502 INDUCTION. 



Suppose that the two circuits have an invariable shape, and that 

 they are so displaced that the two strengths I and I' retain constant 

 values, which are naturally different from the initial or final values ; 

 as the coefficient M is the only one which changes, the term for 

 the potential energy and that of the external work are both reduced 

 to the same value IIWM. We have then, at each moment, 



(El + ET - RI 2 - RT V= 2IIVM. 



It will thus be seen that the excess of the chemical energy 

 furnished by the pile over the energy expended in heat, is equal to 

 twice the external work expended in effecting the displacement; 

 half this energy is used in producing external work, the other in 

 increasing the potential energy of the system. 



This remark, which is due to Sir W. Thomson, should be com- 

 pared with the analogous proposition relative to the displacement of 

 conductors at constant potentials. 



