CH. XIII] DYNAMO FOR ARC LAMPS 47 



decrease in current will be met by an increase in the voltage 

 generated. 



679. Shunt generator. The connections for a shunt genera- 

 tor or dynamo are shown diagrammatically in fig. 257. A is the 

 revolving armature from which the current is drawn. N and S are 

 the poles of the field magnet and F is the field coil which keeps it 

 strongly magnetized. The stronger the magnetization of this field 

 magnet the higher the voltage furnished by the machine. As 

 usually operated the field rheostat R must be continually adjusted 

 so that the right current is supplied to the field coil F to keep the 

 machine at the desired voltage. 



680. Adaptability of a shunt generator for direct connection 

 to an arc lamp. If instead of continually adjusting the rehostat R 

 so that the dynamo will supply a constant potential, the machine 

 is left to itself it will be found that when no current is supplied, i. e., 

 the dynamo is running on no load, the potential difference between 

 the terminals a and b is greatest and consequently the current 

 flowing in the field coil F is greatest. If now current is drawn from 

 the dynamo the potential difference between a and b will drop 

 slightly. This will result in a decrease in the current flowing in the 

 field coil F, a decrease in the magnetization of the field magnets and 

 hence a decrease in the voltage generated. The result is in the 

 direction desired, namely, that an increase in the current will be 

 met by a decrease in the voltage. 



Whether or not a shunt generator connected directly to an arc will 

 work satisfactorily, or whether the arc will be unstable and want to 

 either "run away" or "die out" will depend upon the details of the 

 design of the dynamo ; that is, the voltage at no load, the resistance 

 of the shunt field coils and the resistance of the armature and also 

 on the resistance of the wiring to the arc. Some dynamos have 

 been designed which will work satisfactorily when connected 

 directly to the arc without any intervening resistance. Such 

 dynamos may be run directly by some form of engine or they may 

 be part of a motor-generator set in which high voltage, direct 

 current or alternating current is used to furnish the power. (See 

 also 682, 684). 



