PHOTOMETRY AND ELECTRIC LIGHTING. 329 



When the lamp switch is closed the current which starts to flow 

 draws in the plunger and the clutch catches the carbon and lifts 

 it, thus forming the arc. As the carbons waste away by com- 

 bustion due to the oxygen of the air, the arc lengthens slightly, 

 the current decreases and the consequent movement of the 

 plunger lowers the upper carbon slowly. When the limit of this 

 downward movement is reached the clutch touches the trigger 

 and allows the carbon to fall when the sudden increase of current 

 pulls the plunger and lifts the carbon again as at the start, and 

 so on. 



The alternating-current arc lamp differs somewhat from the 

 direct-current lamp, as explained later. 



Arc lamps which are operated in series by a constant current, 

 direct or alternating, have a more complicated mechanism than 

 that which is described above. The actuating electromagnet 

 usually has two windings, one of which is in series with the arc 

 and the other is shunted across the arc, and a device called a cut- 

 out is arranged for automatically establishing a by-pass for the 

 current when the carbons are burned out or when the lamp gets 

 out of order. 



The operation of the series arc lamp is briefly as follows : 

 The first rush of current through the lamp flows almost wholly 

 through the series magnet-winding and the tendency of this cur- 

 rent is to separate the carbons more and more ; but, as the 

 carbons separate, a larger and larger current flows through the 

 shunt magnet-winding, which, by opposing the series winding, 

 sets a limit to the length of the arc. Then as the carbons waste 

 away the current in the shunt winding still further increases, 

 more and more nearly neutralizing the series winding and allow- 

 ing the upper carbon to move slowly downwards until finally the 

 clutch which grips the carbon rod is released by a trigger and 

 the carbon falls and is lifted again as at the start, and so on. 

 There is a variety of series arc lamp mechanisms and the above 

 description applies to the arc lamp having what is called a differ- 

 ential actuating magnet. 



