240 



OUR PHYSICAL WORLD 



station. All sizes of motors can be built, from those of a frac- 

 tion of one horse-power to those of thousands of horse-power. 

 They can, therefore, be used for the most delicate operations, 

 such as running the dentist's drill, as well as for tasks requir- 

 ing tremendous power, such as the operation of electric trains 

 (Fig. 104). The power is generated so as to produce continuous 

 rotation rather than a back-and-forth motion which, as in the 

 piston rod of an engine, must be transformed into rotation at a 

 considerable loss of energy. Therefore, very high rotary speeds 



FIG. 104. Electric locomotive, weight 265 tons, used on the Chicago, Mil- 

 waukee, and St. Paul Railway over the Cascade Mountains. (Courtesy of the 

 Chicago, Milwaukee & St. Paul Railway.) 



may be achieved. The motor of a rapidly running automobile 

 makes a thousand or more revolutions per minute, while motors 

 built especially for speed, such as those used on drills, centrifugal 

 pumps, etc., may run at rates ten, twenty, or more times as 

 great. 



Faraday, it will be recalled, reasoned that if a current moving 

 in a wire will cause motion in a nearby magnetic needle, then a 

 moving magnet should produce a current in a nearby wire. He 

 verified this hypothesis by introducing a bar magnet into a coil 



