5o6 SCIENCE PROGRESS 



measurement, and pyrometers for which considerable accuracy 

 is claimed are now available for the determination of the highest 

 obtainable terrestrial temperatures, such as that of the crater 

 of the electric arc. These depend on measurements of the 

 radiation given out by the hot body, from which its temperature 

 can be estimated. A convention has arisen by which these 

 pyrometers using visible light only are spoken of as " optical " 

 pyrometers, and those in which the whole heat spectrum is 

 concerned as " radiation " pyrometers. Both types have the 

 advantage over the resistance pyrometer and thermocouple, that 

 no part of the instrument itself is subjected to the destructive 

 action of the high temperature. 



The intensity of the radiation emitted by a hot body increases 

 enormously with rise of temperature, the amount of the increase 

 varying with the wave-length. An incandescent body at 

 2,000° C. emits more than 2,000 times as much red light per 

 unit area as it does at 1,000° C. : hence it would appear that a 

 photometric measurement of the light evolved from any very 

 hot substance should be a sensitive and easy way to measure 

 its temperature. If at a given temperature all substances 

 emitted the same amount of light, this would certainly be the 

 case ; but it has been found that, under ordinary conditions, 

 the nature of the radiating body, especially of its surface, 

 greatly affects the amount of radiation sent out. Carbon or 

 iron emit per unit area considerably more light and heat than 

 incandescent platinum or molten copper at the same tem- 

 perature. The work of Stewart, Kirchoff, Wien and others 

 has shown, however, that if substances of varying emissivity 

 are placed inside a uniformly heated enclosure and looked at 

 through a small aperture in its wall, the amount of radiation 

 sent out under these conditions is independent of the substance 

 and is a function only of the temperature. Such an enclosure 

 is called by Kirchoff a " black body." 



The relation between the temperature and radiation from a 

 black body has been accurately studied, and it has been found 

 that the total energy radiated per unit area is proportional to 

 the fourth power of the absolute temperature.^ Thus if Q be 



' The " absolute " temperature of a body is its temperature as measured not 

 from that of melting ice, but from " absolute " zero. On the Centigrade Scale this 

 point is - 273° C. ; hence absolute temperatures are obtained by addition of 273° 

 to the Centigrade temperature. 



