22 THE DIRECT-CURRENT MOTOI! (Ml. II 



current in the circuit and the induction factor. If the 

 motion is in the direction of the force produced by 

 the current, work is being done by the current; if the motion 

 is in the contrary direction, work is being done on instead 

 of by the moving system. In both cases the rate of 

 working is given by the product of the current and the 

 induced tension. 



Example 11. A dynamo with an induction factor of 

 4 runs at 1,500 revolutions per minute, and has a curivnt 

 of 70 amperes passing in the armature. The induced 

 tension is 100 volts, so that the rate of working is 7.000 

 watts. Nothing is here stated as to whether the dynamo is 

 acting as a motor and doing work, or acting as a generator 

 and having work done on it ; it is sufficient to know that 

 a current of 70 amperes is passing, and that the induced 

 tension is 100 volts, in order to ascertain the rate of 

 working, positive or negative. 



We have now obtained expressions for the rate of 

 expenditure of energy in our circuit in the two possible 

 ways, as heat and as work. The Principle of the 

 Conservation of Energy tells us that the energy supplied 

 to the circuit, per second, must be equal to the energy 

 spent in heating together with that spent in doing work. 

 We are thus able to write down the equation of energy for 



the circuit thus : 



.................. (12). 



This is the energy equation when the line is the source of 

 energy, the current is then doing work and the dynamo is 

 acting as a motor. 



If no work is being done, the induced tension is 



El 



nothing since there is no motion, and consequently c= _1. 



H 



