12 ELECTRIC INERTIA [CH. n. 



is opposed by the reaction of the magnetic lines 

 generated by it. 



Motion is opposed while it is increasing in speed, 

 and it is assisted while it is decreasing in speed 

 an effect precisely analogous to ordinary mechanical 

 inertia ; and therefore force is necessary, and work 

 must be done, either to start or to stop the motion 

 of a charged body. An extra force, that is, by 

 reason of its charge. Whatever ordinary inertia 

 the body may have, considered as a piece of matter, 

 it has a trifle more by reason of its being charged 

 with electricity no matter what the sign of its 

 charge may be. 



The value of this imitation or electrical inertia, 

 for the case of a charged sphere of radius a, was 

 calculated by J. J. Thomson in 1881, and is 



3a 



Electrical Inertia or Mass, continued. 



Since this is very important, I repeat : 

 Just as a changing magnetic field affects an electro- 

 static charge, that is to say generates a feeble field 

 of electric force, into the intensity of which the 

 velocity of light enters squared in the denominator 

 (see Lodge, Phil. Mag., June 1889, p. 472), so it 

 is with a changing electric field, it generates a 

 magnetic field proportional to its velocity of change. 

 And if it is being accelerated, the magnetic field 

 itself varies, and in that case generates an E.M.F. 

 which reacts upon the accelerated moving charge, 

 always in such a way as to oppose its motion 

 by what is called Lenz's law, or simply by the law 

 of conservation of energy : for if it assisted the 



