THE RADIOMETER

The radiometer was invented in 1879 by an Englishman, William Crooks (1832-1919). It consists of an evacuated clear glass stand 5" high - the diameter at the top being 3" - holding a four-bladed vane balanced on a needle. Because opposite surfaces of the vanes are black and white, incident radiant energy is absorbed or reflected to a different degree. The result is a spinning vane. The brighter the light, the faster it spins. Simple but effective. A brief explanatory instruction sheet is included. The vanes revolve when exposed to light. The first radiometer was designed to settle the controversy as to whether light exerts a force. the idea was that a reflecting surface would experience a greater force from the light than an absorbing one. The instrument was therefore constructed in the now familiar form. Unexpectedly, the opposite effect was observed. The blackened vane retreated from the light source. We know now that the black surface is warmer than the shiny one and that gas molecules will recoil faster from a hot surface. The slight difference in molecule recoil is what causes the device to spin. (Later experiments in a much better vacuum have confirmed that light does exert a very small pressure). The action of a radiometer depends upon striking a balance between molecular drag and recoil. At higher pressures drag will dominate and the vanes will fail to spin. At lower pressures, there are too few recoiling molecules to drive the vanes. The optimum balance occurs at a pressure of about 60 m Torr (60 microns Hg- 0.99 mb). By using a suitable tachometer (e.g. a "Strobotach" or an electronic counter that detects the interruption of a light beam by the vanes) it would be possible to measure the change in rotational velocity with changing pressure, providing a constant light input.