428 



ALTERNATING CURRENTS 



The open arc, especially the direct-current type, is used exten- 

 sively for projection purposes, as for stereopticans, moving- 

 picture machines, search lights, etc. The high intensity of the 

 arc, combined with its being concentrated, makes it very desir- 

 able for these purposes. The foregoing types of arc lamps 

 are practically obsolete as far as general illumination is con- 

 cerned, although they are used for street lighting in isolated 

 instances. 



194. Arc Regulation. Arc lamps must be automatically 

 regulated both in starting and in operation. In starting, the 

 carbons must be in contact and then be drawn apart as the arc 

 is formed. The carbons must also be fed together as they are 

 consumed. 



Arc lamps are divided into two 

 general classes, the series lamp and 

 the parallel or multiple lamp. 



In the series lamp, the current is 

 maintained constant by the generat- 

 ing apparatus and this type of lamp 

 requires no ballast. As the current in 

 the series lamp is maintained constant 

 under all conditions, the arc regulation 

 is accomplished primarily by main- 

 taining the proper voltage across the 

 arc. The general scheme is shown in 

 Fig. 385. The coil Si is a series coil 

 of low resistance connected in series 

 with the arc and S% is a shunt coil of 

 high resistance connected across the 

 arc. Si and S 2 operate differentially 

 on the lever L which actuates the 

 clutch. Si tends to lift the clutch 

 and pull the carbons apart and Sz 

 tends to lower the clutch and allow the 

 carbons to come together. Should the arc become too long, the 

 voltage across it rises and strengthens the shunt solenoid $2. 

 This tends to feed down the upper carbon. 



A cut-out switch is necessary to keep the circuit closed should 

 the lamp become open-circuited. When this occurs a high 



Cut-out Switch 



FIG. 385. Mechanism and 

 connections of series arc 

 lamp. 



