With drawn tungsten wire it was easier to coil and therefore concentrate the filament as required by focusing types of lamps. The automobile headlight lamp was among the first of these, which in 1912 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. [Illustration: 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 absorbed 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 used to any extent in this country as an illuminant. They have an efficiency of about 26 lumens per watt. Quartz is very difficult to work, so the cost of a quartz tube is very great. The ordinary bunsen gas flame is used with glass, but quartz will only become soft in an oxy-hydrogen or oxy-acetylene flame. [Illustration: GAS FILLED TUNGSTEN LAMP, 1913. By operating a coiled filament in an inert gas, Dr. Langmuir was able to greatly increase its efficiency, the gain in light by the higher temperature permissible, more than offsetting the loss of heat by convection of the gas. This lamp is in the exhibit of Edison lamps in the Smithsonian Institution.] THE GAS-FILLED TUNGSTEN LAMP