262 SCIENCE OF HOME AND COMMUNITY 



it might injure the motor. In order to avoid this, resistance 

 coils are connected with the motor, so that when the handle 

 is turned to the first notch, the current passes through both 

 the coils and the motor, and thus the motor at first receives 

 only a portion of the full strength of the current. As the 

 handle is turned still further around, these resistance coils 

 are cut out, till finally the motor receives the full strength 

 of current and runs at full speed. 



Circuit breakers. In order to protect the parts of the 

 machinery on the car from being injured by too strong a 

 current, circuit breakers and fuses are provided. These 

 circuit breakers are so arranged that when too strong a 

 current passes through them they automatically open the 

 circuit. Sometimes when riding on the trolleys, one hears 

 a noise like an explosion which is due to the working of this 

 circuit breaker. If you watch the motorman, you will see 

 that he reaches up and turns back a handle connected with 

 the circuit breaker, thus making a connection again. 



Fuse. A fuse serves the same purpose. This is com- 

 posed of certain metals which melt when a strong current 

 passes through them, thus opening the circuit and preventing 

 the current from passing through the remaining portions 

 of the machinery. 



Motor. The machine which actually makes the wheels 

 of the car turn is the motor, which is placed under the car 

 on the axles. The motor is constructed the same as a 

 dynamo ; that is, it is a reversible machine. If it is turned 

 by some outside power, it generates electricity ; if electricity 

 is conducted through it, the parts begin to revolve. So the 

 same machine can be used for either a dynamo or motor. 

 As a matter of efficiency, however, it is found better to make 

 machines intended for motors somewhat different from those 

 intended for dynamos. 



The motor has the same essential parts as the dynamo, 

 that is, an armature and two magnets. When two magnets 



