6 PROPERTIES OF AN ELECTRIC CHARGE [CH. I. 



it is moving. There is, in my opinion, no other 

 kind of magnetic field possible ; and so if ever we 

 come across a magnetic field which we feel entitled 

 to consider " strong," we must conclude that it is 

 associated with the motion of a very considerable 

 charge, at a velocity we may properly style great. 

 But certainly it is true that for any ordinary charged 

 sphere, moving at any ordinary pace, even supposing 

 that it is a cannon-ball shot from the mouth of a 

 gun the concentric circular magnetic field sur- 

 rounding its trajectory is decidedly feeble. Feeble 

 or not, it is there, and to its existence we must trace 

 all the magnetic phenomena of the electric current. 



For just as there is no electrostatic field save that 

 extending from one charged body to another, so 

 there is no electric current except the motion of such 

 a charged body, and no magnetic field except that 

 which surrounds the path of this motion. 



The locomotion of an electric charge is an electric 

 current, and the magnetic field surrounding that 

 current is believed to be the only kind of magnetic 

 field in existence. If any other variety is possible, 

 the burden of proof rests on those who make the 

 positive assertion. 



Transmission of Energy. 



While the charge is stationary everything is steady, 

 and we have an electric field only. 



While the charge is moving at constant speed the 

 current is steady, and we have a steady magnetic 

 field superposed upon a steadily moving electric field; 

 there is likewise a certain conveyance of energy in 

 the direction of the motion. 



This is a special case of the general theorem 

 known as Poynting's : viz. that wherever an electric 



