Crookes radiometer

The Crookes radiometer, also known as a light mill, 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, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity. The reason for the rotation was a cause of much scientific debate in the ten years following the invention of the device,[1][2] but in 1879 the currently accepted explanation for the rotation was published.[3][4] Today the device is mainly used in physics education as a demonstration of a heat engine run by light energy.

It was invented in 1873 by the chemist Sir William Crookes as the by-product of some chemical research. In the course(ilys) 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, generally as an educational aid or as a curiosity.

Crookes radiometer
Crookes radiometer

General description

Radiometer 9965 Nevit
A Crookes radiometer in action

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 vanes spaced equally around the axis. The vanes are polished or white on one side and 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.

Effect observations

The effect begins to be observed at partial vacuum pressures of a few torr (several hundred pascals), reaches a peak at around 10−2 torr (1 pascal) and has disappeared by the time the vacuum reaches 10−6 torr (10−4 pascal) (see explanations note 1). At these very high vacuums the effect of photon radiation pressure on the vanes can be observed in very sensitive apparatus (see Nichols radiometer) but this is insufficient to cause rotation.

Origin of the name

  • The prefix "radio-" in the title originates from the combining form of Latin radius, a ray: here it refers to electromagnetic radiation.
  • A Crookes radiometer, consistent with the suffix "- 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 stroboscope) 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 museums to illustrate "radiation pressure" – a scientific principle that they do not in fact demonstrate.

Thermodynamic explanation

A Crookes radiometer in action with the light switched on and off.(note that the explanation given in the caption to the clip doesn't agree with the explanation below)

Movement with black-body absorption

When a radiant energy source is directed at a Crookes radiometer, the radiometer becomes a heat engine. The operation of a heat engine is based on a difference in temperature 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 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.

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 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. The two sides of each vane must be thermally insulated to some degree so that the polished 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 external air 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.

Movement with black-body radiation

When the radiometer is heated in the absence of a light source, it turns in the forward direction (i.e. black sides trailing). If a person's hands are placed around the glass without touching it, the vanes will turn slowly or not at all, but if the glass is touched to warm it quickly, they will turn more noticeably. Directly heated glass gives off enough infrared radiation to turn the vanes, but glass blocks much of the far-infrared radiation from a source of warmth not in contact with it. However, near-infrared and visible light more easily penetrate the glass.

If the glass is cooled quickly in the absence of a strong light source by putting ice on the glass or placing it in the freezer with the door almost closed, it turns backwards (i.e. the silver sides trail). This demonstrates black-body radiation from the black sides of the vanes rather than black-body absorption. The wheel turns backwards because the net exchange of heat between the black sides and the environment initially cools the black sides faster than the white sides. Upon reaching equilibrium, typically after a minute or two, reverse rotation ceases. This contrasts with sunlight, with which forward rotation can be maintained all day.

Explanations for the force on the vanes

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.[5] 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 test 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 were 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.[6] 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. This results from conservation of momentum - the momentum of the reflected photon exiting on the light side must be matched by a reaction on the vane that reflected it. The actual pressure exerted by light is far too small to move these vanes but can be measured with devices such as the Nichols radiometer.
  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[7] and Lebedev's experiments.[6]
  3. A partial explanation is that gas molecules 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 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 Einstein 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.
  4. The final piece of the puzzle, thermal transpiration, was theorized by Osborne Reynolds[8] in an unpublished paper that was refereed by Maxwell, who then published his paper which contained a critique of the mathematics in Reynolds's unpublished paper.[9] Maxwell died that year and the Royal Society refused to publish Reynolds's critique of Maxwell's rebuttal to Reynolds's unpublished paper, as it was felt that this would be an inappropriate argument when one of the people involved had already died.[3] 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 due to the tangential force of the movement of the rarefied gas moving from the colder edge to the hotter edge.[3]

