136 THE TANTALUM LAMP. 



(25 Hefner candlepower 110 volts). This shape offers a number 

 of noticeable advantages. In the first instance it is very stable 

 and will stand strong shocks without damage to the lamp. A con- 

 siderable number of such lamps sent across the sea to test their ability 

 to withstand the hardshij^s of transport came back unhurt, although 

 they had been packed just like common glow lamps, and no 

 special care in any respect had been taken in their handling. The 

 lamp burns, of course, in any position, and can therefore be held in 

 any kind of fitting. The light is rather white and agreeable, and its 

 effect is i3articularly uniform if the lamp is provided with a ground- 

 glass globe. 



We shall now proceed to describe the electric and photometric 

 properties of the lamp and its behavior in actual use. Numerous 

 trials for lengthy periods of time at 1 to 3 watts per candlepower 

 have proved the vast superiority of the tantalum lamp over the 

 carbon filament lamp under equal electric and photometric conditions. 

 Expressing this fact in figures, we can state that the tantalum lamp 

 consumes about 50 per cent less current at the same voltage, with the 

 same intensity of light and the same useful life; or that, at the same 

 economy, its life is several times that of the carbon type. Moreover, 

 at an initial efficiency of 1 -5 volts per Hefner candlepower the tanta- 

 lum lamp has an average life quite sufficient for all practical require- 

 ments, so that this rating has been standardized for the 110- volt 

 lamp. Trials liave also proved that the lamps have a life of several 

 hundred hours at 1 watt per Hefner candlepower, but in that case 

 they were very sensitive to variations of pressure and often showed 

 an early decrease of illuminating power. The useful life of the 

 tantalum lamp — i. e., the time within which it loses 20 per cent of 

 its initial illuminating power — averages between 400 and 600 hours 

 at 1 -5 watts per Hefner candlepower. Some specimens have proved 

 to have a useful life of as much as 1,200 hours. The absolute life, in 

 general, amounts to 800-1,000 hours under normal working conditions. 

 Further, we have to remark that the tantalum lamp blackens but 

 little unless it has been strongly overheated during work in conse- 

 quence of partial short-circuiting of the filament. 



It is very interesting to observe the behavior of the tantalum 

 lamp during the whole course of its life. The first fact worthy of 

 note is that, like some carbon lamps, the illuminating value increases 

 at the beginning, generally after a few hours, by 15 to 20 per cent. 

 In the same way the consumption of current rises by about 3 to 

 G per cent, Avhile the consumption • of energy drops to 1 -3 to 1 "4 

 watts per candlepower. After that the illuminating value gradually 

 decreases, while a corresponding increase of the consumption of 

 energy occurs. The average behavior of the 25-candlepower lamp 



