88 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



automobile headlight lamp was among the first of these, which in 19 12 

 started the commercial use of electric light on cars in place of oil 

 and acetylene gas. On street railway cars the use of tungsten lamps, 

 made possible on this severe service by the greater sturdiness of the 

 drawn wire, greatly improved their lighting. Furthermore, as the 

 voltage on street railway systems is subject to great changes, the 

 candlepower of the tungsten filament has the advantage of varying 

 but about half as much as that of the carbon lamp on fluctuating 

 voltage. 



Quartz Mercury Vapor Lamp, 1912. 



The mercury arc if enclosed in quartz glass can be operated at 

 much higher temperature and therefore greater efficiency. The light 

 is still deficient in red but gives a considerable amount of ultra 

 violet rays which kill bacteria and are very dangerous to the eye. 

 They can, however, be absorbed by a glass globe. The lamp is not 

 used as an illuminant in this country, but is valuable for use in the 

 purification of water. 



THE QUARTZ MERCURY VAPOR ARC LAMP 



By putting a mercury arc in a tube made of quartz instead of glass, 

 it can be operated at a much higher temperature and thereby obtain 

 a greater efficiency. Such a lamp, however, is still largely deficient 

 in red rays, and it gives out a considerable amount of ultra-violet 

 rays. These ultra-violet rays will kill bacteria and the lamp is being 

 used to a certain extent for such purpose as in the purification of 

 water. These rays are very dangerous to the eyes, but they are ab- 

 sorbed by glass, so as an illuminant, a glass globe must be used on the 

 lamp. These lamps appeared in Europe about 1912 but were never 



