APPENDIXES. 



APPENDIX A. 

 Calculation of the Inertia of an Electric Charge. 



Let a spherical conductor of radius a, carrying a 

 charge of electricity, move forward with moderate speed 

 u\ meaning by moderate speed anything distinctly less 

 than the speed of light: it constitutes a current element 

 of magnitude eu, and its circuit is closed by displace- 

 ment currents in the surrounding dielectric. For electric 

 lines of force arise in the medium in front, and subside 

 in the medium behind, and so a displacement of electricity 

 takes place from fore to aft, to compensate the motion 

 forward ; and the lines of displacement are identical with 

 the magnetic lines due to a short magnet. A charge 

 may be said to travel carrying its electrostatic lines 

 with it, or it may be said to be constantly generating an 

 electrostatic field in front and destroying one behind. 



When an electric field thus moves, partly laterally, it 

 generates a magnetic field in the present instance in 

 circular lines round the line of motion; for the moving 

 charge is an element of a linear current. The generation 

 of these magnetic lines acts so as to oppose the current 

 which produced them ; though so long as they continue 

 steady they exert no effect on it. When they subside, 



