Crookes radiometer

The Crookes radiometer, also known as the light mill or solar engine, consists of an airtight glass bulb, containing a partial vacuum. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light. The reason for the rotation has been the cause of much scientific debate.

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It was invented in 1873 by the chemist Sir William Crookes as the by-product of some chemical research. In the course of very accurate quantitative chemical work, he was weighing samples in a partially evacuated chamber to reduce the effect of air currents, and noticed the weighings were disturbed when sunlight shone on the balance. Investigating this effect, he created the device named after him. It is still manufactured and sold to this day as a curiosity item.

Related Topics:
1873 - Sir William Crookes

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The radiometer is made from a glass bulb from which much of the air has been removed to form a partial vacuum. Inside the bulb, on a low friction spindle, is a rotor with several (usually four) vertical lightweight metal vanes spaced equally around the axis. The vanes are polished or white on one side, black on the other. When exposed to sunlight, artificial light, or infrared radiation (even the heat of a hand nearby can be enough), the vanes turn with no apparent motive power, the dark sides retreating from the radiation source and the light sides advancing. Cooling the radiometer causes rotation in the opposite direction.

Related Topics:
Vacuum - Friction - Sunlight - Infrared

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Although it has the word-element "meter" in its title, a radiometer cannot be used to measure anything by itself. A measurement of the speed of its rotation can, however, be obtained using a spinning slotted disk, which functions as a strobe light does. A strobe light itself would distort the measurement.

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Radiometers are now commonly sold worldwide as an interesting household novelty ornament, no batteries needed, just light to get the vanes to turn; strong light gets them spinning furiously. They come in various forms, as the one pictured, and are also to be found often used in science museums to illustrate the hidden power of light and heat.

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