326 ELEMENTS OF ELECTRICAL ENGINEERING. 



be conveniently classified under three heads, namely, (a) glow 

 lamps, in which the heated element is a solid electrical conductor 

 in the form of a rod or filament ; (fr) vapor lamps, in which the 

 light is given off by a highly incandescent or luminescent column 

 of vapor through which the electric current flows ; and (r) the 

 ordinary carbon-arc lamp, in which the light is given off chiefly 

 by the tips of the carbons which are raised to an extremely high 

 temperature by the faintly luminous but excessively hot column 

 of conducting vapor between them. 



Under the head of glow lamps are included the ordinary car- 

 bon-filament lamp, several varieties of metal-filament lamps, and 

 the Nernst lamp in which the glower is a rod of porcelain-like 

 material made of a mixture of the rare earths. 



Under the head of vapor lamps are included the recent types 

 of luminous-arc lamps in which the electric arc is charged with 

 metallic vapor and is highly luminous, the mercury-vapor lamp, 

 and the Geissler-tube lamp of Macfarlane Moore. 



135. Lamp efficiency. In practice the efficiency of a lamp is 

 always specified by giving the watts consumed in the lamp per 

 spherical-candle of light flux emitted. Thus the carbon-filament 

 glow lamp of the kind that is at present most widely used con- 

 sumes about 3.6 watts per spherical -candle. The actual efficiency 

 of a lamp is of course the ratio of the light energy emitted by the 

 lamp to the total energy supplied to the lamp. The actual effi- 

 ciency is the greater the smaller the so-called efficiency in watts 

 consumed per spherical-candle. 



Angstrom * has found that that part of the horizontal radia- 

 tion of a Hefner lamp which lies within the limits of the visible 

 spectrum represents a flow of 8. 1 ergs per second across one 

 square centimeter at a distance of one meter from the lamp, so 

 that one spherical-hefner corresponds to a flow of 471 x ioo 2 x 8. 1 

 ergs per second or 0.102 watt. This corresponds to o. 1 15 watt 

 for one spherical-candle. The power value of the spherical- 

 hefner (or candle) depends, however, to some extent upon the 



* Physikalische Zeitschrift, Vol. III., p. 257, 1902. 



