of the Electromotive Forces in a Voltaic Cell. 53 



is the place at which the circuit receives or gives up energy as 

 distinct from heat due to resistance, and the amount of the 

 E.M.F. is measured as the amount of energy which enters the 

 circuit per unit of electricity passing that place. Now it is not 

 possible with such a definition of E.M.F. as this, to give any 

 answer to Dr. Lodge's question different from the one he has 

 himself given. The seat of the E.M.F. is practically denned 

 to be the place where the chemical action goes on. It follows, 

 therefore, that the place where the chemical action goes on 

 is the seat of the E.M.F. The answer is given in the defi- 

 nition, and the definition is merely a partial statement of Sir 

 William Thomson's result — that the E.M.F. of a voltaic cir- 

 cuit can be calculated from the chemical action going on. 



The important subject, however, taken up in Dr. Lodge's 

 paper is, as we have already stated, quite a different one. He 

 finds strong reasons for believing that there cannot be a con- 

 siderable difference of potential between zinc and copper 

 in contact. In fact, a large difference of potential is to him 

 logically absurd, and he devotes the paper to speculations as to 

 how experiments could have misled Sir William Thomson and 

 others into believing such an absurdity. The absurdity is 

 so obvious to Dr. Lodge, that it might have struck him, per- 

 haps, that Sir William Thomson might also have seen it. 

 However, as Dr. Lodge has the opinions of Clerk Maxwell 

 and many others on his side, the question is a fair subject 

 for debate, and we shall endeavour to show that our notions on 

 the subject do not lead to any absurdity. With his definition 

 of E.M.F. we have never stated that there is such an E.M.F. 

 due to contact between zinc and copper. We deny that 

 there is such contact E.M.F., but we do say there is contact- 

 difference of potential. We do not say that what has been mea- 

 sured may not be to some, even to a large extent, due to air- 

 effect: indeed, in our third paper on the Contact Theory of 

 Voltaic Action, in Part I. of the Phil. Trans, for 1880, we 

 drew especial attention to the possibility of air-effects, and 

 we showed, exactly as Dr. Lodge now shows, that our measure- 

 ments gave no indication as to whether there is or is not 

 an air-effect. But we contend that no such air-effect has 

 ever been observed, and until it is observed we cannot admit 

 that there is strong reason for believing in its existence, 

 especially since we shall prove that all the phenomena of con- 

 tact-action can be rigorously explained without the necessity 

 of resorting to any air-action. 



Dr. Lodge says that if electricity rises in potential from 

 copper to zinc, there must be a supply of energy from some 

 outside source at the place of contact. We ask him why 



