SM174     Crookes’ Radiometer     circa 1950     Heat

  Crookes discovered a highly remarkable class of phenomena that are due to the radiant action of heated and luminous bodies.   To demonstrate these phenomena he devised an instrument called a radiometer.  

  A bulb blown out of a glass tube has a fine steel point inside which has a small vane consisting of four arms of aluminum wire with a bearing in the centre and with each end holding a mica lozenge or disc.   One side of the mica is blackened using lampblack.   The whole vane is very light (about 2 grams).   The glass envelope is evacuated by means of a pump to a controlled degree.

  Experiments with the degree of evacuation show that if radiation is directed at the apparatus the vane will rotate so that the blackened side moves towards the light until a certain degree of rarefaction is exceeded.   At this stage, the direction of rotation reverses so that the blackened face now moves away from the radiation source.   If the bulb were to be evacuated to a very high degree, the rotation will cease.

  The explanation of the phenomena is that they are due to convection currents in the rarefied gas.   The heat falling on the blackened surface increases its temperature and this in turn heats the adjacent gas, reducing its pressure and causing the movement towards this lower pressure.   The opposite rotation with higher evacuation can be explained by the rapid movements of the gas molecules on the high temperature side pushing the vane when they rebound from the side of the vessel.   This only occurs when the molecules are very free to move, that is with the gas in a highly rarefied condition.   No movement would be detected in a complete vacuum.