PHOTOMETRY AND ELECTRIC LIGHTING. 339 



equivalent resistance when the lamp fails to operate. This auto- 

 matic cut-out, together with the resistance which it substitutes for 

 the lamp, is sometimes mounted in a case entirely separated from 

 the lamp as shown in Fig. iSS. When the circuit in the lamp is 

 broken, for example by the failure of the carbons to feed, the cur 

 rent ceases to flow and the cut-out magnet releases its armature 

 which establishes a by-pass through the substitute resistance. 



When very many lamps are operated in series the automatic 

 cut-out is a necessity, inasmuch as the very high voltage that is 



Fie. 187. 



used in this case would be likely to force the current through an 

 inoperative lamp, especially if the lamp mechanism includes a 

 shunt winding on its electro-magnet, and the lamp would be de- 

 stroyed. 



141. Constant-current series systems of arc lighting. Where 

 many arc lamps are to be operated continuously, as in the light- 

 ing of city streets, it is best to connect the lamps in series and 

 supply them with constant current. This arrangement is exem- 

 plified by numerous old style direct-current open-arc street-light- 

 ing plants now in operation, and by the more recently installed 

 series systems using enclosed-arc lamps or luminous-arc lamps. 



