52 Professors Ayrton and Perry on the Seat 



his paper, that there is a prima facie absurdity in acknow- 

 ledging a considerable difference of potentials between two 

 metals such as zinc and copper in contact with one another. 



He says that all the apparent differences of potential which 

 have been measured in air are due to an air-effect; and when 

 these same differences of potentials are obtained by measure- 

 ments in gases quite different from air, he still says that 

 they are due to an air-effect, and even that when they are 

 obtained from measurements made in a fairly good vacuum, 

 they are still due to air-effects. In fact, he considers that 

 he has such good reason for concluding that the existence of 

 a large contact-potential difference between metals is absurd, 

 and he boldly puts forward the view that if measurements 

 were made in the most perfect vacuum hitherto obtained 

 in the laboratory, and if the results were still the same (as 

 they probably would be) they would still be due to air-effects. 

 Now, when it is borne in mind that all the experimental 

 evidence is in favour of the existence of a real large contact- 

 potential difference between metals, it is quite evident that 

 our most important duty is to examine the basis of the strong 

 reason which has caused Dr. Lodge to discard direct experi- 

 mental results, for it is this strong reason which has caused 

 him to enter into elaborate explanations as to the attraction 

 between gases and metals. 



We will first dispose of that part of Dr. Lodge's paper 

 which is not debatable, viz. the seat of the E.M.F. in a 

 voltaic circuit. So far from differing from Dr. Lodge on this 

 point, we believe that we are in agreement; and we hope to 

 prove that this seat of the E.M.F. in a voltaic circuit has really 

 nothing whatever to do with the potential-difference between 

 zinc and copper in contact. In fact, Dr. Lodge's paper has 

 not got quite its proper title. The subject to which the greater 

 part of his paper is devoted is a debatable one, but it is quite 

 a different subject from that of the seat of the E.M.F. in a 

 voltaic cell. In considering this latter subject we shall, for 

 brevity's sake, regard the small Peltier effect as non-existent. 

 Formerly, any cause of flow of electricity was called an 

 E.M.F., and hence in stating Ohm's law, E.M.F. and potential- 

 difference were used indifferently. But when we adopt, as 

 everybody does nowadays, the exact definition of Clerk Max- 

 well, we must be more careful ; and when we adopt the much 

 more guarded definition of E.M.F. of Dr. Lodge, we must be 

 particularly careful. This definition of Dr. Lodge is given 

 in two quotations from Clerk Maxwell ; and the definition is 

 not merely the definition of E.M.F., but is the definition of 

 the seat of the E.M.F. The seat of the E.M.F. in any circuit 



