DIRECT-CURRENT DYNAMOS AND MOTORS. 33 
2%. Arrangement of Inductors on Smooth-Core 
Armature.—The number of inductors found by for- 
mula (16) must be modified according to the number of 
divisions and to the manner of winding of the armature. 
The number of divisions of a smooth-core armature is 
either equal to, or twice, the number of commutator 
segments, the latter depending upon the voltage and on 
the number of poles of the machine. For machines up 
to 300 volts, the commutator is usually divided into from 
40 to 60 segments per pairof poles. For higher potentials, 
a correspondingly greater number of segments is provided; 
thus, for 500 volts up to 100 segments, and for 7,000 volts 
up to 150 segments, per pair of poles are used. 
In a smootk armature, the number of wires per layer must 
be a whole multiple of the number of armature divisions; 
to find the final number of wires per layer, therefore, 
multiply the circumference of the armature (diminished 
by the space allowed for division strips) by the number 
of wires per inch (column 7, Table 13); divide the result 
by the number of commutator segments, and note the quo- 
tient so obtained. If this quotient is not quite a whole 
number, but so nearly so, that the deficiency can be made 
up by a moderate reduction of the allovrance made for 
division strips, the number per layer is the product of the 
raised quotient by the number of commutator segments. 
If the quotient is but little greater than a whole number, 
the number of wires per layer is the next smaller whole 
number multiplied by the number of commutator seg- 
ments. If the quotient is not sufficiently near a whole 
number to allow one or the other of the above methods 
to be employed, either the next smaller or the next 
larger whole number must be taken and the number of 
commutator segments so adjusted that the product of the 
two numbers is equal to, or slightly less than, the num- 
ber of wires previously obtained by multiplying the 
available circumference by the number of wires per inch. 
The corrected number of wires per layer is then multiplied 
by the number of layers, which, in turn, is found by mul- 
