348 



ELEMENTS OF ELECTRICAL ENGINEERING. 



by Fig. 191 in which the coordinates of the dots represent the 

 candle-power and watts of the individual lamps. 



145, Metal-filament glow lamps.* In the first attempt to make 

 a commercial glow lamp fine platinum wire was used for the 

 lamp filament but it was soon found that the filament was very 

 short-lived at the temperature necessary to give a fairly high 

 efficiency. The next step was to use a carbon filament and from 



5 6 Watts. 57 5 59 



Fig. 191. 



about 1 88 1 to 1900 the carbon-filament lamp was the only rival 

 of the arc lamp. 



It has been long known, however, that many metals have 

 higher melting points than platinum, some of them very much 

 higher ; but the most refractory metals are extremely brittle at 

 ordinary temperatures and the difficulty of forming these metals 

 into fine wires prevented their being even tried as lamp filaments. 

 Now, however, processes have been developed for the formation 



* For a description of the osmium-filament glow lamp see Fritz Blau, Electro- 

 technische Zeitschrift, Vol. 26, pp. 196-199, February 23, 1905. For a description 

 of the tantalum-filament glow lamp see von Bolton and Feuerlein, Electrotechnische 

 Zeitschrift, Vol. 26, pp. 105-108, January 26, 1905. 



