APR A.] CALCULATION OF INERTIA 207 



In other words, the mass equivalent to the charge is such 

 that if it were a piece of matter with constant inertia 

 travelling at the speed of light, its kinetic energy would 

 be half as great again as the potential energy of the 

 electric charge when standing still. 



APPENDIX B. 

 The Electric Field due to a Moving Magnet. 



If a short bar magnet or uniformly magnetised sphere 

 (its moment M being the intensity of magnetisation x the 

 volume of the sphere) moves along axially that is in the 

 direction of its magnetisation with velocity u, it generates 

 circular lines of electric force all centred upon its axis, 

 much as a moving charge generates circular lines of 

 magnetic force. If there is a conducting path round 

 any such circle, then the motion of a magnet along its 

 axis will generate a current in it; but if there be no 

 conductor, the motion will only result in an electric dis- 

 placement which subsides when the magnet stops. 



The intensity of the magnet's field at any point along 

 its axis is well known to be 2M/r 8 ; at any point on its 

 equatorial plane it is M/r 3 ; and in any intermediate 

 direction it is, as regards magnitude alone 



All this holds for the moving as for the stationary magnet, 

 provided its speed does not approach that of light. 

 The electric force at the same point is 



