THE TANTALUM LAMP. 



139 



It must further be mentioned that after serving for some time, 

 say 200 to 300 hours, the tantakmi filament loses a great deal of its 

 mechanical resistance, while, as has been stated by Doctor Von 

 Bolton, tantalum wire, when new, has a greater tensile strength than 

 steel. It becomes brittle and will break easily in the course of its 

 life as a filament. It is therefore advisable when lamps have served 

 for some time not to remove them from their old fittings and put 

 them into new ones, as that might easily cause the filament to break. 

 New lamps are not very sensitive to strong shocks, even while l)urn- 

 ing, but when this alteration in the filament has occurred it is Avell to 

 preserve them from shocks. 



The behavior of the tantalum lamp under a very great increase of 

 voltage is of special interest to the incandescent-lamp maker. As 

 was to be expected, the trials 

 made in this respect have also 

 shown the superiority of this 

 lauip over the carbon lamp. It 

 has been ascertained that tanta- 

 lum lamps for 110 volts, 25 Hef- 

 ner candlepow^er and 1 -5 watts 

 per candlepower only bm-n 

 through at 260 to 300 volts if 

 the pressure is increased slowly 

 and gradually, while with car- 

 bon lamps designed to work un- 

 der the same conditions nothing 

 like that figure can be obtained. 

 The superiority of the tantalum 

 lamp over the carbon lamp with 

 regard to blackening of the glass 

 globe can also be proved in a feAv hours by means of comparative 

 burning tests at about 30 per cent overload. 



Another advantage of the tantalum lamp over the carbon lamp is 

 that the resistance of tantalum, like that of all other metals, strongly 

 increases with the rise of temperature, while carbon is known to 

 diminish in resistance when it is hot. In figure 9 the variation of 

 the resistance of tantalum and of carbon as a function of the voltage 

 is graphically represented, the pressure being assumed as 100 volts 

 and the resistance at 100 arbitrary units when the efficiency is 1-5 

 watts per Hefner candlepower, so that for each per cent of variation 

 of voltage the respective percentage of variation of resistance is 

 shown. It will be seen in the first instance that the resistance of the 

 tantalum increases to more than five times its original value from the 

 cold state to 1 ' 5 watts per Hefner candlepower, while the resistance 

 of the carbon decreases to about one-half of its initial value. It will 



20 40 60 80 100 t2Q 140 760 180 200 

 Voltage 



Fig. 9.— Variation of resistance with voltage of 

 tantalum as compared with carbon. 



