232 



HEAT. 



This statement must not be taken too generally. It requires limita- 

 tion on account of the variation of the absorbing power with the nature 

 of the radiation falling on it, and we shall see later that the limitation is 

 that the law applies to each kind, or wave-length, of radiation emitted. So 

 that a high or low absorber of radiation of a given wave-length is a high 

 or low emitter of that same radiation. If, as in the case of lampblack, 

 nearly all wave-lengths are absorbed to a very great degree, the surface 

 is a high radiator of nearly every kind of radiation. 



The way in which absorbing and radiating powers go together may be 

 illustrated in a very striking way by the experiment represented by 

 Fig. 137. A Leslie cube AB is placed midway between two tin plates 

 A' B'. These have small thermoelectric junctions at the back, and the 

 circuit is completed, as shown in the figure, through a galvanometer G. 

 If the plate B' is at a different temperature from the plate A' a current 

 goes through the galvanometer. If A and A' are lampblacked surfaces, 

 while B and B' are bright tin, no current is generated. A radiates far 



more than B, but B' 

 absorbs [a far less propor- 

 tion of the radiation fall- 

 ing on it than does A', 

 and if E, E' are the emit- 

 ting powers of A and B, 

 while a a' are the absorb- 

 ing powers of A' and B', 



E E' 



A further illustration 

 of this equality of emitting 

 and absorbing powers is 

 given by a thermometer 

 placed in an enclosure at 

 a different temperature. 



If the thermometer is 6" above the enclosure, it will fall towards the 

 temperature of the enclosure at the same rate as it will rise towards it if 

 it is below the enclosure. 



Illustrations of Emission and Absorption of Radiation. 



The low radiating and absorbing power of metals is frequently used in 

 physical experiments. Thus the external surface of a calorimeter is 

 brightly polished, and is usually surrounded in turn by a second metal 

 vessel brightly polished. Most of the small quantity of heat lost by 

 radiation from the calorimeter is reflected by the outer vessel, and the 

 low radiating quality of the outer vessel protects the inner vessel from 

 external changes. It is probably advantageous to make the two vessels 

 of different metals, since, if of the same metal, the outer vessel will 

 absorb more of the rays emitted by the inner one than if it is of a 

 different metal. It should as far as possible reflect them back 



A familiar example of the use of low radiating power of bright metal 

 is afforded by a metal teapot. 



FIG. 137. 



