4 The Rev. T. R. Robinson on the Relation between the Tempe)'ature of 



each molecule, acting by induction through the interval between them in a way 

 corresponding to the charge and disruptive discharge of a coated plate. This 

 in some degree accounts both for the heat produced by the current, and for its 

 increasing with the resistance. It also seems to explain the increase of resis- 

 tance ; for the distance increases with the expansion, and the intensity required 

 for discharge increases \vitli the distance. That intensity must, by the ordi- 

 nary theory, be as the square of the distance between the particles ; and, 

 therefore, the resistance is in the same ratio. Calling it A and the distance z ; 

 if this latter become by expansion - + f, the proportional change of resistance 



A'-A_2r t 

 A ~ z^ 2" 



or, if / + eP be the length of the expanded -wire I, 



,, , , /2eP e=P-\ 



It will, therefore, increase faster than the expansion. 



Or the transfer may be regarded in another view, as merely a momen- 

 tary change of condition propagated in rapid succession like a wave, and 

 modifying as it passes the normal action of the forces which keep the 

 molecules in their state of equilibrium. A statement of this kind is of neces- 

 sity vague, in our present imperfect knowledge (or rather conjectures) ; yet we 

 are not without grounds for believing, that when these forces exert any specific 

 action, they become less efficient as to others, which, under different circum- 

 stances, they would have maintained with energy. Thus, light interferes with 

 affinity, and this with cohesion ; thus also, pressure developes heat and electri- 

 city. It is, therefore, possible, that while producing thermic effects they may 

 be unable to contribute to electric conduction. If so, we may expect that the 

 change will be proportionate to its cause, to the heat developed in the con- 

 ductor. 



However this may be, whatever tends to throw light on a question so im- 

 portant cannot be without its use ; and I hope the experiments which I have the 

 honour to lay before the Academy may in some degree answer this purpose. 



The rheometer and rheostat wliich I employed have been described by 



