ELECTROMAGNETS. MAGNETISM OF IRON. 



355 



The total magnetomotive force acting on any magnetic circuit 

 is equal to the sum of the magnetomotive forces of the coils 

 with which the circuit links. 



It is instructive to inspect Figs. 32-39, Chap. II., and note 

 the manner in which the field coils link with the various mag- 

 netic circuits. Thus in Fig. 33 each of the four magnetic circuits 



Fig. 4. 



links with two field coils, and if, for example, 5,000 ampere-turns 

 are required for each magnetic circuit, then 2,500 ampere-turns 

 must be furnished by each field coil. On the other hand an in- 

 spection of Fig. 38 shows that each of the six magnetic circuits 

 links with but one field coil, and if 5,000 ampere-turns are re- 

 quired for each magnetic circuit, then 5,000 ampere-turns must 

 be furnished by each field coil. 



5. Proposition. The magnetomotive force of a coil is given by 

 the equation : 



in which Z is the number of turns of v/ire in the coil and i is the 



*For proof of this equation see Nichols and Franklin's Elements of P/ivsus,Vol. 



II., p 122. 



