Nichols radiometer

It consisted of a pair of small silvered glass mirrors suspended in the manner of a torsion balance by a fine quartz fibre within an enclosure in which the air pressure could be regulated.

By turning the mirror system around to receive the light on the unsilvered side, the influence of the air in the enclosure could be ascertained.

This influence was found to be of almost negligible value at an air pressure of about 16 mmHg (2.1 kPa; 0.021 atm; 0.31 psi).

The radiant energy of the incident beam was deduced from its heating effect upon a small blackened silver disk, which was found to be more reliable than the bolometer when it was first used.

The original papers, with their historical context, have been re-printed in a chapter of the book Quantum Photonics: Pioneering Advances and Emerging Applications.