THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



JUNE 1874. 



L. On the new Contact Theory of the Galvanic Cell. 

 By J. A. Fleming, B.Sc, F.C.S.* 



THE contest that has for so long been waged between the sup- 

 porters of the two theories of the galvanic cell, the contact 

 and the chemical, can hardly be said to have been brought even 

 now to a decisive issue. For although the contact theory, as 

 originally proposed by Volta, received a fatal blow when the law 

 of conservation of energy became clearly understood, yet in its 

 place a new contact theory has arisen, supported by novel and 

 important experimental evidence, which has again been placed 

 by recent writers on electrical science in formidable opposition 

 to its old rival. 



The old contact theory of Volta had its origin in an entire 

 ignorance of the science of energy. It simply referred the cur- 

 rent produced through the circuit of a pile to the effect of the 

 metallic contacts, and it ignored the thermal and chemical 

 changes which are also necessarily present; but it had to be 

 finally abandoned when once it became clearly understood that 

 the appearance of a current involved the disappearance of some 

 other energy, actual or potential, as an invariable accompani- 

 ment. The new contact theory may be said to have had its 

 source in the discovery of Sir W. Thomson, that there is un- 

 doubtedly a difference of potential produced when dissimilar 

 metals are placed in contact — a fact not only abundantly proved 

 by Thomson by direct experimental evidence, but, as he has 



* Read before the Physical Society, March 21, 1874. Communicated 

 by the Society. 



Phil. Mag. S. 4. Vol. 47. No. 314. June 1874. 2 D 



