Light mill
A light mill (also known as a light wheel or radiometer , seldom a sun mill ) is a glass ball with a movable impeller inside, which is provided with several small plates blackened on one side. When light falls, the wheel begins to turn. The apparatus, which is mostly used for decorative purposes, was invented by William Crookes in 1873 .
construction
A light mill usually consists of a four-armed impeller, which is easily rotated by means of a glass cap on a needle tip. Each of the arms made of wire carries a vertically placed plate made of annealed mica (possibly mirrored), one side of which is blackened with soot, in such a way that the sooty surfaces are all directed in the same direction of rotation.
The structure is enclosed in a hollow glass sphere five to six centimeters in diameter. A glass tube protrudes into the ball from above, preventing the impeller from falling.
The glass flask is evacuated to about 5 Pascal (that is 0.05 millibars) and then sealed. Light mills neither work in a high vacuum nor at normal pressure.
If the light mill is exposed to light or heat radiation, the wheel rotates at a speed that depends on the strength of the radiation, with the non-blackened areas leading the way.
In order to observe a rotation, the friction and air resistance must be very low. This is achieved through the negative pressure inside the glass ball and the low-friction bearing of the rotor.
A light mill only works if the blackened side can absorb energy, is well thermally insulated from the light side and thus heats up. A good light mill rotates quickly in sunlight, but also moves slowly in weak daylight, while room lighting, for example from fluorescent tubes, is usually not sufficient. Since the sensitivity in the infrared range is high, candles, flashlights or even your hands are sufficient to let the wings turn slowly.
Explanation
The gas pressure inside the glass sphere is so low - only one to ten Pascal - that the free path , i.e. the mean distance that a gas molecule travels between two collisions, is on the order of millimeters. For a pressure of 5 Pascal, the mean free path is 1.4 millimeters. Therefore one has to describe the behavior of the gas molecules inside with the law of momentum rather than with flow , convection or thermal expansion . The interaction of the gas molecules with each other is too small for these phenomena. Investigations on the strength of the effect, which the Berlin professor Wilhelm Westphal carried out before 1920, show a maximum of the force acting on the mill at an internal pressure of approx. 1.33 Pa.
The thermal movement of the gas molecules inside leads statistically to the same number of impacts on the light and dark wing surfaces as well as the glass wall when the impeller is not illuminated and there is thermal equilibrium. When exposed to radiation, the sooty surfaces heat up and their molecules and atoms move more strongly ( Brownian molecular movement ). If gas molecules hit fast-swinging particles on the warm side, they receive a stronger impulse when they fly away. The force equilibrium of the wing is no longer given and according to the law of conservation of momentum, the black side experiences a recoil force in the opposite direction of the gas particle flying away.
This theory can be used to explain all of the observed dependencies such as the optimum of the gas pressure, the poorly heat-conducting platelets and the counter pulse on the glass vessel.
The reversal of the direction of rotation of the non-irradiated mill, with the black surfaces first, that takes place when the glass vessel cools, can also be explained in this way. A non-irradiated light mill starts to rotate in the opposite direction, for example when it is placed in a vessel with cold water. The surfaces that are mostly blackened with soot then take on a lower temperature than the bright surfaces because of their higher emissivity not only in the visible light range but also in the mid- infrared range . They lose heat energy through radiation in the disturbed radiation equilibrium inside the sphere; the glass wall reflects less than it absorbs. As a result, the lighter side of the wing becomes the “drive side” because it has a higher temperature and the gas molecules there receive a stronger impulse.
History of the attempted explanation
Various physical principles have been used to explain the cause of the rotation.
Crookes initially believed that the rotational movement was due to the difference in radiation pressure (light energy is reflected on the light side, light energy is absorbed on the dark side). A more detailed analysis (among others by James Clerk Maxwell ) showed, however, that this effect is too small, and that it would also cause a rotation with the blackened side first, which would be in contrast to the direction observed.
A further refutation of the radiation pressure theory was achieved through experiments which showed that an interaction takes place between the impeller and the glass envelope and consequently the movement cannot come from an external force. If you let a light mill, the impeller of which is provided with a light magnetic rod, float in water and stop the rotation of the wheel by means of a magnet approached from the outside, the glass cover rotates in the opposite direction when irradiated.
The radiation pressure hypothesis can easily be refuted by storing the impeller in a vacuum. Due to the lack of air resistance, it would be expected that the wings would now turn faster. However, the pressure is at its optimum; if the internal pressure is too low, no more movement takes place.
The temperature dependence of the rotary movement is a further indication of 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 inside and outside: An unirradiated, stationary light mill starts in the opposite direction, with the black areas first, when you put them in a vessel with cold water - the surfaces, which are mostly blackened with soot, take on a lower temperature than the light surfaces due to their better emissivity in the mid-infrared range.
Another explanation of how it works was published in 1879 by the English engineer Osborne Reynolds . He explained the movement with a temperature difference between the black warm and the white cold surface and the associated gas flow, which leads to a pressure difference in immobile surfaces. When this air flows towards the outer edges of the blades, the warmer, faster molecules brushed the edges at a greater angle than the cooler ones, driving the blades in the direction away from the dark surface.
There are plenty of other attempts to explain rotary motion, some of which may at least contribute to the motion, but not be its main cause. The effects used are
- the outgassing of the black coatings,
- the photoelectric effect ,
- the convection flow .
literature
- Wolfgang Bürger: The light mill , Spectrum of Science 2/2001 , page 104
- Falk Müller: Why does a light mill turn? A historical look. In: Jürgen Renn (Ed.): Albert Einstein. Engineer of the Universe. A hundred authors for Einstein. Wiley-VCH Verlag, Weinheim 2005, ISBN 978-3-527-40579-4 , pp. 48-51.
- Gerhard Wurm: Photophoresis - the power of light and shadow. The importance of the light mill effect for astronomy. in: Stars and Space, April 2008 preview online
Individual evidence
- ↑ See Fig. 16.7 in section " 16.2 Recoil of the gas molecules during reflection, radiometer force " in Klaus Lüders, Robert O. Pohl (Ed.): Pohls Introduction to Physics . 21st edition. tape 1 . Springer-Verlag, Berlin Heidelberg 2017, ISBN 978-3-662-48662-7 .
- ↑ The light mill. In: Wolfgang Bürger, Spektrum.de. February 1, 2001, accessed October 10, 2019 .
Web links
- Philip Gibbs: How does a light-mill work? , Usenet Physics FAQ, 1997 (English)
- Does the rotor container in the light mill have to be a sphere? , Association of German Glass Blowers, 1998