306 
ture may be expressed merely as black body temperature, Kirchoff’s abso- 
lute seale. 
Laws of Black Body Radiation.—Stefan deduced from experiment, and 
3oltzmann deduced from thermodynamic considerations, the law that the 
total radiant energy emitted from a black body is proportional to the fourth 
power of the absolute temperature, or, 
4) = IND 
where K is a constant. 
The radiant energy emitted by a heated body is in the form of waves 
of diverse wave length. Most 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 
Gistribution 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, 7m in 
spectrum is a constant. , ‘ess 
AmT = 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. 
JmaxI~> = 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 
74 
where J is the energy corresponding to wave length “ and T is the 
i (Oise 
(1) 
absolute temperature. Ci and C, are constants and e is the base of the 
natural system 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 Féry pyrometer which allows radiations of all wave lengths 
to fall upon a sensitive thermo-couple connected to a direct reading gal- 
vyanometer. In the second case some particular wave length is used and 
