Crookes radiometer

A light mill ( also referred to as Lichtrad or radiometer ) is a glass ball inside which a movable vane is provided with several one-sided blackened flakes. When light falls, the wheel begins to turn. The most decorative purposes serving apparatus was invented in 1873 by William Crookes.

Construction

There is usually a light mill of a four-arm impeller that is mounted easily rotatable by means of a glass Hütchens on a needle tip. Each of the existing wire arms carries at its end a vertically excepted annealed mica platelets (if mirrored ), wherein one side is blackened with carbon black, and such that the sooted surfaces are all facing the same direction of rotation.

The structure is enclosed in a hollow glass sphere of five to six inches in diameter. From above, stands a glass tube into the ball, which prevents the fall of the impeller.

The glass bulb is about 5 Pascal (ie 0.05 millibar ) evacuated and then sealed. Light mills work either in a high vacuum or at atmospheric pressure.

Putting the light mill light or heat radiation, so the wheel at a rate dependent on the intensity of radiation speed turns, wherein the non- blackened faces precede.

To observe a rotational motion, friction and air resistance must be very low. This is accomplished by the vacuum in the interior of the glass ball, and by low friction bearing of the rotor.

A light mill only works if the blackened side can absorb energy, is well thermally isolated from the bright side and thus heats up. A good light mill rotates in the sunlight quickly, but also moves in weak sunlight slowly, while a room lighting for example fluorescent tubes usually not sufficient. Since the sensitivity in the infrared region is large, but sufficient candles, flashlights or even hands to rotate the wings slowly.

History of the attempts to explain

To explain the cause of the rotation of different physical principles have been used over time.

Crookes believed initially that the rotational motion due to the different radiation pressure (on the light side of the light energy is reflected, absorbed in the dark ) is formed. A more accurate analysis ( for example by James Clerk Maxwell ) showed, however, that this effect is too small, also would cause rotation of the blackened side first, which would be opposed to the observed direction.

Another refute the radiation pressure theory achieved by experiments, which showed that between the impeller and the glass envelope takes place interaction and thus can not come from an external force, the movement. If we let namely a light mill, the impeller is provided with a slight magnetic bar, swim in water and keeps the rotary motion of the wheel by an approximated from the outside magnet on, the glass envelope rotates when irradiated in the opposite direction.

The radiation -pressure hypothesis can be easily refuted by storing the impeller in a vacuum. Due to the now omitted air resistance would be expected that the blades rotate faster now. However, the pressure has an optimum level is too low internal pressure no movement takes place.

The temperature dependence of the rotational movement is further evidence against the radiation pressure - the direction of rotation of the light mill depends on the radiation balance inside and thus also on the temperature difference between the inside and the outside: An unirradiated, stationary light mill begins in the reverse direction, with the black areas above, to turn when you put them in a container with cold water - the most blackened with soot surfaces take account of their better also in the mid-infrared emissivity while a lower temperature than the bright areas.

Another explanation of the operation was published in 1879 by the British engineer Osborne Reynolds. He explained the movement with a temperature difference between the warm black and white cold surface and the associated flow of gas leads for immovable space to a pressure difference. If this air flowed toward the outer edges of the blades spread the warmer, faster molecules at a greater angle than the edges of the cooler, which exaggerate the blades in the direction facing away from the dark area.

There is a wealth of other explanations for the rotational movement, some of which at least provide a contribution to the movement, however, can not be the main cause. The effects are relied

• Outgassing black coatings,
• The photoelectric effect,
• The convection.

Current Statement

The gas pressure in the interior of the glass sphere is low - only one to ten Pascal - to the free path, that is the average distance traveled by a gas molecule between two collisions is on the order of millimeters. For a pressure of 5 pascals the mean free path is as high as 1.4 millimeters. So you have better described with the law instead of pulse flow, convection or thermal expansion behavior of the gas molecules inside. The interaction of gas molecules with one another is too low for these phenomena.

The thermal motion of the gas molecules inside leads unlighted impeller and thermal equilibrium statistically the same number of collisions at the bright and the dark wing surfaces and the glass wall. Upon irradiation, the soot-covered surfaces heat up, and their molecules and atoms result in a stronger motion ( Brownian motion ) from. Meeting now gas molecules to rapidly vibrating particles of the warm side, they get a stronger pulse when flying away. The force equilibrium of the wing is no longer given and the black side experiences by the momentum conservation law a repulsive force in the opposite direction of the departing gas particle.

With this theory can be observed all dependencies such as optimum gas pressure, possible poor heat conducting plates as well as the counter- impulse to explain to the glass vessel.

Also taking place during cooling of the glass vessel reversing the direction of rotation of the unirradiated mill, with the black areas above can be explained this way. A non-irradiated stationary light mill starts to rotate in the reverse direction when it is liberated, for example, in a container with cold water. The most blackened with soot surfaces will then due to their higher not only in the visible range of light, but also in the mid-infrared emissivity at a lower temperature than the bright areas. You lose heat energy by radiation in the disturbed equilibrium of radiation inside the sphere, the glass wall radiates back less than they absorbed. Thereby, the brighter side of the wing to the " drive side ", since it has a higher temperature, where the gas molecules get a stronger pulse.