196 



Profs. W. E. Ayrton and John Perry. [Mar. 21, 



March 21, 1878. 



Sir JOSEPH HOOKER, K.O.S.I., President in the Chair. 



The Presents received were laid on the table and thanks ordered for 

 them. 



The following Papers were read : — 



I. " Contact Theory of Voltaic Action." Parts I and II. By 

 W. E. Ayrton and John Perry, Professors in the Imperial 

 College of Engineering, Tokio, Japan. Communicated by 

 Professor Sir W. Thomson, F.R.S. Received October 2, 

 1877. 



Part I. 

 I. 



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, equal- 

 izing the potentials of any two metals connected by it. The electro- 

 motive 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. INow 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 what- 

 ever. In fact, the explanation of voltaic action given in the latest 



