106 ELECTRICAL MACHINERY 



of resistance can be varied by a movable shoe (carried on 

 an arm that rotates) which makes contact with any one of 

 many taps on the resistance. This adjustable resistance is 

 called a field rheostat. A diagram 

 of the connections of the movable 

 contact and the taps of the resistance 

 is shown in Fig. 63. Figs. 64 and 65 

 show the external appearance of the 

 rheostat, the connections of which are 

 given in Fig. 63. The cast-iron plate 

 Rheostat serves as a mechanical support for 



FiG.62.-Connectionof the enamel and resistance wire, and 

 Field Rheostat. also serves to radiate the heat gener- 

 ated in the resistance wire. 



Tapering a Rheostat. The size of wire used in a field 

 rheostat is " tapered; " the wire is much larger on one 

 end than on the other. If a rheostat is connected in 

 series with a shunt field (as in Fig. 62) and the amount 

 of resistance in the rheostat is varied evidently the cur- 



O 



Terminals 



FIG. 63. Inside Connections of a Field Rheostat. 



rent through the rheostat and field winding will vary. 

 As the rheostat is " cut out " (i.e., as its resistance is 

 lowered) the current in the circuit increases; for this 



