n 



°EC is 1890 



THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



DECEMBER 1890. 



LI. On the Dynamical Theory of Electromagnetic Action. 

 By Professor Andrew Gray, ALA* 



THE use of the term potential energy in electromagnetic 

 theory is somewhat vague. For example, an expression 

 for the potential energy of a circuit and a magnetic shell is 

 obtained by replacing the former by its equivalent shell and 

 proceeding as in the magnetic theory of two shells. This 

 expression gives the potential energy of the system in terms 

 of the configuration of the two shells, and so enables the 

 mutual forces between the shell and the circuit, and there- 

 fore also the work done in any relative displacement, to be 

 calculated. But, in the presentation of the theory, the 

 assumption is then made that this work is wholly due to the 

 expenditure of the potential energy of the substances in the 

 battery, and nothing is credited to a change in the intrinsic 

 or potential energy which the system possesses irrespective of 

 the store of energy contained in the battery. 



But when we consider the case of two circuits, we have 

 always to take into account the change in intrinsic energy. 

 For example, when tw 7 o rigid circuits are allowed to approach 

 one another infinitely slowly, the battery, as is well known, is 

 drawn upon for twice as much energy as is required for the 

 work done by electromagnetic forces between the two circuits, 

 the balance going to increase the intrinsic energy of the system. 



* Communicated by the Author. 

 Phil. Mag. S. 5. Vol. 30. No. 187. Dec. 1890. 2 I 



