78 ELEMENTS OF ELECTRICAL ENGINEERING. 



current-resistance curve a horizontal straight line. The dotted 

 part of the shunt generator curve A in Fig. 67 is very far from 

 being a horizontal straight line, while a large portion of the un- 

 dotted part of the shunt generator curve is not greatly inclined. 

 On the other hand the dotted part of the series generator curve 

 B is not greatly inclined, while the undotted portion is greatly 

 inclined. Therefore, the nearest approximation to an automatic 

 constant current generator would be a shunt generator designed 

 so as to operate normally on the undotted portion of its charac- 

 teristic curve as shown in Figs. 64 and 67, or a series generator 

 designed so as to operate normally on the dotted part of its char- 

 acteristic curve as shown in Figs. 66 and 67. 



As a matter of fact, however, the series generator is always 

 used for constant direct-current supply for the following reason : 

 The voltages used for series constant current arc lighting are 

 high, ranging from 1,000 to 6,000 volts per lamp-circuit. A 

 high voltage shunt generator is expensive to construct because of 

 the great quantity of very fine and very highly insulated wire 

 required for its field winding, whereas the high voltage series 

 generator has a comparatively coarse wire field winding which 

 need not be so thoroughly insulated as the shunt winding. 



Generators which are used for constant current distribution are 

 always provided with automatic devices for keeping the current 



output constant, in spite of 

 changes of resistance of the 

 receiving circuit.* 



42. Connections of a series 

 generator and its receiving 

 circuit. Fig. 68 is a diagram 

 of connections of a series 



Fig. 68. 



generator, some of its acces- 

 sory apparatus, and a circuit of arc lamps. An ammeter A is 



*See chapter on Arc- Lighting Dynamos in S. P. Thompson's "Dynamo Electric 

 Machinery." 



