260 THE DIRECT-CURRENT MOTOR CH. XII 



while this method gives us the form of the magneti- 

 sation curve, we must make a separate experiment to tind 

 its area in terms of lines of force. 



The area of each polar portion of this curve represents 

 N, the total useful lines of force per pole, passing through 

 the armature. If we observe the speed and the total 

 induced tension between a and b as measured in the usual 

 way, we can find N from Equation 8. 



The only variable in the expression for the induction 

 factor is the term N, and this is represented by the 

 area of the magnetisation curve ; it follows that any 

 change in the area of this curve involves a change in the 

 value of the induction factor. We shall now discuss the 

 influences tending to alter the area of this curve. 



The intensity of magnetisation at any point h produced 

 by a wire say at A:, in Fig. 64, can be found by considering 

 the lines of force in the magnetic circuit passing through 

 h, due to i amperes at A 1 . The lines of force passing 

 through one square centimetre at h will circulate in some 

 path round A 1 , the form of which need not concern us if we 

 assume that the greater part of its reluctance consists 

 of the air gap which is crossed twice ; if for the present 

 we neglect all other reluctance but this, we can write 

 down the lines per square centimetre at h thus : 



where B is the width of the gap in centimetres, and i is 

 the number of amperes in the wire at A - . 



This equation shows that the intensity due to i amperes 

 at the point A 1 is uniform between k and the tip of the 

 pole, that the intensity on one side of the wire is equal but 



