Governing of Electromotors. 67 



If n is to be constant as well as V, the left-hand side of the 

 equation is constant, whereas the right-hand side varies with 

 A the current through the armature, which must alter as the 

 power given out by the motor alters. One solution for 

 making this equation always true independent of the value of 

 A is the well-known one of making a very small. When, 

 however, a is not very small, and we have shown in detail 

 elsewhere why we consider it a mistake to make the resistance 

 of the armature as small in a motor as in a dynamo, equation 

 (10) cannot be fulfilled for constant values of n and V, and 

 a variable value of A ; and in order that the equation may be 

 made true a negative term proportional to A must be intro- 

 duced on the left-hand side. The simplest way of doing this 

 is to add a series demagnetizing-coil. Then (9) becomes 



E = ;i (p — qA + tZ), 

 and (10) 



n (p — q A + t~\=V—A(a + s), 



an equation which will be always satisfied for any value of 

 A if 



a -\-s 



But when the supply is a constant current C supplied to the 

 motor, the equation for a simple shunt-motor is 



E = n (p + tZ), 



E = Zz-Aa, 



C=A+Z; 



and, therefore, 



n{p + t(C-A)\=(C-A)z-Aa. . . . (11) 



A now T appears with a negative sign on both sides of the 

 equation (11), and this equation will always be satisfied for 

 any value of A if 



2 + a /ION 



without there being any necessity for using a series demagne- 

 tizing-coil. At first sight, then, it appears as if an entirely new 

 solution for governing a motor supplied with a constant 

 current had been arrived at, consi>ting simply of using a 

 shunt-motor. But on further examination it is seen that if 

 we substitute in (11) the value given in (12) for n, we obtain 

 for C the current for which the motor governs : 



F 2 



