WHAT MAKES THE TROLLEY CAR GO. 



321 



magnetj and tlie rotating part is called the armature, while the 

 device by means of which the direction of the current through the 

 armature coil is reversed is called the commutator. In this last fig- 

 ure it will be noticed that the coils wound upon the field magnet are 

 represented as of wire much finer than that wovmd upon the arma- 

 ture. In actual practice machines are sometimes wound in this 

 way, and sometimes the field wire is twice as large as that on the 

 armature. When the field wire is very much finer than that of 

 the armature the machine is what is known as shunt wound, which 

 means that only a small portion of the current that passed through 

 the armature passes through the field coils. Although with this 

 type of winding the current that passes through the field coils is 

 very weak, the magnetism developed thereby can be made greater 

 than that of the armature if desired. This result is accomplished 

 by increasing the number of turns of wire in the field coils. Thus 

 if the current through the armature is one hundred times as strong 

 as that through the field coils, the latter can be made to equal the 

 effect of the former by increasing the number of turns in the pro- 

 portion of one hundred to one, and if the increase is still greater 

 the field coils will develop the strongest magnetism. The reason 

 why a small current passing around a magnet a great many times 



.if 



Fig. 9. Fig. 10. 



FifiS. 9. 10. — Diagrams ilu'strating the Principles of the Electric Motor. 



will develop as strong a magnetization as a large current, can be 

 readily understood when we say that the magnetism is in propor- 

 tion to the total strength of the electric current that circulates 

 around the magnet. Suppose we have two currents, one of which is 

 one thousand times as strong as the other, then if the weak one is 

 passed through a coil consisting of one thousand turns it will de- 

 velop just as strong a magnetization as the large current passing 



