CH.L] CHARGE IN MOTION 5 



The answer is that during the motion of the 

 charged body, and even after the cessation of its 

 motion, until the disturbance has had time to die 

 away and everything to settle down into static con- 

 dition again, the phenomena of magnetism make 

 their appearance : a new set of lines of force quite 

 different from the electrostatic lines (although they, 

 too, exhibit a tension along them and a pressure at 

 right angles) come into temporary being. These do 

 not like the electric ones originate at one place 

 and terminate at another : they are always and 

 necessarily closed curves or rings, and in the present 

 simple case they are circles all centred upon the path 

 of motion of the charged body. At any point of space 

 there are now three directions to consider : (l) there 

 is the original direction of the electrostatic field the 

 original electric line of force ; (2) there is the direction 

 of the motion that is, a direction parallel to the 

 movement of the charged sphere ; and (3) there is the 

 direction at right angles to these two ; this last being 

 the direction of the magnetic lines of force the 

 direction of the magnetic field. 



I spoke of the magnetic field as temporary, but 

 that is on the assumption that the charged body is 

 merely displaced merely shifted from one position to 

 another ; if it is not stopped, but keeps on moving, 

 then the magnetic lines continue as long as the 

 motion lasts. The strength of the magnetic field, 

 at any point with polar coordinates r, 0, is 



TT eu a 



H = r sin 6. 



r 2 



If we are asked whether such a magnetic field is 

 weak or not, I have to reply that that depends 

 entirely on how strong the charge is and how quickly 



