CHAP, v.] ELECTRICITY AND MAGNETISM. 291 



(i.) Two parallel portions of a circuit attract one 

 another if the currents in them are flowing in the same 

 direction, and repel one another if the currents Jlow in 

 opposite directions. 



This law is true whether the parallel wires be parts 

 of two different circuits or parts of the same 

 circuit. The separate turns of a spiral coil, like 

 Fig. 1 1 6, for example, when traversed by a 

 current attract one another because the current 

 moves in the same direction in adjacent parts of 

 the circuit ; such a coil, therefore, shortens when 

 a current is sent through it. 



(ii.) Two portions of circuits crossing one another 

 obliquely attract one another if both the currents run 

 either towards or from the point of crossing, and repel 

 one another if one rtms to and the other from that 

 point. 



Fig. 119 gives three cases of attraction and two of 

 repulsion that occur in these laws. 



(iii.) When an element of a circuit exerts a force on 

 another element 

 of a circuit, that 

 force always 

 tends to itrge the 

 latter in a direc- 

 tion at right 

 angles to its own 

 direction. Thus, 

 in the case of two 



parallel circuits, ^ -^- 



the force of at- 

 traction or repul- 



. , Fig. 1 19. 



sion acts at right- 

 angles to the currents themselves. 



An example of laws ii. and iii. is afforded by the 

 case shown in Fig. 120. Here two currents db 



