40 THK DIRECT-CniHKXT MOTOR < H. II 



load did thus vary, the current would diminish from I>H 

 to nothing, and the speed would increase from nothing to 

 DO. We here assume that the normal resistance of the 

 motor is increased by inserting a starting rheostat in the 

 circuit, so that the current drawn from the line when the 

 motor is at rest is not greater than the maximum safe 

 current. If this rheostat were left in the circuit, the motor 

 would attain a speed given by Equation 13, where 7? is the 

 resistance of the motor and the starting rheostat. In 

 practice, when a sufficient speed has been attained, and 

 the current reduced, the starting rheostat is taken out, 

 until finally the resistance of the motor only is left in the 

 circuit. Our diagram will now serve to show the variations 

 of the speed with the current, but only a small portion of 

 the speed curve will be used, and it will be inclined to the 

 horizontal at an angle whose tangent is equal to the 

 resistance of the motor only. 



The general equation of energy involves three terms : 

 1. The term c", expressing the energy per second 

 from tl-3 line if c is negative, and into the line if c is 

 positive, the former being the case of the motor, and the 

 latter that of the generator. 2. The term ce, expressing 

 the product of the current and the induced tension, and 

 hence the work done. 3. The term c*R, giving the rate 

 of expenditure of energy in the form of heat. 



We have (a) the case of the motor when c is negative ; 

 current is passing from the line, and the induced tension is 

 opposed to that of the line. (/3) c positive ; the case of the 

 generator when c is positive ; the direction of rotation and the 

 induced tension are the same as in the motor, but the sign of 

 the current is now changed, since the current is passing into 

 the line, (y) c nsgative, but with the sign of e changed ; 



