A Radiometer is a device used for measuring the power of electromagnetic radiation. Usually a radiometer is an infrared radiation detector or ultraviolet detector. The first radiometer was developed by Sir William Crookes as a by-product of his chemical research in 1873 (so quite a while ago). The Crookes Radiometer – as it was dubbed, for obvious reasons – is made from a glass bulb which has all the air removed to create a vacuum. The science museum currently has one on display from 1877! Inside the bulb, is a rotor with several (4 on this model) vertical lightweight metal vanes that are spaced out along the axis of a low friction spindle. The vanes are white on one side and black on the other. 

How Does A Radiometer Work?

When exposed to any form of light (even the heat of a hand nearby may be enough), the vanes turn without any visible means, the dark sides being pushed from the source. Cooling the radiometer causes rotation in the opposite direction. The more radiation it is exposed to, the faster it will spin, giving you a good way to measure the electromagnetic radiation.

When a source of electromagnetic radiation is pointed at a Crookes Radiometer, it becomes a heat engine (a system which turns heat into mechanical energy, movement). The black side of the vane becomes hotter than the other side, as the radiation heats the black side faster than the white side. Due to the change of heat and resulting change in pressure the vanes start to spin on the spindle. The more radiation the source gives off the faster the vanes will spin. The Science museum also has one of the first ever Radiometers made by Crookes himself!

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