ILLUMINATION AND PHOTOMETRY 423 



first rush of current . although the fuses were of ample capacity to 

 take care of the steady operating current of the lamp. 



190. Gas-filled Lamps; Type C. Remembering that the 

 light emitted by an incandescent body increases as the fourth 

 power of its absolute temperature, it is obvious that there is still 

 opportunity for increasing the efficiency of tungsten lamps if 

 the temperature of the filament can be safely increased. The 

 factor which limits the operating temperature of the Type-A 

 and Type-B filaments is the volatilization or evaporation of 

 the filament itself. This volatilization has two effects. The 

 evaporation removes useful material from the filament, resulting 

 in a reduction of efficiency and in an ultimate burning out of the 

 lamp. The tungsten vapor condenses on the bulb, blackening 

 it and so cutting down the useful light by absorption. This last 

 factor has in part been remedied by injecting a chemical into the 

 hull) which tends to keep the tungsten deposit semi-transparent. 



The gas-filled or Type-C Mazda lamp is based on the principle 

 of vapor pressure. The higher the pressure, the higher the 

 temperature at which water and other substances evaporate. 

 Water will hoil at a very low temperature in a rarefied atmos- 

 phere, whereas under pressures greater than atmospheric its 

 boiling temperature may become quite high. The same rule 

 applies to tungsten. In a vacuum, 

 the 1 1 mgsten evaporates quite read- 

 ily, which results in a more rapid 

 deterioration of the filament. In 

 Mazda-C lamps, the bulb is 

 filled with an inert gas such as nit ro- 

 gen. This gas does not enter into 

 chemical reactions wit h the filament 

 and yet it causes sufficient pressure 

 to materially increase the evapor- 

 ation temperature of the filament. '" :i. GM-IHI,,! 



watt) and section of film 

 Due tO the higher temperature of 



evaporation, the filament may be operated at a higher tempera- 

 ture without rapid deterioration. Tin Mot only increased 

 efficiency hut also a whiter liu hi . more nearK - resemMiim ^unliulit . 

 Thi .t her jnii It \\ ill l>e noted from 

 Fig. 381 that this type Of lamp has a long narrow neck. The 



