6 SCIENCE PROGRESS 



In comparison with the Nernst lamp, there could be claimed 

 for the tantalum lamp the same or even greater efficiency — 

 a useful life of 1,000 hours, equal to that of carbon filament 

 lamps and considerably longer than that of the Nernst lamp ; 

 and entire freedom from all the complications of the Nernst 

 lamp, the construction being the same in all outward details 

 as the carbon filament lamp, for which it could be substituted 

 in the lamp holder without trouble. 



The tantalum lamp was not destined to remain long in the 

 field unrivalled. The patent records for the years 1904-6 show 

 that many inventors were busily engaged in improving the 

 osmium lamp by using other metals and other methods for 

 the preparation of the filament. Before long tungsten emerged 

 from these experiments as the most suitable metal and two or 

 three satisfactory methods of manufacturing tungsten filaments 

 were worked out. Once again the Welsbach Company in 

 Germany were to the front ; the Osram lamp (a name com- 

 pounded of osmium and wolfram) it produced being the first 

 and still the most successful tungsten lamp. 



The tungsten lamp does not differ in general characteristics 

 from tantalum or carbon filament lamps. The filament is 

 enclosed in an evacuated bulb and mounted on a wire frame 

 in somewhat the same way as the tantalum filament : the 

 tungsten filament, however, is not flexible and instead of 

 winding a single thread over the frame, separate horseshoe 

 filaments are mounted upon it and connected in series by the 

 wires of the frame. The efficiency of the tungsten lamp 

 approaches twice that of the tantalum lamp, the consumption 

 averaging V2 to 1*4 watts per candle over a life of at least 

 1,500 hours. The life of the tungsten lamp can hardly be said 

 to have been determined: 1,500 hours is probably a conservative 

 estimate, lamps lasting 2,000, 3,000 and even as long as 5,000 

 hours without serious deterioration. Considerable difficulty 

 was experienced at first in manufacturing lamps for high 

 voltages (200 volts and over) but these difficulties are now 

 overcome and lamps of the most common candle-powers are 

 made for all voltages. The tungsten lamp is also made in very 

 high candle power sizes, 500 to 1,000 candle power ; in this form 

 it is coming steadily into use for street lighting and the lighting 

 of large interiors, a field hitherto occupied by the arc lamp 

 alone. We shall refer to this matter later on. 



