Electromotive Forces of Contact. 49 



For consider a circuit consisting of two metals A and 

 B (fig. 3) and an inert gas C, all at the same temperature. 



Fig. 3. 



Suppose that there is on the whole a finite E.M.F. in the 

 circuit ACBAj i. e. that the potential of the gas C varies as 

 we pass from the plate A to the plate B. Now, it is evident 

 that if the gas C has any trace of conductivity, or any 

 power whatever of equalizing potentials by convection, a 

 feeble current will flow continuously around the circuit 

 ACBA, and will derive its energy from absorption of heat at 

 one or more of the junctions, which would be a violation of 

 the Second Law of Thermodynamics. If the metal A were 

 furnished with points as fine as possible, and if these points 

 were brought within an extremely small distance of B, they 

 would greatly favour any conductive or convective tendency 

 that might exist ; and if by such an arrangement a convective 

 discharge of only one molecule of electricity in a century 

 were established, it would constitute a violation of the Second 

 Law of Thermodynamics. 



If we admit that in an inert gas C the difference of poten- 

 tial between the layers in contact with A and B is finite, we 

 must assume that the gas is an absolutely perfect insulator for 

 such an E.M.F. Even if this property is assumed it does 

 not follow that Yolta's Law is inapplicable to the case of 

 gases and metals. 



I have endeavoured to show how a Peltier E.M.F. depends 

 on a tendency to absorb heat-energy and convert it into 

 electrical energy. Now from the law of successive contacts 

 of chemically inactive conductors, it is evident that, the 

 temperature being fixed, we can connect with each substance 

 a constant (called its " Peltier-constant ") such that the 

 E.M.F. between any two inactive conductors is equal to the 

 difference of their respective constants, which is only another 

 way of stating Volta's Law. 



Now 7 it seems not unlikely that the Peltier constant of a given 

 conducting substance, at a given temperature, depends only 

 on the size, form, kc. of its molecule-, and the nature of their 

 heat-motion ; and, if so, each non-conductor should also have 



Phil. Mag. S. 5. Vol. 26. No. 158. July 1888. E 



