SOMP: NKW modes of LIGHTlNCi. 



273 





tainetl by introducinf; n solf-iiuluctioii coil in (he lamp circuit, and 

 shunting across the lamp a (juick-acting oil interrupter. The extra 

 current at break suffices to start the arc. which thus primarily- excited 

 is able to continue, and the self-induction thereafter plays the part 

 merely of reducing variations of the current. Figure -i shows the 

 disposition of circuit adopted by Cooper-Hewitt. 



Usually the tubes have 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 pass between iron and mercury or between two 

 mercury electrodes as desired. In the former case the distance be- 

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

 lamps are exhausted to about the same degree as ordinary incandes- 

 cent electric lights, and they nuiy be run at any current strength 

 between 0.5 and 5 amperes. Below^ 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 82 to 87 volts 

 for U tubes. The effi- 

 ciency of these lam])s 

 is said to be ui)\vard 

 of three times as high 

 as that of the ordinary 

 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 theivfore appears that there 

 are ready means of overcoming 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 Oerman exjjeriments 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 

 tmt by interposing a red mantle, but 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 



Piti. 4.— Different forms of the Men-ury vapor lamp. 



