20 THE DIRECT-CURRENT MOTOR CH. II 



CHAPTER II 



CONDITIONS OF UNIFORM MOTION 



WHEN a conducting circuit of R ohms resistance is con- 

 nected to a line having a tension of E volts, the rate of 

 communication of energy to the circuit is measured by 

 the product of the tension of the line into the current 

 flowing ; if this current is c amperes, energy is supplied 

 to the circuit at the rate of cE watts. 



The energy thus supplied is expended in two ways, 

 in heating the circuit and in doing work. The rate of 

 expenditure of energy in the form of heat we know to be 

 equal to the product of the resistance into the square of 

 the current, or c 2 R watts. We now have to find an 

 expression for the rate of expenditure of energy in doing 

 work. 



Let us suppose that part of the conducting circuit is 

 placed near some magnetic field, so that when a current is 

 flowing in the circuit the action of the current on the 

 magnetic field produces a tendency to move the circuit. 

 We will further suppose that the circuit is contrived in 

 such a way that continuous motion is possible. 



The action of the current on the field may be repre- 

 sented by the moment of a force of F pounds acting at a 



