Lichtmühle
The lightmill and how it works.
When sunlight or light from a light bulb hits the impeller of the lightmill it starts to turn. One gets the impression that the impeller turns through the “pressure” of the light, just like a water wheel or a windmill turn through the pressure of the respective elements. Warm hands or warm water touching the flask are also capable of making the impeller turn. If you cool off the flask by using cold water or coolant spray the lightmill will even turn in the other direction. When using only the light of fluorescent lamps the windmill will turn quite slow or will not at all.
Explication:
The flask is filled with air. The entering thermal radiation heats up the dark side of the wings faster than the light side. Thereby the air layer near the dark side gets warmed up faster than the layer on the opposite side. The gas pressure on the warmer and darker side is slightly higher than on the cooler and lighter side. This is when the impeller starts to turn. The bigger the difference in temperature on the wings gets, the bigger gets the force moving the impeller. This means that if you want to build the perfect windmill, one side needs to convert heat radiation into heat as good as possible while the other side should convert it as bad as possible. Furthermore, the heat conductance of the wings should be as little as possible. The impeller should be light and almost fritionless to achieve a high acceleration and consequently an impressive speed. The wings are usually made of thin and light mica lamination with a low thermal conductivity. The silver side reflects the thermal radiation quite good and therefor stays cool, while the black and sooted side does not reflect it but absorbs it and heats up. The gas pressure on the sooted side is higher than on the silver side. This is why the silver side gets pushed away. This movement gets chocked off by the friction between the gas and the impeller. By increasing the gas pressure inside the flask, the friction becomes higher and prevails. If the gas pressure, however, is being reduced the gas friction declines as well. In a perfect vacuum it would totally disappear. Yet, in this case the movement of the lightmill wouldn´t increase. Actually it would stop moving because with no gas at all there would be no pressure gradient.
There is a competition between repulsion and friction. The best possible pressure for our windmill is when repulsion and friction are in a perfect balance: 5*10-2 to 6*10-2 mbar (ca 55 mTorr).
False explanations:
A popular but false explanation for the mode of operation of the Magic windmill assumes that the impeller is set in motion by light pressure. Such a radiation pressure acutally existsts but is way too low to for our purposes. Furthermore the lightmill would move into the other direction because the light pressure on the reflective side is higher than on the absorbing side.
The lightmill is also known as radiometer, Light-Mill, Crookes´ Radiometer or solar engine.
