90 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



used. The gas circulating in the bull:) has the disadvantage of conduct- 

 ing heat away from the filament so that the filament w^as coiled. This 

 presented a smaller surface to the currents of gas and thereby reduced 

 this loss. The lamps were commercially introduced in 191 3 and a 

 patent was granted in April, 1 91 6. 



An increased amount of electrical energy is required in these lamps 

 to offset the heat being conducted away by the gas. This heat loss 

 is minimized in a vacuum lamp, the filament tending to stay hot on the 

 principle of the vacuum bottle. This loss in a gas filled lamp becomes 

 relatively great in a filament of small diameter, as the surface in pro- 

 portion to the volume of the filament increases with decreasing diam- 



r'r*^. 



Gas Filled Tungsten Lamp, 1923. 



This is the form of the lamp as at present made. For iio-volt 

 circuits the sizes range from 50 to 1000 watts. 



eters. Hence there is a point where the gain in temperature is offset 

 by the heat loss. The first lamps made were of 750 and 1000 watts for 

 I lo-volt circuits. Later 500- and then 400-watt lamps were made. The 

 use of argon gas, which has a poorer heat conductivity than nitrogen, 

 made it possible to produce smaller lamps, 50-watt gas-filled lamps for 

 iio-volt circuits now being the smallest available. In the present 

 state of the art, a vacuum lamp is more efficient than a gas-filled lamp 

 having a filament smaller than one consuming about half an ampere. 

 Thus gas-filled lamps are not now practicable much below 100 watts 

 for 220 volts, 50 watts for no volts, 25 watts for 60 volts, 15 watts 

 for 30 volts, etc. 



From the foregoing it will be seen that the efficiency of these lamps 

 depends largely on the diameter of the filament. There are other 



