124 INCREASE OF INERTIA [CH. XIL 



Its effect will be, as the annexed figure shows, to 

 alter the arrangement of the lines of force, making 

 them move away from the poles and concentrate 

 towards the equator of the charged sphere, when 

 the speed is very great ; ultimately becoming wholly 

 concentrated upon, or parallel to, the equatorial 

 plane, in the limit ; if the speed could attain that of 

 light. And the electric lines of force would then 

 be opened out into a fan or equatorial brush, like 

 the spokes of a wheel which is rushing furiously 

 along an elongated axle, the circumference of the 



FIG. 16. A is the charge, AB its line of motion, and AE its electric force 

 in a certain direction when stationary ; EF is the magnetically induced 

 electric component due to the motion and AF is the resultant electric 

 force which replaces the original force AE. The magnetic force, to the 

 motion of which EF is due, is perpendicular to the paper, and is itself 

 caused by the motion ; hence EF is a quantity of the second order and 

 is small for speeds distinctly less than that of light. 



wheel representing the direction of the magnetic 

 field ; but this very condensation so intensifies the 

 field as to make the inertia ultimately infinite. 



It might be supposed that rearrangement of the 

 lines means that the distribution of the charge itself 

 is altered by the motion, so that all the charge is 

 concentrated upon the equator, whence the lines 

 of force would start normal to the surface as usual. 

 There are many difficulties about such a conception 

 however (see Appendix K), and it is easier to 

 suppose that the charge retains its distribution un- 

 altered, on the surface of the sphere, and that each 

 line of force starts from its original point ; but that 



