Jump to navigation Jump to search An example of a Crookes radiometer. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity. A radiometer or roentgenometer is a device for measuring the radiant flux power of electromagnetic radiation. Generally, a radiometer is an infrared radiation detector or an ultraviolet detector. While the term radiometer can refer to any device that measures electromagnetic radiation e. A common belief one originally held even by Crookes is that the momentum of the absorbed light on the black faces makes the radiometer operate.

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See Article History Radiometer, instrument for detecting or measuring radiant energy. The term is applied in particular to devices used to measure infrared radiation.

Radiometers are of various types that differ in their method of measurement or detection. Commonly used thermal detectors include the thermocouple , which produces a voltage when heated, and the bolometer , which undergoes a change in electrical resistance when heated.

The photoelectric cell is the basis of many current quantum detectors. The term radiometer is often used to refer specifically to a type of detector invented by Sir William Crookes in the late s.

It is rarely used as a scientific instrument, because it was found to be insensitive and not easily calibrated , but it paved the way for the more exact instruments in use today. 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 to one another on a central pivot; the rotor can turn freely in the horizontal plane.

At the outer end of each arm is mounted a vertical metal vane. Each vane has one side polished and the other side blackened; the vanes are arranged so that the polished side of one faces the blackened side of the next.

When radiant energy strikes the polished surfaces, most of it is reflected away, but when it strikes the blackened surface, most of it is absorbed, raising the temperature of those surfaces. The air near a blackened surface is thus heated and exerts a pressure on the blackened surface, causing the rotor to turn.

Learn More in these related Britannica articles:.


How does a Crookes' radiometer work?

A Crookes radiometer in action with the light switched on and off. Movement with black-body absorption[ edit ] When a radiant energy source is directed at a Crookes radiometer, the radiometer becomes a heat engine. In this case, the black side of the vane becomes hotter than the other side, as radiant energy from a light source warms the black side by black-body absorption faster than the silver or white side. The internal air molecules are heated up when they touch the black side of the vane. The details of exactly how this moves the warmer side of the vane forward are given in the section below. This heat loss through the glass keeps the internal bulb temperature steady with the result that the two sides of the vanes develop a temperature difference. The white or silver side of the vanes are slightly warmer than the internal air temperature but cooler than the black side, as some heat conducts through the vane from the black side.



How Does Crookes Radiometer What is Crookes Radiometer? How does it work? Have you seen a light bulb shaped, paper thin glass envelope that contains what looks like a weather vane with four diamond shaped metal leaves inside? Black on one side and white or silvered on the other side? When someone takes it from a shelf and puts it in the window, it spins in the sun. It is named for Sir William Crookes who first devised it in


Crookes radiometer


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