CH. II.] ELECTRIC INERTIA 13 



motion, the action and reaction would go on in- 

 tensifying themselves, until any amount of violence 

 was reached. 



The magnetic lines generated by a rising current, 

 that is by a positively accelerated charged body, 

 react back upon the motion which produced them 

 in such a way as to oppose it ; to oppose it 

 actively or elastically, not passively or sluggishly 

 as by friction. The reaction ceases the instant the 

 motion becomes steady : it is not analogous to 

 friction therefore, but to inertia ; it is the coeffi- 

 cient of an acceleration term. 



The magnetic lines generated by a falling current, 

 that is by a negatively accelerated or retarded 

 charged body, react oppositely, and tend to con- 

 tinue the motion : thus here also we have a term 

 corresponding to inertia. And the charged body 

 may be said to have extra momentum, by reason 

 of its charge, while it is moving. The value of 

 the momentum is proportional to the velocity, so 

 long as the velocity is not excessively great ; and 

 accordingly the inertia term is constant, i.e. inde- 

 pendent of speed, under the same restriction. It 

 may therefore be considered to be in existence 

 even when the charge is stationary, and thus it 

 simulates exactly the familiar mechanical inertia 

 of a lump of ordinary matter. 



In Appendix A, is given the simplest form of 

 the quantitative relations here indicated, and the 

 inertia due to an electric charge is there calculated. 

 It is to be understood that whatever inertia a 

 material sphere may possess, considered as matter, 

 it will possess more when it is charged with elec- 

 tricity, and this no matter whether the charge 

 be positive or negative. The amount of extra or 



