500 report— 1884. 



for every junction in the chain: neglecting magnetic or impressed E.M.F. 

 [Verified most completely by Ayrton and Perry.] 



B. — Thomson. 



iv. The E.M.F. in any closed circuit is equal to the energy conferred 

 on unit electricity as it flows round it. 



[Neglect magnetic or impressed E.M.F. in what follows.] 



v. At the junction of two metals any energy conferred on, or with- 

 drawn from, the current must be in the form of heat. At the junction* 

 of any substance with an electrolyte, energy may be convej^ed to or from 

 the current at the expense of chemical action as well as of heat. 



vi. In a circuit of uniform temperature ; if metallic, the sum of the- 

 E.M.F.'s is zero by the second law of thermodynamics ; if partly electro- 

 lytic, the sum of the E.M.F.'s is equal to the sum of the energies of 

 chemical action going on per unit current per second. 



vii. In any closed conducting circuit the total intrinsic E.M.F. is equal 

 to the dynnmical value of the sum of the chemical actions goiDg on per 

 unit electricity conveyed (2'JW), diminished by the energy expended in 

 algebraical^ generating reversible heat. 



viii. The locality of any E.M.F. may be detected, and its amounk 

 measured, by observing the reversible heat or other form of energy there 

 produced or absorbed per unit current per second. [This is held by 

 Maxwell, but possibly not by Thomson, 1 though its establishment is due- 

 to him.] 



II. — Statements believed by the writer to be false though 



ORTHODOX. 



ix. Two metals in air or water or dilute acid, but not in contact, are- 

 practically at the same potential. 2 [Sir Wra. Thomson, Clifton, Pellat.] 



x. Two metals in contact are at seriously different potentials (i.e. 

 differences of potential greater than such milli-volts as are concerned in 

 thermo-electricity.) [This is held by nearly everybody.] 3 



xi. The contact force between a metal and a dielectric, or between a 



1 The only reason which I can think of as likely to have caused Sir Wra, Thomson to 

 doubt or deny the validity of this proposition is given and, I hope, refuted at sections 

 (10) and (11). 



2 The truth or falsity of this statement may be held to depend on a question of 

 words, viz. : — the definition of potential. Sir Win. Thomson at the meeting said he 

 had always denned potential as the work done in bringing a unit charge close up to, 

 but not into, the body. This definition explains some apparent inconsistency in one 

 or two of his utterances which I had never quite understood. But seeing that there is 

 no difficulty whatever in giving a charge up to a metal body, but rather the contrary, 

 why not define its potential in the more simple manner which followers of his have 

 unconsciously, and I believe universally, adopted, not knowing that they were thus 

 putting themselves out of harmorjy with him. Given his definition, so that the- 

 potential of a bod* means really not its potential but the potential of the medium' 

 close to it, statements Nos. is. and x. are undoubtedly true ; and No. xi. is also true, 

 I suppose, for it then only means that there is not much E.M.F. between the medium 

 close to a metal and that at a little distance. 



3 It is much more natural to suppose that the potential of a metallic conductor i s 

 uniform, whether it is homogeneous or not. Indeed, it is not only more natural,, 

 but it is true, that two parts of a conductor can only differ in potential by reason 

 of an E.M.F. located at the junction. Now there usually is an E.M.F. at a junc- 

 tion, but it is only of such a magnitude as is concerned in thermoelectricity. It, 

 indeed, does produce a difference of potential between the metals, but nothing 

 else can. N.B. -Always provided that by ' the potential of a metal ' is meant 

 that potential, and not the potential of air near it. 



