation. This molecular agitation goes on at all times independently of the action of electromotive force. The diffusion of one fluid through another is brought ∗ Pogg. Ann. CI. 338 (1857).ELECTROLYSIS.

about by this molecular agitation, which increases in velocity as the tem- perature rises. The agitation being exceedingly irregular, the encounters of the molecules take place with various degrees of violence, and it is proba- ble that even at low temperatures some of the encounters are so violent that one or both of the compound molecules are split up into their constituents. Each of these constituent molecules then knocks about among the rest till it meets with another molecule of the opposite kind and unites with it to form a new molecule of the compound. In every compound, therefore, a certain proportion of the molecules at any instant are broken up into their constituent atoms. At high temperatures the proportion becomes so large as to produce the phenomenon of dissociation studied by M. St. Claire Deville∗ .