All-black light mill

To rotate, a light mill does not have to be coated with different colors across each vane. In 2009, researchers at the University of Texas, Austin created a monocolored light mill which has four curved vanes; each vane forms a convex and a concave surface. The light mill is uniformly coated by gold nanocrystals, which are a strong light absorber. Upon exposure, due to geometric effect, the convex side of the vane receives more photon energy than the concave side does, and subsequently the gas molecules receive more heat from the convex side than from the concave side. At rough vacuum, this asymmetric heating effect generates a net gas movement across each vane, from the concave side to the convex side, as shown by the researchers' Direct Simulation Monte Carlo (DSMC) modeling. The gas movement causes the light mill to rotate with the concave side moving forward, due to Newton's Third Law. This monocolored design promotes the fabrication of micrometer- or nanometer- scaled light mills, as it is difficult to pattern materials of distinct optical properties within a very narrow, three-dimensional space.[10][11]

Nanoscale light mill

In 2010 researchers at the University of California, Berkeley succeeded in building a nanoscale light mill that works on an entirely different principle to the Crookes radiometer. A gold light mill, only 100 nanometers in diameter, was built and illuminated by laser light that had been tuned. The possibility of doing this had been suggested by the Princeton physicist Richard Beth in 1936. The torque was greatly enhanced by the resonant coupling of the incident light to plasmonic waves in the gold structure.[12]

See also


  1. ^ Worrall, J. (1982), "The pressure of light: The strange case of the vacillating 'crucial experiment'", Studies in History and Philosophy of Science, 13 (2): 133–171, doi:10.1016/0039-3681(82)90023-1
  2. ^ ilys The Electrical engineer Check |url= value (help), London: Biggs & Co., 1884, p. 158
  3. ^ a b c Gibbs, Philip (1996). "How does a light-mill work?". Usenet Physics FAQ. Retrieved 8 August 2014.
  4. ^ "Light-Mills discussion; The n-Category Cafe". Retrieved 29 April 2017.
  5. ^ Crookes, William (1 January 1874). "On Attraction and Repulsion Resulting from Radiation". Philosophical Transactions of the Royal Society of London. 164: 501–527. doi:10.1098/rstl.1874.0015..
  6. ^ a b Lebedew, Peter (1901). "Untersuchungen über die Druckkräfte des Lichtes". Annalen der Physik. 311 (11): 433–458. Bibcode:1901AnP...311..433L. doi:10.1002/andp.19013111102.
  7. ^ Brush, S. G., and C. W. F. Everitt. “Maxwell, Osborne Reynolds, and the Radiometer.” Historical Studies in the Physical Sciences, vol. 1, 1969, pp. 105–125.
  8. ^ Reynolds, Osborne (1 January 1879). "On certain dimensional properties of matter in the gaseous state …". Philosophical Transactions of the Royal Society of London. 170: 727–845. doi:10.1098/rstl.1879.0078.; Part 2.
  9. ^ Maxwell, J. Clerk (1 January 1879). "On stresses in rarefied gases arising from inequalities of temperature". Philosophical Transactions of the Royal Society of London. 170: 231–256. doi:10.1098/rstl.1879.0067.
  10. ^ Han, Li-Hsin; Shaomin Wu; J. Christopher Condit; Nate J. Kemp; Thomas E. Milner; Marc D. Feldman; Shaochen Chen (2010). "Light-Powered Micromotor Driven by Geometry-Assisted, Asymmetric Photon-heating and Subsequent Gas Convection". Applied Physics Letters. 96 (21): 213509(1–3). Bibcode:2010ApPhL..96u3509H. doi:10.1063/1.3431741. Archived from the original on 22 July 2011.
  11. ^ Han, Li-Hsin; Shaomin Wu; J. Christopher Condit; Nate J. Kemp; Thomas E. Milner; Marc D. Feldman; Shaochen Chen (2011). "Light-Powered Micromotor: Design, Fabrication, and Mathematical Modeling". Journal of Microelectromechanical Systems. 20 (2): 487–496. doi:10.1109/JMEMS.2011.2105249.
  12. ^ Yarris, Lynn. "Nano-sized light mill drives micro-sized disk". Physorg. Retrieved 6 July 2010.
General information
  • 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
  • US 182172, Crookes, William, "Improvement In Apparatus For Indicating The Intensity Of Radiation", issued 12 September 1876

External links

Crookes (disambiguation)

Crookes is a suburb of Sheffield, England; also:.

