The improvement over the customary forms of the Nichols radiometer consists in the reduction of the moment of inertia of the rotating system to such a value that the system has a period of eight seconds and a sensitivity equal to that of the thermopile or bolometer under their best working conditions. A comparison of the thermopile and the radiometer is given.
B. J. SPENCE, "AN IMPROVED FORM OF NICHOLS RADIOMETER," J. Opt. Soc. Am. 6, 625-625 (1922)