ILLUMINATION AND PHOTOMETRY 



427 



It will 1)0 noted that most of tho light is thrown downward. The 

 jxKsitive carbon is consumed more rapidly than the negative and 

 so requires more frequent renewals. 



If alternating rurrent ho substituted for direct current, it will 

 be found that both carbons consume equally and have the same 

 shu|x of crater, as is indicated in Fig. 384. Hence, as much 

 light is directed upward as downward, as shown by the light - 

 distribution curve. Such a light needs, therefore, a reflector to 

 intercept this upwardly directed light and reflect it downwards 

 where it can be effectively used. The alternating-current 

 multiple arc has one advantage over the direct-current arc in 



A.C. Supply 



oooooooooo x 



Inductive Ballast 

 Fio. 384. AlternatiiiK-r-unvnt arc and candlepower (li>tril>uti<>n rurv,-. 



that it has an inductive ballast which consumes no appreciable 

 power, although it does lower the power-factor. 



193. The Enclosed Arc. The next improvement in arc 

 lighting was to enclose the arc in a small opalescent globe. 

 Thi> enclosing globe prevent- free access of the oxygen of the air 

 to the arc and so reduces the carbon consumption and therefore 

 the number of trims. Tho opalescent globe cuts off con 

 able linht and so reduces the lamp efficiency, but the total cost 

 of illumination is reduced because of the lesser labor charge inci- 

 dent to trimming. The enclosed arc will burn 100 hours per 

 trim when- the <>p<>n arc burn- but S hours. Tin- enclosed arc is 

 also steadier than the open arc. gives a more diffused light and so 

 reduces the object ioiiat.l if the open arc. 



