15* 



ELEMENTARY LESSONS ON [CHAP. in. 



rule, then, a N. pole would be urged downwards through 

 the loop, while a S. pole would be urged upwards. In 

 fact the space enclosed by the loop of the circuit behaves 



Fig. 86. 



like a magnetic shell (see Art. 107), having its upper face 

 of S. -seeking magnetism, and its lower face of N. -seeking 

 magnetism. It can be shown in every case that a closed 

 voltaic circuit is equivalent to a magnetic shell whose 

 edges coincide in position with the circuit, the shell being 

 of such a strength that the number of its lines of force is 

 the same as that of the lines of force due to the current 

 in the circuit. The circuit acts on a magnet attracting 

 or repelling it, and being attracted or repelled by it, just 

 exactly as its equivalent magnetic shell would do. Also, 

 the circuit itself, when placed in a magnetic field, experi- 

 ences the same force as its equivalent magnetic shell 

 would do. 



193. Maxwell's Rule. Professor Clerk Maxwell, 

 who devised this method of treating the subject, has 

 given the following elegant rule for determining the 

 mutual action of a circuit and a magnet placed near it. 

 Every portion of the circuit is acted upon by a force 

 urging it in such a direction as ta make it enclose 

 within its embrace the greatest possible number of lines of 



