DIRECT CURRENT DYNAMOS AND MOTORS. 95 
or in symbols: 
where # = terminal voltage of machine; 
d,, = the diameter of the magnet wire, in mils; 
and 1,” = mean length of one turn, in inches. 
By transposition of (48) we obtain the cross-section of 
the magnet wire required: 
in which AT = total magnetizing force required, in am- 
pere-turns; 
l,” = average length of one turn of wire in 
magnets, in inches; 
EK’ = voltage between terminals of shunt 
winding. | 7 
In applying the above formula to a shunt winding for a 
generator, allowance must be made for the resistance of 
the rheostat, which is put in the shunt circuit to regulate 
the E. M. F. This resistance will consume a portion of 
the voltage amounting to from 10 to 20 percent. There- 
fore, the voltage H’ substituted in the formula should be 
10 to 20 per cent. lower than the E. M. F. of the machine. 
For a multipolar machine, H' in the above formula is the 
voltage corresponding to one set of coils. Thus, if all 
the coils are connected 7n serves, and if there are two 
coils in each magnetic circuit, the value of H’ in a four- 
pole machine is one-half the machine-voltage (dimin- 
ished by 10 or 20 per cent. in case of a generator); in a 
six-pole machine, EH’ is one-third; in an eight-pole 
dynamo, one-fourth of the terminal voltage, etc. In 
the case that the field coils are connected an groups 
across the machine terminals, the proper value of i" is 
found by multiplying the number of coils constituting 
one magnetic circuit by the number of parallel groups, 
and dividing the product so obtained by the total number 
of coils on the machine; the result is the factor by which 