Crookes may also refer to:

Crookes (ward), an electoral ward in Sheffield, England

Crookes Cemetery, a cemetery in Sheffield

Crookes Valley Park, a public park in Sheffield

Crookes (crater), a lunar crater

The Crookes, a pop music band

Crookes tube, an experimental electrical discharge tube

Crookes radiometer

The Crookes (film), a 1974 Iranian Persian-genre crime film

Crookes tube

A Crookes tube (also Crookes–Hittorf tube) is an early experimental electrical discharge tube, with partial vacuum, invented by English physicist William Crookes and others around 1869-1875, in which cathode rays, streams of electrons, were discovered.Developed from the earlier Geissler tube, the Crookes tube consists of a partially evacuated glass bulb of various shapes, with two metal electrodes, the cathode and the anode, one at either end. When a high voltage is applied between the electrodes, cathode rays (electrons) are projected in straight lines from the cathode. It was used by Crookes, Johann Hittorf, Julius Plücker, Eugen Goldstein, Heinrich Hertz, Philipp Lenard, Kristian Birkeland and others to discover the properties of cathode rays, culminating in J.J. Thomson's 1897 identification of cathode rays as negatively charged particles, which were later named electrons. Crookes tubes are now used only for demonstrating cathode rays.

Wilhelm Röntgen discovered X-rays using the Crookes tube in 1895. The term Crookes tube is also used for the first generation, cold cathode X-ray tubes, which evolved from the experimental Crookes tubes and were used until about 1920.

Index of physics articles (C)

The index of physics articles is split into multiple pages due to its size.

To navigate by individual letter use the table of contents below.

Index of solar energy articles

This is a list of solar energy topics.

Infrared astronomy

Infrared astronomy is the branch of astronomy and astrophysics that studies astronomical objects visible in infrared (IR) radiation. The wavelength of infrared light ranges from 0.75 to 300 micrometers. Infrared falls in between visible radiation, which ranges from 380 to 750 nanometers, and submillimeter waves.

Infrared astronomy began in the 1830s, a few decades after the discovery of infrared light by William Herschel in 1800. Early progress was limited, and it was not until the early 20th century that conclusive detections of astronomical objects other than the Sun and Moon were made in infrared light. After a number of discoveries were made in the 1950s and 1960s in radio astronomy, astronomers realized the information available outside the visible wavelength range, and modern infrared astronomy was established.

Infrared and optical astronomy are often practiced using the same telescopes, as the same mirrors or lenses are usually effective over a wavelength range that includes both visible and infrared light. Both fields also use solid state detectors, though the specific type of solid state detectors used are different. Infrared light is absorbed at many wavelengths by water vapor in the Earth's atmosphere, so most infrared telescopes are at high elevations in dry places, above as much of the atmosphere as possible. There are also infrared observatories in space, including the Spitzer Space Telescope and the Herschel Space Observatory.


Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is the visible spectrum that is visible to the human eye and is responsible for the sense of sight. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), or 4.00 × 10−7 to 7.00 × 10−7 m, between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths). This wavelength means a frequency range of roughly 430–750 terahertz (THz).

The main source of light on Earth is the Sun. Sunlight provides the energy that green plants use to create sugars mostly in the form of starches, which release energy into the living things that digest them. This process of photosynthesis provides virtually all the energy used by living things. Historically, another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. With the development of electric lights and power systems, electric lighting has effectively replaced firelight. Some species of animals generate their own light, a process called bioluminescence. For example, fireflies use light to locate mates, and vampire squids use it to hide themselves from prey.

The primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, and polarization, while its speed in a vacuum, 299,792,458 metres per second, is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation (EMR), is experimentally found to always move at this speed in a vacuum.In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays, microwaves and radio waves are also light. Like all types of EM radiation, visible light propagates as waves. However, the energy imparted by the waves is absorbed at single locations the way particles are absorbed. The absorbed energy of the EM waves is called a photon, and represents the quanta of light. When a wave of light is transformed and absorbed as a photon, the energy of the wave instantly collapses to a single location, and this location is where the photon "arrives." This is what is called the wave function collapse. This dual wave-like and particle-like nature of light is known as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics.

Nichols radiometer

A Nichols radiometer was 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 pressure could be regulated. The torsion head to which the fiber was attached could be turned from the outside using a magnet. A beam of light was directed first on one mirror and then on the other, and the opposite deflections observed with mirror and scale. By turning the mirror system around to receive the light on the unsilvered side, the influence of the air in the enclosure could be ascertained. This influence was found to be of almost negligible value at an air pressure of about 16 mmHg (2.1 kPa). The radiant energy of the incident beam was deduced from its heating effect upon a small blackened silver disk, which was found to be more reliable than the bolometer when it was first used. With this apparatus, the experimenters were able to obtain an agreement between observed and computed radiation pressures within about 0.6%. The original apparatus is at the Smithsonian Institution.This apparatus is sometimes confused with the Crookes radiometer of 1873.

Novelty item

A novelty item is an object which is specifically designed to serve no practical purpose, and is sold for its uniqueness, humor, or simply as something new (hence "novelty", or newness). The term also applies to practical items with fanciful or nonfunctional additions, such as novelty slippers. The term is normally applied to small objects, and is generally not used to describe larger items such as roadside attractions. Items may have an advertising or promotional purpose, or be a souvenir.

Perpetual motion

Perpetual motion is motion of bodies that continues indefinitely. A perpetual motion machine is a hypothetical machine that can do work indefinitely without an energy source. This kind of machine is impossible, as it would violate the first or second law of thermodynamics.These laws of thermodynamics apply regardless of the size of the system. For example, the motions and rotations of celestial bodies such as planets may appear perpetual, but are actually subject to many processes that slowly dissipate their kinetic energy, such as solar wind, interstellar medium resistance, gravitational radiation and thermal radiation, so they will not keep moving forever.Thus, machines that extract energy from finite sources will not operate indefinitely, because they are driven by the energy stored in the source, which will eventually be exhausted. A common example is devices powered by ocean currents, whose energy is ultimately derived from the Sun, which itself will eventually burn out. Machines powered by more obscure sources have been proposed, but are subject to the same inescapable laws, and will eventually wind down.

In 2017, new states of matter, time crystals, were discovered in which on a microscopic scale the component atoms are in continual repetitive motion, thus satisfying the literal definition of "perpetual motion". However, these do not constitute perpetual motion machines in the traditional sense or violate thermodynamic laws because they are in their quantum ground state, so no energy can be extracted from them; they have "motion without energy".

Peter Tait (physicist)

Peter Guthrie Tait FRSE (28 April 1831 – 4 July 1901) was a Scottish mathematical physicist and early pioneer in thermodynamics. He is best known for the mathematical physics textbook Treatise on Natural Philosophy, which he co-wrote with Kelvin, and his early investigations into knot theory,

His work on knot theory contributed to the eventual formation of topology as a mathematical discipline. His name is known in graph theory mainly for Tait's conjecture.


Photophoresis denotes the phenomenon that small particles suspended in gas (aerosols) or liquids (hydrocolloids) start to migrate when illuminated by a sufficiently intense beam of light. The existence of this phenomenon is owed to a non-uniform distribution of temperature of an illuminated particle in a fluid medium. Separately from photophoresis, in a fluid mixture of different kinds of particles, the migration of some kinds of particles may be due to differences in their absorptions of thermal radiation and other thermal effects collectively known as thermophoresis. In laser photophoresis, particles migrate once they have a refractive index different from their surrounding medium. The migration of particles is usually possible when the laser is slightly or not focused. A particle with a higher refractive index compared to its surrounding molecule moves away from the light source due to momentum transfer from absorbed and scattered light photons. This is referred to as a radiation pressure force. This force depends on light intensity and particle size but has nothing to do with the surrounding medium. Just like in Crookes radiometer, light can heat up one side and gas molecules bounce from that surface with greater velocity, hence push the particle to the other side. Under certain conditions, with particles of diameter comparable to the wavelength of light, the phenomenon of a negative indirect photophoresis occurs, due to the unequal heat generation on the laser irradiation between the back and front sides of particles, this produces a temperature gradient in the medium around the particle such that molecules at the far side of the particle from the light source may get to heat up more, causing the particle to move towards the light source.If the suspended particle is rotating, it will also experience the Yarkovsky effect.

