I. Contact theory of voltaic action. Parts I and II

Abstract

The contact theory of voltaic action seems to have undergone no development since the date of Sir W. Thomson’s experiment, which consisted in connecting a plate of zinc and a plate of copper by means, of a drop of water, when it was found that the metals were brought to the same electric potential, although when metallically connected they were at different potentials. He believed that any electrolyte would behave in exactly the same way as the water of his experiment, equalizing the potentials of any two metals connected by it. The electromotive force of a simple cell, ought, in accordance with the theory, to be equal to the difference of potentials between zinc and copper in; contact. A test founded on this deduction was very difficult to apply, because there was no exact determination of the difference of potential of zinc and copper in contact, Sir W. Thomson, in his experiment, having really measured the difference of potential between air at the surface of a zinc plate, and air at the surface of a copper plate. In the absence of this test, the equality of the electromotive forces of simple cells in which zinc and copper are the metals (the liquids being water, dilute sulphuric acid, and sulphate of zinc) was held as a proof of the theory. Now it is known that when two pieces of the same metal are dipped into any two liquids, which are diffusing into one another, a difference of potentials is established between the metals, and the electromotive force of a cell of this kind can in no way depend on a difference of potentials due to metallic contact. So that although in such a cell there is an action which is somewhat the same as the action in a simple voltaic cell, the theory took no account of it whatever. In fact, the explanation of voltaic action given in the latest treatises on electricity is felt to be incomplete, even by the writers of such treatises, and the present investigation has been entered upon in consequence. Sir W. Thomson’s result, and our own experiments lead us to magine that when zinc and copper are immersed in water there are three successive states to be noticed:—At the instant of immersion the zinc and copper may be reduced to the same potential, so that the electromotive force of the voltaic cell E is equal to the difference of potential ZC — between zinc and copper in contact; the zinc now becomes negative to the copper, so that E reaches a limit which is greater than ZC — ; lastly, if a current passes, polarization occurs and the zinc becomes gradually less negative to the copper, E diminishing, therefore, from its maximum value# But when a saturated solution of zinc sulphate is employed instead of water, the first state, if it exists at all, exists for so short a time that practically, zinc and copper in zinc sulphate are never at the same potential. Thus (see Table X ) when care is taken to keep the zinc and copper in a water cell well insulated from one another, E is found to increase from a value very little greater than ZC — , the electromotive force of contact of zinc and copper, to a limit, but in a zinc sulphate cell no such great increase is observed.