DIRECT-CURRENT DYNAMOS AND MOTORS. 
45. 
where M = Mass of iron in armature core, in cubic inches; 
D, = diameter of armature core, in inches; for a 
toothed armature, D, is the diameter at the 
root of the teeth; 
B, = radial thickness of arene core, in inches; 
L,, = length of armature core, in inches. 
In toothed armatures the hysteresis loss in the teeth, calcu- 
lated by (29), in which the mass M is then that of the 
teeth, must be added to the hysteresis loss in the core. 
The hysteresis factor 7 depends upon the magnetic density; 
its numerical value for any density is obtained in ergs 
per cubic centimetre by multiplying the 1.6th power of 
this density (expressed in lines per square centimetre) by 
the hysteresis constant or coefficient of hysteresis for 
the respective material; see Par. 262, Book 13. Prac- 
tical values of 7, in watts per cubic inch, for various flux 
densities in sheet tron, are given in Table 19. 
Power Lost by Eddy Currents.—The power con- 
sumed in setting up induced eddy currents in a body of 
iron subjected to reversing or varying magnetization 
increases with the square of the frequency and i is propor- 
tional to the mass of the iron: 
where w. = eady current loss, in watts; 
é eddy current factor, or eddy current loss in 1 
cubic inch of iron at a frequency of 1 cycle 
per second, see Table 20; 
f = frequency, in cycles per second, see formula 
(30); 
M = mass of iron, incubic inches, see formula ($1). 
The eddy current factor, ¢, increases with the square of the 
flux density and with the square of the thickness of the 
sheets employed in building up the iron body. Table 
20 gives the values of ¢ in watts per cubic inch at unit 
frequency for flux densities of from 10,000 to 145,000 