Discovery of photophoresis is usually attributed to Felix Ehrenhaft in the 1920s, though earlier observations were made by others including Augustin-Jean Fresnel.

Psi wheel

A psi wheel is pyramid-shaped top-like device consisting of a small piece of paper or foil balanced on the tip of a pointed object (such as a toothpick or needle). It is commonly used in attempts to prove the validity of telekinesis, by rotating the wheel using the power of the mind.

Radiation pressure

Radiation pressure is the pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength which is absorbed, reflected, or otherwise emitted (e.g. black body radiation) by matter on any scale (from macroscopic objects to dust particles to gas molecules).The forces generated by radiation pressure are generally too small to be noticed under everyday circumstances; however, they are important in some physical processes. This particularly includes objects in outer space where it is usually the main force acting on objects besides gravity, and where the net effect of a tiny force may have a large cumulative effect over long periods of time. For example, had the effects of the sun's radiation pressure on the spacecraft of the Viking program been ignored, the spacecraft would have missed Mars orbit by about 15,000 km (9,300 mi). Radiation pressure from starlight is crucial in a number of astrophysical processes as well. The significance of radiation pressure increases rapidly at extremely high temperatures, and can sometimes dwarf the usual gas pressure, for instance in stellar interiors and thermonuclear weapons.

Radiation pressure can equally well be accounted for by considering the momentum of a classical electromagnetic field or in terms of the momenta of photons, particles of light. The interaction of electromagnetic waves or photons with matter may involve an exchange of momentum. Due to the law of conservation of momentum, any change in the total momentum of the waves or photons must involve an equal and opposite change in the momentum of the matter it interacted with (Newton's third law of motion), as is illustrated in the accompanying figure for the case of light being perfectly reflected by a surface. This transfer of momentum is the general explanation for what we term radiation pressure.


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. Microwave radiometers operate in the microwave wavelengths.

While the term radiometer can refer to any device that measures electromagnetic radiation (e.g. light), the term is often used to refer specifically to a Crookes radiometer ("light-mill"), a device invented in 1873 in which a rotor (having vanes which are dark on one side, and light on the other) in a partial vacuum spins when exposed to light.

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.

If this were true however, the radiometer would spin away from the non-black faces, since the photons bouncing off those faces impart more momentum than the photons absorbed on the black faces.

Photons do exert radiation pressure on the faces, but those forces are dwarfed by other effects.

The currently accepted explanation depends on having just the right degree of vacuum, and relates to the transfer of heat rather than the direct effect of photons. A Nichols radiometer does demonstrate photon pressure. It is much more sensitive than the Crookes radiometer and it operates in a complete vacuum, whereas operation of the Crookes radiometer requires an imperfect vacuum.

The MEMS radiometer, invented by Patrick Jankowiak, can operate on the principles of Nichols or Crookes and can operate over a wide spectrum of wavelength and particle energy levels.

Sir George Stokes, 1st Baronet

Sir George Gabriel Stokes, 1st Baronet, (; 13 August 1819 – 1 February 1903) was an Anglo-Irish physicist and mathematician. Born in County Sligo, Ireland, Stokes spent all of his career at the University of Cambridge, where he was the Lucasian Professor of Mathematics from 1849 until his death in 1903. As a physicist, Stokes made seminal contributions to fluid dynamics, including the Navier-Stokes equation, and to physical optics, with notable works on polarization and fluorescence. As a mathematician, he formulated "Stokes' theorem" in vector calculus and contributed to the theory of asymptotic expansions.

