402 Mr. J. A. Fleming on the new 



o 



pointed out, confirmed in a remarkable way by the phenomena 

 of the Peltier effect, which, when interpreted by the dynamical 

 theory of heat, furnish the most reliable measure of its amount. 

 These facts, together with others presently to be referred to, 

 have been made to furnish the key to a fresh explanation of the 

 dynamics of the galvanic cell, which I have ventured to call 

 the new contact theory, as opposed to the old or voltaic one. 



It is not possible, however, to define in a few words the pre- 

 cise details of the new theory ; they can only be arrived at by 

 collecting together the statements as we find them laid down by 

 their authors. The object of the present paper is to draw the 

 attention of those interested in this question to the objections 

 that may be raised against this new contact theory — objections 

 based on facts, some old and some which perhaps may prove new, 

 but all of which alike seem to throw fresh difficulties in the way 

 of this theory, although capable of simple explanation by the old 

 chemical hypothesis. It will be necessary then to review briefly 

 the precise statements of this new contact theory, in order to 

 show exactly what are the points against which objection may 

 be taken. This will be best accomplished by collecting the 

 statements of its principal supporters and arranging together 

 their explanations of the phenomena which arise 



(1) when dissimilar metals at the same temperature are placed 

 in contact, 



(2) \ibsn one insulated metal is placed in a liquid capable of 

 acting chemically upon it, 



(3) irYi*n two different metals are placed insulated and uncon- 

 nected in one such liquid, 



(4) when the two metals are joined across by a metallic arc or 

 when two or more cells are joined up in series. 



1. That the contact of metals is always attended with the 

 production of a difference of potential between them was for a 

 long time denied by ardent supporters of the chemical theory. 

 De la Rive endeavoured to show that the effects observed might be 

 attributed to oxidation j but his experiments are not conclusive ; 

 and to Sir W. Thomson belongs the credit of having established 

 the fact by experiment, irrespective of his theoretical deductions 

 from the facts of thermoelectricity. He thus describes his de- 

 cisive experiment : — 



" A metal bar insulated so as to be movable about an axis 

 perpendicular to the plane of a metal ring, made up half of cop- 

 per and half of zinc, the two halves being soldered together, 

 turns from the zinc towards the copper when positively elec- 

 trified, and from the copper towards the zinc when negatively 

 electrified"*. The difference of potential he finds to be about *6 

 * Reprint of papers on Electrostatics and Magnetism, p. 316, § 400. 



