Often called the Crookes radiometer, from the name of the British chemist and physicist Sir William Crookes (1832-1919) who invented it in 1875, it is the first vane radiometer used as a detector in the visible and in the infrared. When exposed to radiations, the rotor revolves at a speed depending on the intensity and nature of the radiation striking its vanes; to be more specific, the speed of rotation depends on the heat-producing effects of the radiation used and decreases from the red to the violet.
Rotation is due to the presence of residual gas in the bulb, which can be air or another gas at low pressure (about 1 mm of mercury). The molecules that are near the blackened faces, which heat up by absorbing radiation, acquire a speed of thermal agitation greater than that of the molecules that are close to the face that is not blackened; on striking the vanes, the former transfer an amount of motion greater than the amount transferred by the latter in the opposite direction and the rotor begins rotating in the sense that leaves the blackened faces behind.
The name radiometer was given to the apparatus by Crookes who thought he could use it in measuring the intensity of radiations on the basis of the speed of rotation which, within certain limits, is proportional to incident radiation. ...continues