OPERATION OF THE DYNAMO AS A GENERATOR. 89 



Manipulation of the field rheostat is of course necessary in case 

 of a shunt generator, when it is desired to keep the terminal volt- 

 age constant with varying current output. In case of the flat- 

 compound generator, also, the terminal voltage always varies to 

 some extent with variation of current output, and the compound 

 generator is therefore always provided with a field rheostat in its 

 shunt field circuit so as to enable an attendant to control its 

 terminal voltage at will. 



The usual form of field rheostat consists of coils or zigzags of 

 wire or metal ribbon connected in series, and arranged with a 

 number of contact points over which a metal arm may be moved, 

 so as to include any desired portion of the rheostat in series with 

 the field winding. The insulation of the rheostat is generally 

 made of fireproof material, such as mica or asbestos. Special 

 provision is made for cooling, either by ventilating or by great 

 expanse of radiating surface. 



There is a wide variety in the design of commercial field rheo- 

 stats. Fig. 70 is a general view of a field rheostat manufactured 

 by the Wirt Electric Company. Part of the front plate is cut 

 away to show the resistance strips, EE E. This resistance strip 

 is a long ribbon of high resistance alloy, coated on one side 

 with fireproof insulation, such as flake mica, and wound in a long 

 helix on a mandril of flat-oval section. This flat helix is then 

 slipped off the mandril, and arranged in a circle between two flat 

 circular plates, MJ t and insulated from the plates by sheets of 

 mica, F G. The two plates are then clamped together with con- 

 siderable pressure, and a steel band N is spun over the outer 

 edge of the plates to hold them together. The metal plates are 

 deeply ribbed, front and back, to facilitate the radiation of heat. 

 One end of the resistance helix is connected to one of the ter- 

 minal binding posts, shown at the bottom of the figure, and the 

 other binding post connects to the rotating metal arm C which 

 makes contact with the inner ends of the convolutions of the flat 

 helix. The rheostat is connected in series with the shunt field 

 winding of the generator by wires attached to the rheostat binding 



