200 APPLIED SCIENCE 



temperature rise of any part of the motor not exceeding 

 70 C. A car motor usually has its four poles covered so 

 as to be waterproof. It transmits power by means of a 

 single reduction gear. The motor is suspended at one end 

 upon the car axle, and the spring is suspended at the other 

 end. 



243. Resistance Box. A device to resist or check the 

 flow of current is commercially called a resistance box. It 

 generally consists of an insulated wire, wound in a spool, 

 the ends or terminals of which are fastened to large brass 

 blocks. If the spools contain a large amount of silk, mois- 

 ture tends to accumulate and cause inaccuracy. The plugs 

 should be cleaned with coarse paper. 



244. Rheostat. A rheostat consists of a number of 

 coils of wire connected in series for the purpose of introducing 

 resistance into the circuit. An adjustable device allows the 

 resistance to be varied by cutting out as many of the coils 

 as is desired. 



245. Starting Box and Controller. A starting box is a 

 rheostat used to cut down the voltage in the line, when start- 

 ing a motor. The current should flow through it only while 

 the motor is attaining its normal speed, the resistance being 

 decreased as the speed of the motor increases. 



A controller is a rheostat used in connection with a motor 

 to cut down the voltage, and thereby to control the speed of 

 the motor. It differs from the starting box in that it is in- 

 tended for continuous service. 



246. Efficiency of Dynamo. The efficiency of a dynamo 

 is the quotient obtained by dividing the amount of electric 



