26 THE DIRECT-CURRENT MOTOR OH. II 



This equation is true for all forms of communication of 

 energy, as heat or as work. When we can measure t we 

 are doing work, but when the energy is all going in the 

 form of heat, t cannot be measured. In the case of the 

 68-watt lamp, all that we know is that the product of a 

 torque of t inch-pounds into a speed of n revolutions per 

 second is 96 ; if we knew t, we could deduce ?i, and vice 

 versa. 



The molecular movements constituting heat cannot 

 therefore assist or resist motion ; if we could marshal all 

 the molecules, assuming that we had command of 

 sufficiently delicate mechanism, and oblige them to move 

 in the same direction, we could then make them resist or 

 assist motion ; but if we could do this, we could also apply 

 a force to them and stop their motion completely, thus 

 taking all the heat out of a body and making it perfectly 

 cold. Our physical inability to accomplish this result is 

 stated in what is called the Second Law of Thermodynamics. 

 (See Professor Clerk Maxwell's ' Theory of Heat.') 



In considering the energy supplied to an electric motor 

 we require to know 'precisely how much of the energy is 

 spent in overcoming resistance to motion. It is not 

 sufficient to know that the energy used in overcoming 

 friction, for instance, is eventually dissipated in heat, 

 we must know whether friction offers a resistance to 

 motion. Similarly, we must not consider the energy used 

 in heating the armature as spent in overcoming resistance 

 to motion. 



In the two cases here selected it is not difficult to see 

 which should be classed as heat and which as work. But 

 cases may arise when it is not easy to effect the right 

 classification. For instance, we know that when a mass of 



