ENERGY AND ITS RELATION TO USE OF MACHINES 551 



that the horse can also convert about 20 per cent, of the 

 energy in its food into work (see Art. 658). A steam engine 

 will convert from 4 to 10 per cent, of the energy in coal into 

 work; therefore it has from 4 to 10 per cent, efficiency. A 

 few years ago in a series of tests made by the Northern 

 Pacific Railroad it was found that the efficiency of its best 

 freight locomotives was but 3.8 per cent. Good gas and 

 gasoline engines have* an efficiency of from 20 to 35 per cent. 

 Electric motors frequently develop an efficiency of 75 to 

 90 per cent. From these figures one might conclude that 

 the electric motor was the least expensive motor and that the 

 steam engine was the most expensive motor for us to employ 

 to do our work. Such a conclusion, however, is hasty and 

 incorrect. The fact is that the steam engine is one of the 

 least expensive motors to operate while the electric motor is 

 rather expensive to operate. Can you suggest any reason 

 why this should be so? Before we finish this chapter we 

 shall see what the explanation really is. 



654. The Work Equivalent of a Calorie of Heat. Many 

 experiments have been made to determine the amount of 

 work which is equivalent to a calorie of heat. In 1840, an 

 Englishman, Joule, made such a determination. He sus- 

 pended masses of iron by means of cords in such a manner 

 that they might slowly fall and in so doing revolve a set of 

 paddles immersed in a vessel of water. The paddles, in stirring 

 the water, produced friction which produced heat and therefore 

 raised the temperature of the water. He noted the weight 

 of the iron masses and the distance through which they fell. 

 From these figures he determined the amount of work done. 

 He also noted the weight of the water and the number of 

 degrees of temperature through which it rose. From these 

 figures, he determined the number of calories of heat produced. 

 Other experimenters have used other methods of determining 

 this relation. It is now known that 1 greater calorie (1 Cal.J 

 (Art. 121) is equal to 3080 ft.-lb. (Art. 637) of work. This 

 means that, if all the energy in food or fuel could be converted 



