PHOTOMETRY AND ELECTRIC LIGHTING. 349 



of fine compact filaments of the most refractory and brittle metals, 

 and there are now on the market tantalum-filament glow lamps 

 and osmium-filament glow lamps. These lamps are characterized 

 by the very long and very fine filaments required even for vol- 

 tages as low as 37 volts, they are not so sensitive to changes of 

 voltage as the carbon-filament glow lamp on account of the fairly 

 rapid increase of resistance of the metal filaments with rise of 

 temperature, and they can be operated satisfactorily at a higher 

 efficiency than the carbon-filament glow lamp. The osmium 

 lamp, for example, can be operated at 1.6 watts per spherical- 

 candle with a useful life of 1,500 hours or more, and the tanta- 

 lum lamp can be operated at 1.6 watts per spherical -candle with 

 a useful life of 500 or 600 hours. By useful life is meant the 

 time of continuous operation before the candle-power falls below 

 80 per cent, of its initial value. 



146. The Nernst lamp.* The luminous element or glower of 

 the Nernst lamp is a small rod of porcelain-like material com- 

 posed of a mixture of the oxides of the metals of the yttrium 

 group. This rod is as good an insulator as glass or porcelain at 

 ordinary temperatures ; like glass or porcelain it becomes a fairly 

 good conductor at a low red heat ; and its resistance decreases very 

 rapidly as the temperature rises. Therefore the glower must be 

 heated to a low red heat by external means before current starts 

 to flow through it, and a ballast resistance must be connected in 

 series with the glower to prevent the current from rising indefi- 

 nitely when once it starts to flow. The initial heating of the 

 glower is accomplished by a fine coil of platinum wire, called the 

 " heater," which is covered with a protecting paste somewhat sim- 

 ilar to ordinary whitewash, and when the current starts to flow 

 through the glower it actuates an electromagnet which breaks the 

 heater circuit. The ballast is made of very fine iron wire 



*A full discussion of the Nernst lamp is given by A. J. Wurts, Transactions of 

 the American Institute of Electrical Engineers, Vol. XVIII. , pp. 545-587, 1901. 



The process of manufacture of the Nernst lamp is described in The Electrical 

 World and Engineer, Vol. XLIII., pp. 981-985, May 21, 1904. 



