The Crookes radiometer is well known to the physics student and in science shops as a fascinating toy (Figure 13). It is a rotator with vanes polished on one side and black on the other. These are placed on a free shaft in a glass bulb which has been evacuated to a pressure of 10^-3 to 10^-4 atmospheres. It was the first demonstration of the conversion of light into mechanical energy. There was vigorous debate in the 1870’s over how it worked¹.  The traditional explanation involves collision of air molecules with the hot black surface causing it to recoil, but this is incorrect².  Reynolds and Maxwell proposed an explanation involving ‘thermal transpiration’ but even today there is still no complete explanation of how this little toy works.

The vanes rotate very rapidly in bright sunlight making several thousand revolutions per minute. Crookes³ measured the ‘radiometer force’ and found it to be several orders of magnitude greater than the ‘light pressure’ anticipated by Maxwell. There has been no attempt to harness the rotational energy to measure the efficiency of conversion but I suspect that solar is converted into rotational energy with very high efficiency in the radiometer.

Crookes⁴ also developed the otheoscope (Figure 14). Quoting from the original paper, "The fixed copper disc is lampblacked on the upper side with the vanes made of mica for the sake of lightness. The vanes are as numerous as can be conveniently put together and being set at an angle, the pressure from the upper plate drives them round with great speed when set in action with even a faint light."

There are several parallels between Figure 14 and the ideas outlined in this paper. The energy changes involve a constant volume closed cycle. The absorber surface is flat and stationary. The vanes rotate because of collisions with air molecules which act simply as a conduit for converting heat energy in the disc into rotational energy of the vanes. I believe each of these steps takes place with high efficiency.