422 



ALTERNATING CURRENTS 



standing very high temperatures. It is therefore well adapted for 

 use as the filament in incandescent lamps. In the early days, it 

 was not possible to draw it through a die as ordinary wire is 

 drawn. Powdered tungsten was mixed with an organic binder, 

 such as cellulose, and forced through a die. The binder was 

 then driven out by heating, leaving a porous and pitted tungsten 

 filament which was very fragile. This type of filament was 

 used in the Mazda-A lamp. In 1911, the process of drawing 

 tungsten wire was perfected, so that the present-day filaments 

 are rugged. Lamps using this drawn filament in a vacuum are 

 called Mazda-B lamps. 



Maximum Current 

 at closing of Switch 



.01 .02 



.07 .08 



.03 .04 .05 .06 

 Time in Seconds 

 Fio. 380. Current variation in tungsten lamp when switched in circuit. 



Type-B tungsten lamps have an efficiency of about 1.2 watts 

 per mean horizontal candlepower. In the large sizes, 150 watts 

 and above, the efficiency may be as high as 0.90 watt per mean 

 horizontal candlepower. The reason for this high efficiency is 

 the very high temperature (about 3,000C.) at which it is possi- 

 ble to operate the tungsten without too rapid deterioration. The 

 life of tungsten lamps is longer than that of most other lamps, the 

 guaranteed life being about 1,000 hours. 



Tungsten has a positive temperature coefficient so that its 

 resistance at operating temperatures is several times that when 

 cold. This results in a high initial current when the lamp is 

 first switched in circuit. This is called "overshooting." The 

 relation of the current to time is shown in Fig. 380. Fuses in 

 tungsten-lamp circuits have been known to blow as a result of this 



