432 



ALTER \.\ TI\<; CURRENTS 



by the negative electrode and are attracted by the positive 

 electrode. Therefore, the arc stream moves from negative to 

 positive. The copper must always be positive. If it becomes 

 negative, the arc will then consist of luminescent copper-vapor 

 and be green in color. In connecting a lamp in circuit, care must 

 be taken to insure correct polarity. 



As the electrodes are comparatively cool under operating 

 conditions, there is not sufficient heat to maintain the arc if the 

 electric power is interrupted even for an instant. Hence, out- 



-4 Series Magnet 



Copper Contact 

 Cut out 

 Carbon Contact 



*i Shunt Magnet 



FIG. 389. Mechanism and connections of luminous arc lamp, series type. 



side the question of the greenish arc resulting from the copper op- 

 erating as cathode, this type of lamp cannot -be used with 

 alternating current. 



The feed mechanism is slightly different in principle from 

 that of other types of arc lamp. Were the ordinary type of 

 feed used, the hot metal of the cathode or negative electrode 

 would weld to the copper and "freeze" when the current was 

 turned off. Therefore, the mechanism is so designed that the 

 feed is intermittent, the arc being maintained by re-striking. 

 When the lamp is out of circuit its electrodes are separated by 



