49 



FARM MOTORS 



^nnnnn 



DYNAMO MOTOR 



FIG. 368 WIRING SYSTEM FOR DYNAMO AND MOTOR 



the motor has attained its speed for this point it is 

 moved on up to point two, then three, and so on until 

 the last point is reached and the motor is directly con- 

 nected to the feed wire. To stop the motor, switch A 

 should be opened, and if the arm B is not an automatic 

 shifter, it should be thrown back to its original position 

 ready for starting the next time. Most of these arms are 

 now made so they work against a spring, and when the 

 last point is reached an electromagnet attracts the arm 

 sufficiently to hold it in position ; then when the circuit 

 is broken the magnet loses its attraction for the arm, and 

 the spring draws it back. 



66 1. The electric arc. When a current of from 6 to 

 10 amperes under a pressure of about 45 volts is passed 

 through two rods of carbon with their ends first in con- 

 tact, then gradually drawn apart to a distance of about 

 1/8 inch, a brilliant arc of flame is established between 

 them. This arc, known as the electric arc, is made of 

 a vapor of carbon. As the current passes across the con- 

 tact points the high resistance produces enough heat to 



