434 



CURRENTS AND MAGNETIC SHELLS. 



If there were no other forces in the field than those arising from 

 these two circuits, the relative energy of the two currents is 



W= -PM. 



As the energy tends towards a minimum, the movable part would 

 displace itself, so that the flow of force M is a maximum. 



\ 



Fig. 100. 



If the two contours s and s' are plane, it is clear that the movable 

 part s would place itself in the plane s 1 so as to form its prolongation : 

 it is seen by the action which the two equivalent magnetic shells 

 would exert on each other. 



If the general circuit consists of a flexible wire of a given length, 

 the action of a current on itself would tend to give it the greatest 

 surface that is to say, to make it take the form of a circumference 

 of a circle. 



If the wire is elastic it will elongate .until the elasticity balances 

 the electromagnetic forces. 



456. FARADAY'S EXPERIMENTS. In certain cases the funda- 

 mental formula 



W= -IQ 



appears not to hold, as continuous movements can be produced, even 

 when the flow of force through the system is zero or is invariable. 



Let us consider, for instance, an arc ACB of a plane curve 

 (Fig. 10 1) movable about a straight line AB, passing through the 

 axis of a magnet PP', one of the ends being placed between the two 



