24 THE DIRECT-CURRENT MOTOR CH. II 



a tension tending to send a current in a direction 

 opposing the motion. 



In the case with which we shall be concerned chiefly, 

 when the dynamo is used as a motor, the current in the 

 armature does not flow in the direction of the induced 

 tension, so that it is important to remember that a tension 

 is induced in virtue of the motion of the conductors in 

 the magnetic field, even though the current that actually 

 flows is not in the direction of this induced tension. 



We have stated that energy is communicated to a 

 motor in two ways as heat and as work We must 

 now inquire more precisely into the distinction here as- 

 sumed. We know that energy in any form can be expressed 

 as the product of a force into a distance. Thus, if a 

 body weighing ten pounds is raised through a distance of 

 five feet, the energy communicated to the body is fifty 

 foot-pounds. If the body is raised through this distance 

 in one second, the rate of communication of energy is fifty 

 foot-pounds per second, or 68 watts. This would be the 

 rate of communication of energy to an incandescent lamp 

 taking O68 ampere at 100 volts, but in this case the 

 energy would be communicated in the form of heat. There 

 is this important difference : in the case of the weight the 

 energy is expended in overcoming a force acting in a 

 definite direction, whereas in the lamp there is no such 

 force to be overcome. 



When energy is spent in overcoming a resisting force 

 we say that work is being done. Thus we say that we are 

 doing work when we are raising the weight, because we 

 are overcoming a resisting force. With the lamp, how- 

 ever, we cannot say that we are overcoming any resisting 

 force ; it is true that we are communicating energy, but we 



