The
Crookes radiometer, also known as the
light mill, consists of an airtight glass bulb, containing a partial
vacuumIn everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty," but in reality, no volume of space can ever be perfectly empty...
. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity. The reason for the rotation has historically been a cause of much scientific debate.
It was invented in 1873 by the chemist Sir William Crookes as the by-product of some chemical research.
The
Crookes radiometer, also known as the
light mill, consists of an airtight glass bulb, containing a partial
vacuumIn everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty," but in reality, no volume of space can ever be perfectly empty...
. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity. The reason for the rotation has historically been a 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 as a novelty item.
General description
The
radiometerA radiometer is a device for measuring the radiant flux of electromagnetic radiation. Generally, the term “radiometer” denotes an infrared radiation detector, yet it also comprises detectors operating on any electromagnetic wavelength, e.g...
is made from a glass bulb from which much of the air has been removed to form a partial
vacuumIn everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty," but in reality, no volume of space can ever be perfectly empty...
. Inside the bulb, on a low
frictionFriction is the force resisting the relative lateral motion of solid surfaces, fluid layers, or material elements in contact. It is usually subdivided into several varieties:...
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
sunlightSunlight, in the broad sense, is the total spectrum of the electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the atmosphere, and the solar radiation is obvious as daylight when the Sun is above the horizon. Near the poles in summer, the days are longer and the...
, artificial light, or
infraredInfrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves...
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.
The effect begins to be seen at partial vacuum pressures of a few
mmThe millimetre is a unit of length in the metric system, equal to one thousandth of a metre, which is the current SI base unit of length.Equal to 1000 micrometres.Equal to 1000000 nanometres....
of mercury (
torrThe torr is a non-SI unit of pressure defined as of a standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimeter of mercury, i.e. a pressure of 1 Torr is approximately equal to 1 mmHg...
) , reaches a peak at around 10
−2 torr and has disappeared by the time the vacuum reaches 10
−6 torr (see explanations note 1). At these very high vacuums the effect of photon
radiation pressureRadiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
on the vanes can be observed in very sensitive apparatus (see
Nichols radiometerA Nichols radiometer is the apparatus used by Ernest Fox Nichols and Gordon Ferrie Hull in 1901 for the measurement of radiation pressure. It consisted of a pair of small silvered glass mirrors suspended in the manner of a torsion balance by a fine quartz fibre within an enclosure in which the air...
) but this is insufficient to cause rotation.
The word-element "radio-" in the title originates from the combining form of Latin
radius, a ray. Here it refers to
electromagnetic radiationElectromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation...
. A Crookes radiometer, consistent with the word-element "meter" in its title, can provide a quantitative measurement of electromagnetic radiation intensity. This can be done, for example, by visual means (e.g., a spinning slotted disk, which functions as a simple
stroboscopeA stroboscope, also known as a strobe, is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. The principle is used for the study of rotating, reciprocating, oscillating or vibrating objects...
) without interfering with the measurement itself.
Radiometers are now commonly sold worldwide as a novelty ornament; needing no batteries, but only light to get the vanes to turn. They come in various forms, such as the one pictured, and are often used in
science museumA science museum or a science centre is a museum devoted primarily to science. Older science museums tended to concentrate on static displays of objects related to natural history, paleontology, geology, industry and industrial machinery, etc. Modern trends in museology have broadened the range of...
s to illustrate "
radiation pressureRadiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
" – a scientific principle that they do not in fact demonstrate.
External radiant source motion
When a
radiant energyRadiant energy is the energy of electromagnetic waves. The quantity of radiant energy may be calculated by integrating radiant flux with respect to time and, like all forms of energy, its SI unit is the joule. The term is used particularly when radiation is emitted by a source into the...
source is directed at a Crookes radiometer, the radiometer becomes a
heat engineA heat engine is a physical or theoretical device that converts thermal energy to mechanical output. The mechanical output is called work, and the thermal energy input is called heat. Heat engines typically run on a specific thermodynamic cycle...
. The operation of a heat engine is based on a difference in
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
that is converted to a mechanical output. 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
moleculeA molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense...
s are "heated up" (i.e. experience an increase in their speed) when they touch the black side of the vane. The details of exactly how this moves the hotter side of the vane forward are given in the section below.
The internal temperature rises as the black vanes impart heat to the air molecules, but the molecules are cooled again when they touch the bulb's glass surface, which is at ambient temperature. This heat loss through the glass keeps the internal bulb temperature steady so that the two sides of the vanes can 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. The two sides of each vane must be thermally insulated to some degree so that the silver or white side does not immediately reach the temperature of the black side. If the vanes are made of metal, then the black or white paint can be the insulation. The glass stays much closer to ambient temperature than the temperature reached by the black side of the vanes. The higher external air pressure helps conduct heat away from the glass.
