306 



tiire may be expressed merely as black body temperature, Kirchoff"s absb-- 

 lute scale. 



Laics of Black Body Radiation. — Stefan deduced from experiment, and^ 

 Boltzmaim deduced from thermodynamic considerations, the law that the 

 total radiant energy emitted from a black body is proiwrtional to the fourth 

 power of the absolute temperature, or, 



J = KT* 

 where K is a constant. 



The radiant energy emitted bj' a heated body is in the form of waves' 

 of diverse wave length. ^Nlost of the radiant energy is due to waves that 

 are too long to affect the eye. As the temperature of the body is increased, 

 the energy of all the emitted waves is increased, but the energy of the 

 shorter waves increases more than that of the longer waves. That is, the 

 distribution of energy among the waves of different lengths depends upon 

 the temperature of the body. 



Wien has also shown that the product of the absolute temperature T 

 of some source and the wave length having maximum energy, /m in 

 spectrum is a constant. 



/mT = constant A 



This is generally known as the displacement law or Wien's First Law. 

 Wien also combines his first law with the Stefan-Boltzmann Law giving 

 his second law. 



JmaxT"-^ =^ constant B 

 Llis most important investigation, however, was the investigation of spec- 

 tral distribution of energy in the radiation of a black body in which he 

 shows that for any particular wave length the relation between the energy 

 emitted and the absolute temperature is as follows : 



— C2 



.J = Ci /-' 'e ( 1 ) 



/-T 



where J is the energy corresponding to wave length / and T is the 

 absolute temperature. Ci and C2 are constants and e is the base of the 

 natural S3"stem of logarithms. 



The working principles of the following experiments are based upon 

 these two laws, i. e., the total radiation and spectral radiation laws. In 

 the first case black body temperature is determined by measuring the total 

 energy, as in a Fery pyrometer which allows radiations of all wave lengths 

 to fall upon a sensitive thermo-couple connected to a direct reading gal- 

 vanometer. In the second case some particular wave length is used and 



