596 Mr. Alexander Siemens [April 23, 



After trying various methods of housing the long Ta filament in 

 a glass bulb of approximately the same dimensions as the carbon glow 

 lamps, the present form was arrived at during the year 1904. In this 

 lamp, which was adopted as standard, the length of the filament was 

 650 mm., its diameter 0*05 mm., and its weight 0*U22 grammes, so 

 that about 45,000 of these lamps contain 1 kg. of Ta. 



Since then these dimensions have been modified to a certain extent, 

 for instance the diameter of the filament is now only * 0:-) mm., but 

 the external shape has not been altered. 



It was soon found that after burning a short time the filament 

 underwent certain structural changes and lost its great tensile strength. 

 Examination under a microscope revealed the fact that in about 

 1000 hours the smooth cylindrical filament shows signs of capillary 

 contraction, as if the cylinder was going to break up into a series of 

 drops, and the surface, from being dull, commences to glitter. This 

 contraction of the filament after being heated is readily recognised by 

 comparing a new lamp with an old one. On the stars of the new 

 lamp the filament hangs loosely, while in the old lamp the filament is 

 evidently in tension. 



The characteristic difference between carbon filaments and tanta- 

 lum filaments is shown by a diagram representing the influence of 

 temperature on the electric resistance of the two filaments in propor- 

 tion to each other. 



In order to have the differences at once shown in per cents., the 

 normal pressure and the normal resistance of both filaments, when 

 giving the light of 1 candle for 1*5 watt, is marked as 100, and it is 

 immediately seen that the resistance of Ta alters directly and that of 

 carbon inversely as the temperature. Owing to this quality, a Ta 

 filament is better able to resist overheating than a carbon filament, as 

 the following experiment shows, where two lamps, one Ta and one C, 

 burning normally at 110 volts with 1*5 watt per candle power, are 

 gradually exposed to higher voltages. The C lamp breaks while the 

 Ta lamp stands up to 200 volts, the highest voltage available to-night. 

 Of course its useful life wiU be shorter than at its normal voltage. 



As stated at the beginning of the discourse, the primary object of 

 all the research was to find a filament more economical in the con- 

 sumption of electrical energy than the C filament, and the following 

 experiments will show that the Ta filament is, in this respect, a great 

 improvement on the C filament. 



To begin with, a comparison can be made by burning a Ta and 

 a C lamp under water, each being immersed in a vessel containing the 

 same quantity of water. 



Owing to the C lamp requiring more energy to give the same light 

 as the Ta lamp, the temperature of the water in the C vessel rises 

 quicker than in the other vessel. 



Another way of showing the difference is by measuring the current 

 taken by each of the two lamps when giving approximately the same 