The air pressure inside the bulb needs to strike a balance between too low and too high. A strong vacuum inside the bulb does not permit motion, because there are not enough air molecules to cause the air currents that propel the vanes and transfer heat to the outside before both sides of each vane reach thermal equilibrium by heat conduction through the vane material. High inside pressure inhibits motion because the temperature differences are not enough to push the vanes through the higher concentration of air: there is too much air resistance for "eddy currents" to occur, and any slight air movement caused by the temperature difference is damped by the higher pressure before the currents can "wrap around" to the other side.
Motion without external radiation
When heating the radiometer in the absence of a light source, it turns in the forward direction (i.e. the black sides trailing). You can place your hands around but not quite touching the glass and it will turn slowly or not at all, but if you touch the glass to warm it quickly, it will turn more noticeably. The directly heated glass gives off enough infrared radiation to turn the vanes, but if the hands are not touching the glass, the glass blocks much of the far-infrared radiation. Near-infrared and visible light more easily penetrate the glass.
If you cool the glass quickly in the absence of a strong light source by placing ice on the glass or in the freezer with the door most of the way closed, it turns backwards (i.e. the silver sides are trailing). This demonstrates black-body radiation from the black sides of the vanes rather than black-body absorption. It turns backwards because the black sides give off more heat and cool more quickly than the other side.
The rotation lasts only as long as the temperature of the glass is increasing or decreasing fast enough to overcome the friction of the spindle and faster than the temperature conduction through the vanes can cause the two sides of the vanes to reach equal temperature.
Explanations for the force on the vanes
Over the years, there have been many attempts to explain how a Crookes radiometer works:
- Crookes incorrectly suggested that the force was due to the pressure of light
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
. This theory was originally supported by James Clerk MaxwellJames Clerk Maxwell was a Scottish theoretical physicist and mathematician. His most significant achievement was the development of the classical electromagnetic theory, synthesizing all previous unrelated observations, experiments and equations of electricity, magnetism and even optics into a...
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 Sir Franz Arthur Friedrich Schuster FRS was a versatile German-born British physicist known for his work in spectroscopy, electrochemistry, optics, X-radiography and the application of harmonic analysis to physics...
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 LebedevPyotr Nikolaevich Lebedev was a prominent Russian physicist.He made his doctoral degree in Strasbourg under the supervision of August Kundt in 1887–1891. In 1891 he started working in Moscow State University in the group of Alexander Stoletov. There he made his famous experimental studies of...
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 photonIn physics, a photon is an elementary particle, the quantum of the electromagnetic field and the basic "unit" of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s 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 is far too small to move these vanes but can be measured with devices such as the Nichols radiometerA Nichols radiometer is the apparatus used by Ernest Fox Nichols and Gordon Ferrie Hull in 1901 for the measurement of radiation pressure. It consisted of a pair of small silvered glass mirrors suspended in the manner of a torsion balance by a fine quartz fibre within an enclosure in which the air...
.
- 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.
- A partial explanation is that gas molecule
A molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense...
s hitting the warmer side of the vane will pick up some of the heat, bouncing 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 while the faster moving molecules produce more force, they also do a better job of stopping other molecules from reaching the vane, so the net force on the vane should be exactly the same — the greater temperature causes a decrease in local density which results in the same force on both sides. Years after this explanation was dismissed, Albert EinsteinAlbert Einstein was a theoretical physicist. His many contributions to physics include the special and general theories of relativity, the founding of relativistic cosmology, the first post-Newtonian expansion, explaining the perihelion advance of Mercury, prediction of the deflection of...
showed that the two pressures do not cancel out exactly at the edges of the vanes because of the temperature difference there. The force predicted by Einstein would be enough to move the vanes, but not fast enough.
- The final piece of the puzzle, thermal transpiration, was theorized by Osborne Reynolds
Osborne Reynolds was a prominent innovator in the understanding of fluid dynamics. Separately, his studies of heat transfer between solids and fluids brought improvements in boiler and condenser design.-Life:...
, but first published by James Clerk MaxwellJames Clerk Maxwell was a Scottish theoretical physicist and mathematician. His most significant achievement was the development of the classical electromagnetic theory, synthesizing all previous unrelated observations, experiments and equations of electricity, magnetism and even optics into a...
in the last paper before his death in 1879. Reynolds found that if a porous plate is kept hotter on one side than the other, the interactions between gas molecules and the plates are such that gas will flow through from the cooler to the hotter side. The vanes of a typical Crookes radiometer are not porous, but the space past their edges behaves like the pores in Reynolds's plate. On average, the gas molecules move from the cold side toward the hot side whenever the pressure ratio is less than the square root of the (absolute) temperature ratio. The pressure difference causes the vane to move cold (white) side forward.
Both Einstein's and Reynolds's forces appear to cause a Crookes radiometer to rotate, although it still isn't clear which one is stronger.
External links