NO. 2 HISTORY OF ELECTRIC LIGHT SCHROEDER 83 



experiments, he heated some carbon filaments and found that the high 

 temperature changed their resistance " characteristic " from negative 

 to positive. The ordinary carbon filament has a resistance v^hen hot 

 that is less than when it is cold, which was reversed after heating it 

 to the high temperature Dr. Whitney was able to obtain. These fila- 

 ments were made into lamps for no- volt service and it was found 

 that they could be operated at an efficiency of 4 lumens per watt. The 

 lamps also blackened less than the regular carbon lamp throughout 

 their life. 



This lamp was put on the market in 1905 and was called the Gem 

 or metallized carbon filament lamp as such a carbon filament had a 



Gem Lamp, 1905. 



This incandescent lamp had a graphitized carbon filament obtained 

 by the heat of an electric furnace, so that it could be operated at 

 25 per cent higher efficiency than the regular carbon lamp. This lamp 

 is in the exhibit of Edison lamps in the Smithsonian Institution. 



resistance characteristic similar to metals. At first it had two single 

 hair pin filaments in series which in 1909 were changed to a single 

 loop filament like the carbon lamp. 



In 1905 the rating of incandescent lamps was changed from a 

 candlepower to a wattage basis. The ordinary i6-candlepower carbon 

 lamp consumed 50 watts and was so rated. The 50-watt Gem lamp 

 gave 20 candlepower, both candlepower ratings being their mean 

 candlepower in a horizontal direction. The Gem lamp was made for 

 no-volt circuits in sizes from 40 to 250 watts. The 50-watt size 

 was the most popular, many millions of which were made before the 

 lamp disappeared from use in 1918. The lamp was not quite as 

 strong as the carbon lamp. Some Gem lamps for series circuits were 