Stokes was made a baronet (hereditary knight) by the British monarch in 1889. In 1893 he received the Royal Society's Copley Medal, then the most prestigious scientific prize in the world, "for his researches and discoveries in physical science". He represented Cambridge University in the British House of Commons from 1887 to 1892, sitting as a Tory. Stokes also served as president of the Royal Society from 1885 to 1890 and was briefly the Master of Pembroke College, Cambridge.

Solar engine

A solar engine can refer to:

Crookes radiometer, a light mill composed of an airtight glass bulb containing a partial vacuum.

Solar engine, a circuit which stores energy from solar cells in a capacitor and releases it in pulses to animate toys.

Stirling engine, a heat engine of the external combustion piston engine type whose heat-exchange process allows for near-ideal efficiency in conversion of heat into mechanical movement.

Thermal transpiration

Thermal transpiration or thermal diffusion refers to the thermal force on a gas due to a temperature difference. Thermal transpiration causes a flow of gas in the absence of any pressure difference, and is able to maintain a certain pressure difference (called thermomolecular pressure difference) in a steady state. The effect is strongest when the mean free path of the gas molecules is comparable to the dimensions of the gas container.

Thermal transpiration appears as an important correction in the readings of vapor pressure thermometers, and the effect is historically famous as being an explanation for the rotation of the Crookes radiometer.


Vacuum is space devoid of matter. The word stems from the Latin adjective vacuus for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often discuss ideal test results that would occur in a perfect vacuum, which they sometimes simply call "vacuum" or free space, and use the term partial vacuum to refer to an actual imperfect vacuum as one might have in a laboratory or in space. In engineering and applied physics on the other hand, vacuum refers to any space in which the pressure is lower than atmospheric pressure. The Latin term in vacuo is used to describe an object that is surrounded by a vacuum.

The quality of a partial vacuum refers to how closely it approaches a perfect vacuum. Other things equal, lower gas pressure means higher-quality vacuum. For example, a typical vacuum cleaner produces enough suction to reduce air pressure by around 20%. Much higher-quality vacuums are possible. Ultra-high vacuum chambers, common in chemistry, physics, and engineering, operate below one trillionth (10−12) of atmospheric pressure (100 nPa), and can reach around 100 particles/cm3. Outer space is an even higher-quality vacuum, with the equivalent of just a few hydrogen atoms per cubic meter on average in intergalactic space. According to modern understanding, even if all matter could be removed from a volume, it would still not be "empty" due to vacuum fluctuations, dark energy, transiting gamma rays, cosmic rays, neutrinos, and other phenomena in quantum physics. In the study of electromagnetism in the 19th century, vacuum was thought to be filled with a medium called aether. In modern particle physics, the vacuum state is considered the ground state of a field.

Vacuum has been a frequent topic of philosophical debate since ancient Greek times, but was not studied empirically until the 17th century. Evangelista Torricelli produced the first laboratory vacuum in 1643, and other experimental techniques were developed as a result of his theories of atmospheric pressure. A torricellian vacuum is created by filling a tall glass container closed at one end with mercury, and then inverting it in a bowl to contain the mercury (see below).Vacuum became a valuable industrial tool in the 20th century with the introduction of incandescent light bulbs and vacuum tubes, and a wide array of vacuum technology has since become available. The recent development of human spaceflight has raised interest in the impact of vacuum on human health, and on life forms in general.

William Crookes

Sir William Crookes (; 17 June 1832 – 4 April 1919) was a British chemist and physicist who attended the Royal College of Chemistry in London, and worked on spectroscopy. He was a pioneer of vacuum tubes, inventing the Crookes tube which was made in 1875. In 1913, Crookes invented 100% ultraviolet blocking sunglass lens. Crookes was the inventor of the Crookes radiometer, which today is made and sold as a novelty item. Late in life, he became interested in spiritualism, and became the president of the Society for Psychical Research.

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