338 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



eral use, namely, the glow lamp, or incandescent lamp, and the 

 arc lamp. 



77*4* glow lamp consists of a fine filament or wire of highly re- 

 fractory material which is enclosed in a glass bulb from which the 

 air is exhausted. In the older type of glow lamp the filament is 

 made of charred vegetable material upon which a dense deposit of 

 carbon is formed by heating it in the vapor of gasoline. The 

 heating is accomplished by sending an electric current through 

 the filament. The carbon-filament glow lamp consumes from 

 three to four watts for each candle of light emitted. Thus, a 16- 

 candle carbon filament lamp consumes about 5 5 watts. 



Recently several varieties of metal-filament glow lamps have 

 been placed on the market. The earliest of these was the 

 osmium lamp, the filament of which is made of metallic osmium 

 which is sufficiently refractory to stand a temperature high enough 

 to emit one candle of light with a consumption of about I */ 

 watts. The scarcity of metallic osmium, however, was a serious 

 obstacle in the way of extensive use of the osmium lamp. The 

 next metal filament lamp to be placed on the market was the 

 tantalum lamp, in which the filament consists of a wire of metallic 

 tantalum. In the tungsten lamp, which is now coming into ex- 

 tensive use in Europe and America, the filament consists of me- 

 tallic tungsten. The carbon filament lamp consumes from 3 to 4 

 watts for each candle of light emitted, the osmium lamp con- 

 sumes about I y 2 watts per candle of light emitted, the tantalum 

 lamp consumes about 2 watts per candle, and the tungsten lamp 

 consumes about I ^ watts per candle. The greatest difficulty 

 with the metal filament lamps is that the filament must be exces- 

 sively fine to give a low candle power lamp with the standard volt- 

 ages now in use for lighting purposes (110 volts and 220 volts), 

 because of the low specific resistance of metals as compared with 

 carbon. This difficulty is greatly enhanced in the case of the 

 tungsten lamp by the excessive brittleness of the material. 



The arc lamp. When an electric arc is formed between carbon 

 points as described in Chapter VIII, the carbon points become 



