- DIRECT-CURRENT DYNAMOS AND MOTORS. 79 
by casting or attaching polepieces to the cores, as in Fig. 
26, the latter should have a circular section. 
CALCULATION OF MAGNETIZING FORCE. 
62. Total Magnetizing Force.—The number of ampere- 
turns required to produce the necessary flux through the 
magnetic circuit is the sum of the ampere-turns required 
to overcome the reluctances of the various parts of the 
circuit and of the ampere-turns needed to counteract the 
effect of the armature reaction on the magnetic fied. 
The formula for the totai magnetizing force, therefore, is: 
= at, -+ alg te Obey FH Gby. west esc: (3'7) 
in which AT = total number of ampere-turns required; 
at, = ampere-turns necessary to overcome the 
reluctance of the air gaps; 
at, = ampeve-turns to overcome reluctance of 
armature core; , 
at, = ampere-turns to overcome reluctance of 
magnet frame; 
at, = ampere-turns necessary to compensate for 
armature reaction (Book 15, Pars. 
56 to 65). 
63. Magnetizing Force Required for any Part of a 
Circuit.—Each of the three terms af,, at, and at, in 
the above formula (37) can be obtained from the law of 
the magnetic circuit stated in Par. 35, Book 15: 
Fluc = Magnetomotive Force 
Reluctance 
which can also be written in the form: 
Magnetomotive Force = Flux x Reluctance. .(38) 
According to Par. 32, Book 15, the recipiodal of re- 
luctance is called permeance, hence © 
