Vol. 6, 1920 
PHYSICS: L. PAGE 
117 
To obtain Ampere's law, consider the increase in electric intensity at 
0 (fig. 1) taking place in the time dt. During this time the lines of force 
FIG 1 
at P will have moved up to^O. If these lines are closer together at P 
than at 0, E will change by an^^amount 
Furthermore E will suffer a change if the velocity Cp of the moving ele- 
ments at P has a different direction from that at 0, or if these lines twist 
as they move upward. The increase in E due to these two causes is 
(— EV-c + ^'Vc)dt. 
Therefore, in all, 
E = — cVE — EVc + EVc 
= — cV-E + V X (c X E). 
So far, no account has been taken of the new lines of force produced by 
the emission of moving elements in the neighborhood of a charged particle. 
P / 
FIG. 2 
Let the dots in figure 2 represent sources emitting lines of force in the di- 
rection of the broken lines, and let these sources have a velocity v to the 
