186 JAMES CLERK MAXWELL 



of equations connecting together the electric and 

 magnetic quantities involved. 



The equations deduced from the above four 

 principles, together with the condition implied in 

 the continuity of an electric current, constitute 

 Maxwell's equations of the electro-magnetic field 



If we are dealing only with a dielectric medium, 

 the reciprocal relation between the third and fourth 

 principle may be made more clear by the following 

 statement : 



(A) The work done at any moment in carrying 

 a unit quantity of magnetism round a closed circuit 

 in a field in which electric displacement is varying, is 

 equal, to the rate of change of the total electric 

 displacement through the circuit multiplied by 4?r.* 



(B) The work done at any moment in carrying a 

 unit quantity of electricity round a circuit in a tield 

 in which the magnetic induction is varying, is equal 

 to the rate of change of the total magnetic induction 

 through the circuit. 



. From these two principles, combined with the 

 laws connecting electric force and displacement, 

 magnetic force and induction, and with the condition 

 of continuity, Maxwell obtained his equations of the 

 field. 



Faraday's experiments on electro-magnetic induc- 

 tion afford the proof of the truth of the fourth 

 principle. . It follows from those experiments that 

 when the number of lines of magnetic induction 



* The 4 v is introduced because of the system of units usually 

 employed to measure eWtricnl quantities. If we adopted Mr. Oliver 

 Ileaviside's "rational units," it would di*appe:ir, ns it does in (H). 



