Crookes radiometer

instrument

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July 23, 2026

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The Crookes radiometer is an instrument invented by Sir William Crookes in the late 1800s to measure radiant energy, particularly infrared radiation. It consists of a glass bulb with a partial vacuum and a freely rotating rotor inside. This rotor has four arms, each with a metal vane. One side of each vane is blackened to absorb radiant energy, while the other is polished to reflect it. When light strikes the blackened sides, they heat up, causing the air near them to expand and exert pressure. This pressure differential makes the rotor spin, converting light radiation into rotary motion. While not a precise scientific instrument today, it was a precursor to modern, more accurate radiometers.12

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gas
Free-molecule gas
A radiometer is a four-vaned mill that depends essentially on free-molecule effects. A temperature difference in the free-molecule gas causes a thermomolecular pressure difference that drives the vanes. The radiometer will stop spinning if enough air leaks into its glass envelope. (It will also stop spinning…
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A radiometer is a four-vaned mill that depends essentially on free-molecule effects. A temperature difference in the free-molecule gas causes a thermomolecular pressure difference that drives the vanes. The radiometer will stop spinning if enough air leaks into its glass envelope. (It will also stop spinning…
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radiometer
A Crookes radiometer consists of a glass bulb from which most of the air has been removed, thereby creating a partial vacuum, and a rotor that is mounted on a vertical support inside the bulb. The rotor bears four light, horizontal arms mounted at right angles…
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A Crookes radiometer consists of a glass bulb from which most of the air has been removed, thereby creating a partial vacuum, and a rotor that is mounted on a vertical support inside the bulb. The rotor bears four light, horizontal arms mounted at right angles…
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Sir William Crookes
…discovered the principle of the Crookes radiometer, a device that converts light radiation into rotary motion. The principle of this radiometer has found numerous applications in the development of sensitive measuring instruments. Crookes was knighted in 1897.
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…discovered the principle of the Crookes radiometer, a device that converts light radiation into rotary motion. The principle of this radiometer has found numerous applications in the development of sensitive measuring instruments. Crookes was knighted in 1897.
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