Cavendish Laboratory : Crookes radiometer

Cavendish Laboratory

<p style='text-align: justify;'>The radiometer invented by William Crookes consists of an airtight glass bulb containing a partial vacuum. It encloses a set of vanes mounted on a spindle. One side of each vane is painted black and the other side white or reflecting. When radiation is incident upon the vanes, the vanes rotate about the spindle axis. Originally it was thought that the rotation was due to radiation pressure, but the effect decreases at very low pressures and should occur in the opposite direction of rotation from that observed. The effect is the result of the heating of the black side to a higher temperature than the white or reflecting side because the black side is a more effective absorber of the incident radiation. The rotation is caused by the greater rate of emission of thermal particles on the black side as compared with the white.</p>