Electromotive Forces of Contact. 51 



energy being furnished by the partial satisfaction of the said 

 affinity. The method further assumes that when the metals 

 are fused together it is this same affinity which, being satisfied 

 as completely as possible, gives rise to an evolution of heat. 

 But if (as appears from what precedes) the energy required 

 to produce the difference of potentials is supplied by absorp- 

 tion of heat at the junction, and if the contact E.M.F. is due 

 to a tendency to convert heat-energy into electrical energy, 

 then the heat evolved on fusing the metals together can have 

 no direct connexion with E.M.F. 's of contact, and the method 

 would not apply to the case of two metals. 



It would, however, obviously apply where the E.M.F. was 

 due to chemical affinity, as in the case of zinc and oxygen 

 proposed by Professor Lodge (Phil. Mag. May 1885, p. 363). 

 In all these investigations the electrostatic capacities are cal- 

 culated on the assumption that the specific inductive capacity 

 of the conductors is unity. I have not been able to find any 

 experimental justification for this assumption. 



Electrification of Non-conductors. 



When two non-conductors are brought into contact, there 

 are other sources of energy which may cause electrification. 

 For if the bringing of two free surfaces into contact, or the 

 striking or rubbing of the bodies together, should give rise 

 to electrification, the charges so produced would remain 

 separated, owing to the insulating properties of the substances. 

 This source of electrification is absent in conductors, as any 

 electrification produced between them by sudden violence 

 would be destroyed by conduction, the final difference of 

 potentials between the bodies being always that due to a 

 definite contact E.M.F. 



The difference between the two cases may be illustrated by 

 considering the case of a spring. If a force be applied to the 

 spring a definite compression will ensue, supposing that there 

 is no friction ; and if by any temporary impulse the com- 

 pression be increased or diminished, it will regain its 

 former value when the impulse ceases to act. The inverse of 

 the strength of the spring represents the capacity of the 

 system, the force compressing the spring the E.M.F. of con- 

 tact, the distance through which the spring is compressed the 

 quantity of electricity displaced, and the potential energy of 

 the compressed spring the electrostatic potential energy of 

 the two conductors. 



If we suppose compression or extension of the spring to be 

 accompanied by friction, a compression effected by a sudden 

 shock will be maintained. This illustrates what perhaps 



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