468 FARM MOTORS 



directly proportional to the E.M.F. in the circuit and in- 

 versely proportional to the resistance. 



Hence if 



Likewise, 



C := current, 

 E = E.M.F., 

 R = resistance, 



C = 5. or current = E ' M - R . 

 R Resistance 



Amperes 



Volts 

 Ohms 



620. Rheostats. The common method for controlling 

 the current required for various electrical purposes is 

 either to insert or to remove resistance. By Ohm's law 



C = | (A) 



If E is kept constant and R is varied, C will also be 

 varied but with an inverse ratio. Any instrument which 

 will change the resistance in a circuit without breaking 

 it is known as a rheostat. A rheostat can be constructed 



FIG. 348 PRINCIPLE OF RHEOSTAT FIG. 349 COMMERCIAL RHEOSTAT 



of various substances : coils of iron wire, iron plates or 

 strips, carbon, columns of liquids, etc. Fig. 348 illus- 

 trates a commercial rheostat. The current enters at A< 



