SOME NKW MODES OE I.IC I I'll N< i 



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Fui. 4.— Different forms of the Mercury vapor laiiip. 



^^ 



taincd hy iiili'odiiciiiijf a sclf-iiuliictioii coil in the laiuj) cii-ciiil. iiml 

 sliuntiiii:" across the hinip a (luick-actini; oil iii(i'rni])t('i'. The cxti'a 

 cnriviit at break sulliccs to stai't (he ai-c. wliicli lliiis i)riinarilv cxcitcH] 

 is al)l(' to coiiliiiiH', and the sclf-iiidiiclioii (Iicrcartcr plays tlu' pari 

 iuorcly of rcducini/; variations ol' the ciiriH-iil. Fi<rurc ■) shows tlu^ 

 disposition of circnit adopted by C'ooper-IIcwitt. 



Usually the tubes ha\'e the form of an inverted U, with short arms. 

 Each arm contains mercury, and at the top is an iron electrode. 

 Thus the arc may i)ass between iron and merc^ury or between two 

 mercury electrodes as desiivd. Tn (he foi'iner case the distance be- 

 tween electrodes is about 7 cm. and in the latter about 9 cm. These 

 hinii^s are exhausted to about the same deoree as ordinary incandes- 

 cent electric lights, and they may be run at any current s(i-eiiiitli 

 between 0.5 and 5 amperes. IJelow 0.5 amperes the arc ceases, and 

 above 5 amperes the tube breaks, owing to the heat produced. The 

 difference of potential 

 between terminals va- 

 ries from 8*2 to 87 volts 

 for U tubes. The effi- 

 ciency of these lamps 

 is said to be upward 

 of three times as high 

 as that of the ordinai-y 

 arc. 



Other methods of 

 starting the mercury 

 arc have been proposed 

 by various physicists, 

 and one of the most 

 simple, devised by Doc- 

 tor Weintraub, involving the use of a small auxiliary arc, avoids re- 

 course to a high voltage at starting. It therefore appears that there 

 are ready means of oAercoming this primary obstacle to the use of the 

 mercury arc; but a second and very serious objection lies in the green 

 color of the light produced. Being wholly without red rays, this 

 light gives a ghastly appearance to the objects illuminated. On the 

 other hand, according to (lerman ex])eriments the light has a great 

 power of penetration, and it is besides highly actinic, and thus suit- 

 able for photographic purposes, and, moreover, the color is not 

 fatiguing to the eyes. It has been proposed to introduce a warmer 

 tint l)y interposing a red mantle, l)ut this device greatly diminishes 

 the efficiency. Perhaps better results may be reached by introducing 

 traces of certain neutral gases, or substituting some amalgam instead 

 of pure mercury. 

 SM 1904 18 



