THE MEASUREMENT OF HIGH TEMPERATURES 239
whereby the user can verify accurately if he is within the reguisite working distance.
Fig. 140.—Foster Radiation Pyrometer.
THEORETICAL CONSIDERATIONS
It is well known that the hotter a body is the more heat it radiates. When the body is of sufficient heat (i.e., above about 500°) a portion of the energy issuing from it is visible as light. The intensity of brightness of a luminous body is measured by the rate of emission of luminous radiance per unit of area, and depends, therefore, upon temperature. It must be borne in mind, however, that in addition to the energy radiated as visible light there is another form of radiated energy, the wave lengths ■of which are too long to be seen by the eye. It so happens that in those instances met with in practice the invisible radiated energy is much greater than the energy derived from visible light, but in the modem form of radiation pyrometer radiated ■energy of all frequencies—whether visible or invisible—is almost entirely absorbed and converted into heat. It is in this respect that the thermo-electric instrument ■differs from the optical type, for the latter is dependent solely upon the visible radiations. Again, in tlie optical instrument it is customary to bring the visible radiations to a focus by means of a convex lens or a concave mirror, and exactly the same principle may be applied in the thermo-electric pyrometer with. the invisible energy.
The Stefan-Boltzmann law states the relation between the temperature of a hot body and the energy which it emits. From this law it may be seen that the energy radiated is proportional to the fourth power of the difference in temperature :—■
Ek=K (T/-T2‘)
Where ER=the radiated energy emitted.
„ K =a known constant.
,, T1 =the temperature of the hot body (absolute).
,, T2 =the temperature of the instrument (absolute).
Under ordinary conditions the temperature of the instrument becomes a negligible