132 THE TANTALUM LAMP. 



tiirino- department. As soon as Doctor I>olton's experiments showed 

 that the originality brittle tantalum could be made ductile enough to 

 draw into wire by the usual methods, and that this wire could be bent 

 and coiled like a thin steel wire, it became possible to test it thor- 

 oughly as to its usefulness for incandescent lamps. The first trials 

 with wires of about '3 millimeter diameter gave most promising 

 results. They confirmed the fact that tantalum has a very high melt- 

 ing point and that it is but slightly subject to disintegration in a 

 vacuum, even when subjected to a heavy current. 



The first tantalum lamp that proved moderately satisfactory in that 

 it admitted of an exact measurement of the electric photometric 

 conditions and stood a burning test for some time, was completed 

 just over two years ago, viz, on December 28, 1902. This lamp had 

 a loop-shaped filament made of the first tantalum wire ever drawn. 

 The diameter of the wire was •28 millimeter, its effective lighting 

 length 54 millimeters, and its electrical resistance when cold 0'29 

 ohm. This corresponds to a specific resistance (1 meter length, 1 

 square millimeter section) of O'^Sl. The photometric measurements 

 made at efficiencies of 2, IJ-, and 1 watt per Hefner candlepower 

 showed potential differences of 4*9, 4 "95, and 5*9 volts, currents of 

 5, 5 46, and 0-2 amperes, and illuminating values of 11, 18, and 37 

 Hefner candlepower, respectively. On being burnt at 1 watt per 

 candlepower the lamp had a life of twenty hours, during which it 

 blackened considerably. 



As the chemical and mechanical manufacturing processes devel- 

 oped and the material became purer and the wires more uniform, the 

 results obtained also improved. The lamps lasted longer and black- 

 ened less; at the same time the specific resistance decreased until it 

 had dropped to the present figure of •1(35 for the pure metal. It is 

 clear that the material used for the first lamps still contained a con- 

 siderable quantity of impurities, probably niobium and carbides, 

 which caused the great disintegration and the nearly double specific 

 resistance. During these first trials we looked very carefully into 

 the question as to what dimensions the filament of a tantalum lamp 

 ought to have for ordinary voltages and illuminating values. From 

 the dimensions of the filament used in the first lamp we calculated 

 that, with this rather impure material, we should require a filament 

 about 520 millimeters long and '06 millimeter diameter for a lamp 

 for 110 volts, 32 Hefner candlepower, and 1*5 watts per candle- 

 power-. These unusual figures increased when the specific resistance 

 of the material had diminished to tlie present value of -165, at which, 

 for a 32 Hefner candlepower lamp, a filament of about TOO milli- 

 meters in length by 0*055 millimeter in diameter was required; for 

 a 25 Hefner candlepower lamp a filament of about 050 milli.neters 

 by 0"05 millimeter diameter was required. Thus, in order to con- 



