Crookes radiometer

The Crookes radiometer , also known as the light mill or solar engine, consists of an airtight glass bulb with a set of vanes inside 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.

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.

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Crookes Radiometer

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.

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.

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.

Attempted explanations

Over the years, there have been many attempts to explain how a Crookes radiometer works:

1. Crookes incorrectly suggested that the force was due to the pressure of light. This theory was originally supported by James Clerk Maxwell who had predicted this force. This explanation is still often seen in leaflets packaged with the device. The first experiment to disprove this theory was done by Arthur Schuster in 1876, who observed that there was a force on the glass bulb of the Crookes radiometer that was in the opposite direction to the rotation of the vanes. This showed that the force turning the vanes was generated inside the radiometer. If light pressure was the cause of the rotation, then the better the vacuum in the bulb, the less air resistance to movement, and the faster the vanes should spin. In 1901, with a better vacuum pump, Pyotr Lebedev showed that in fact, the radiometer only works when there is low pressure gas in the bulb, and the vanes stay motionless in a hard vacuum. Finally, if light pressure were the motive force, the radiometer would spin in the opposite direction as the photons on the shiny side being reflected would deposit more momentum than on the black side where the photons are absorbed. The actual pressure exerted by light, though it exists, and can be measured with devices such as the Nichols radiometer, is far too small to move these vanes.

2. Another incorrect theory was that the heat on the dark side was causing the material to outgas, which pushed the radiometer around. This was effectively disproved by both Schuster's and Lebedev's experiments.

3. A partial explanation is that gas molecules hitting the warmer side of the vane will pick up some of the heat i.e. will bounce off the vane with increased speed. Giving the molecule this extra boost effectively means that a minute pressure is exerted on the vane. The imbalance of this effect between the warmer black side and the cooler silver side means the net pressure on the vane is equivalent to a push on the black side, and as a result the vanes spin round with the black side trailing. The problem with this idea is that the faster moving molecules produce more force, they also do a better job of stopping other molecules from reaching the vane, so the force on the vane should be exactly the same -- the greater temperature causes a decrease in density which results in the same force on both sides. Years after this explanation was dismissed, Albert Einstein showed that, because of the temperature difference at the edges, the two pressures do not cancel out exactly at the edges of the vanes. The force predicted by Einstein would be enough to move the vanes, but not fast enough.

4. The final piece of the puzzle, thermal transpiration, was theorized by Osborne Reynolds, but first published by James Clerk Maxwell in the last paper before his death in 1879. In addition to the temperature differential at the edges that Einstein considered, there is an additional factor due to how the gas interacts with the surface. Reynolds original work was on how a porous plate could be used to pump gas by heating one side and cooling the other. Reynolds found that the fast moving molecules could more easily move through a pore in a plate, or more easily move over a surface. Maxwell showed how this would cause an air current within a Crookes radiometer. On the black side, the slower moving molecules at the edges get caught in the microscopic bumps on the surface, while the faster moving molecules in the center can skip over the bumps. This causes a slight outward air flow, and creates a lower pressure in the center of the black face. Similarly on the white face, there is a slight inward air flow and a corresponding higher pressure.

Both Einstein's and Reynolds' force appear to cause a Crookes radiometer to rotate, although it still isn't clear which one is stronger. [See discussion.]


  • Loeb, Leonard B. (1934) The Kinetic Theory Of Gases (2nd Edition);McGraw-Hill Book Company; pp 353-386
  • Kennard, Earle H. (1938) Kinetic Theory of Gases; McGraw-Hill Book Company; pp 327-337

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