14 THE DIRECT-CURRENT MOTOR CH. I 



representing as before the number of polar divisions of the 

 armature thus connected in series, we may write : 



e=pAnNlO~ (8). 



We have assumed that the conductors are spaced evenly 

 round the armature. A similar proof would hold good if 

 the conductors lay two or more deep, with equal spaces 

 between those in each layer. 



If in Equation 8 we insert M in place of pAN10~*, 



we get 



e=Mn (9). 



From this we see that the induced tension in a dynamo 

 is given by multiplying the induction factor by the number 

 of revolutions per second. The induction factor may thus 

 be defined as the induced volts divided by the number of 

 revolutions per second. 



We must not however suppose that because M is thus 

 defined it depends on the motion of the armature. Equa- 

 tion 6 shows that M depends only upon p, A, and N, and 

 does not in any way involve the speed. 



We have here a second method for finding the 

 value of the induction factor namely, to drive the 

 dynamo as a generator, and observe the induced tension at 

 the terminals of the machine, and the revolutions per second. 



In making this test we must be careful to insure that 

 the tension recorded is the true induced tension, expressed 

 by Equation 7. The voltmeter will only indicate this 

 tension provided that the brushes are placed precisely at 

 the neutral points, for in this position only will all the 

 conductors between two brushes be generating a tension 

 of like sign. Any forward or backward lead will place two 

 sets of conductors with opposing tensions between two 



