H. XIL] EFFECT OF RAPID MOTION 123 



normal to the line of movement, then a greater 

 component of their motion would be of a kind 

 suitable to excite a magnetic field ; moreover, both 

 the fields would by this concentration increase in 

 intensity, and the inertia would increase. 



Thus, then, it may be possible that electric inertia 

 may depend in some fashion on speed, a thing 

 unknown in ordinary mechanics. I do not say 

 that such dependence must be untrue in ordinary 



! mechanics ; on the contrary, I feel reasonably san- 

 guine that it will be found true for matter also, 

 when moving sufficiently fast say over a thousand 

 miles a second, though it is unlikely that it can 

 have a practical influence in any actual known case 

 of rapid movement in astronomy. But however this 



l may be, there is no doubt that theory points to an 

 increase of electro- magnetic inertia at excessively 

 high speeds, and Mr. Heaviside long ago calculated 

 its amount. 



It will be observed that when a charge moves, 

 it generates circular magnetic lines of force. Now 

 these magnetic lines are not stationary, but are them- 

 selves moving at the same rate as the body ; hence 

 they generate fresh electrostatic lines, i.e., cause an 



j electric displacement away from the axis, which dis- 

 placement is superposed upon the original radial 

 displacement (away from or toward the centre) due 

 to the charge. 



At ordinary, at even violent speeds, this second- 

 order electric effect is insignificant, but it is there all 

 the time, and must not be ignored when the speed 

 becomes extravagantly high. It rapidly rises into 

 prominence when the speed approaches the velocity of 

 light, but at any speed much smaller than this such a 

 second-order effect is negligibly small. 



