Equivalent Shell of a Circular Current. 143 



and negative charges being completely explained by the 

 notion of "displacement/' Taking a positive electric ring 

 and a positive magnetic ring coincident with one another, 

 the electric current corresponds to the whirling round of the 

 electric ring attended by a displacement of the magnetic 

 ring as in fig. 4 a. The magnetic current corresponds to the 

 whirling round of the magnetic ring attended by* a displace- 

 ment of the electric ring as in fig. 4 b. An inspection of the 



Ffr. 4. 



figure will show that both the cases are essentially identical, 

 being the case of an electric and a magnetic ring spinning 

 round in directions opposite to one another and pulled apart 

 by the spinning. The displacement and therefore the static 

 charge is proportional to the velocity and so is the current. 

 In the electric current we view the system magnetostatically 

 and in the magnetic current electrostatically. 



If m and <? u be the static charges when v = l, the force of 

 attraction between two such identical systems of " electro- 

 magnetic doublets" of infinitely small radius placed 1 cm. 

 apart, 





4-0 



m 2 v 



P 



+ c'° 



K 



according as we treat the systems electrostatically or mag- 

 netostatically. C is a function of the radius of the rings 



