Kirchhoff, Boltzmann, Professors Wien and Planck, and taking their
starting-point from the laws of thermodynamics, have given formulas
which establish the radiating power of a dark body as a function of
the temperature and the wave-length, or, better still, of the total
power as a function of the temperature and wave-length corresponding
to the maximum value of the power of radiation. We see, therefore, the
possibility of appealing for the measurement of temperature to a
phenomenon which is no longer the variation of the elastic force of a
gas, and yet is also connected with the principles of thermodynamics.

This is what Professors Lummer and Pringsheim have shown in a series
of studies which may certainly be reckoned among the greatest
experimental researches of the last few years. They have constructed a
radiator closely resembling the theoretically integral radiator which
a closed isothermal vessel would be, and with only a very small
opening, which allows us to collect from outside the radiations which
are in equilibrium with the interior. This vessel is formed of a
hollow carbon cylinder, heated by a current of high intensity; the
radiations are studied by means of a bolometer, the disposition of
which varies with the nature of the experiments.

It is hardly possible to enter into the details of the method, but the
result sufficiently indicates its importance. It is now possible,
thanks to their researches, to estimate a temperature of 2000° C. to
within about 5°. Ten years ago a similar approximation could hardly
have been arrived at for a temperature of 1000° C.


§ 6. DERIVED UNITS AND THE MEASURE OF A QUANTITY OF ENERGY