All bodies lose and gain thermal energy by means of electromagnetic radiation. The rate of energy loss depends on the temperature of the body and the rate of energy gain depends on the temperature of the surroundings. Stefan's Law states that the power radiated by a body is proportional to the 4th power of the absolute temperature. Thus
where e is the emissivity of the surface of the body,
is the Stefan-Boltzmann constant
equal to 
, and A is
the surface area of the body. The value of the emissivity is
between
zero and one, and equals one for a perfect black body. Bodies
can, of course, lose heat by other mechanisms (conduction and
convection), but at high temperature radiation becomes dominant.
The phenomenon can be studied using a light-bulb filament as the radiating body since it can be easily heated to a very high temperature. The power can be determined from the voltage and current to the filament. The temperature of the filament has to be found indirectly by first computing the electrical resistance and then using a standard resistance-versus-temperature relationship. To a fair degree of accuracy the temperature is given by
where 
is the resistance of the element when its temperature
is equal to 
. It is
convenient to take 
and 
at room temperature
which is 300 K.