PHOTOMETRY AND ELECTRIC LIGHTING. 327 



composition (color) of the light. The value of o. 1 1 5 watt per 

 spherical -candle applies to the orange-colored light from the 

 Hefner lamp, and this value can be considered only as an approx- 

 imation to the power value of one spherical -candle of yellowish 

 light, or of pure white light, or of bluish light. 



Taking o. 1 1 5 watt as the approximate power value of one 

 spherical-candle, it follows that the ordinary carbon-filament 

 glow lamp which takes 3.6 watts per spherical-candle (about 3.1 

 watts per mean horizontal candle) has an actual efficiency of 

 about 3 per cent., an ordinary enclosed-arc lamp which takes 2.2 

 watts per spherical-candle has an actual efficiency of about 5.2 

 per cent, a luminous-arc lamp which takes 1.2 watts per spherical- 

 candle has an actual efficiency of about 9.6 per cent., and a mer- 

 cury-vapor lamp which takes 0.6 watt per spherical-candle has 

 an actual efficiency of about 19.2 per cent. 



The thermodynamics of radiation. If the light-giving element of a lamp were 

 to conform to the laws of radiation which apply to an ideal black body, then the 

 efficiency of the lamp would be a definite and well-known function of the tempera- 

 ture * of the element, increasing with the temperature and reaching a maximum of 90 

 or 95 per cent, at a temperature of about 4600 C. This temperature is far greater 

 than any known solid or liquid substance can stand. 



All substances depart more or less from the laws of radiation which apply to an 

 ideal black body. This is especially the case with gaseous substances, and the light 

 efficiency of the radiation emitted by such substances is not a well defined function of 

 the temperature. Indeed, the idea of temperature applies strictly to substances in 

 thermal equilibrium, only, and the column of luminescent vapor in a mercury-vapor 

 lamp, for example, cannot be said to have any temperature at all, that is to say, the 

 luminescent state of the vapor could not be reproduced by raising the vapor to any 

 assignable temperature whatever. 



136. The carbon-arc lamp. The arc lamp is the earliest form 

 of electric lamp. It consists of two carbon rods arranged so that 

 an electric current may flow from the one to the other forming 

 what is called an electric arc between them, and a more or less 

 elaborate mechanism for moving one or both of the carbon rods 

 so as to keep the arc fairly steady. The electric arc consists of 



* A very good resumS of the thermodynamics of the electric lamp is given by E. 

 F. Roeber in the Transactions of the American Electrochemical Society, Vol. VIII., 

 pp. 243-267, 1905. 



