Talk:Crookes radiometer

In fact, it has always been wrong. Crookes said it was radiation pressure. Maxwell was pleased that it demonstrated a prediction of his theory of electromagnatism. Maxwell was embarassed to discover the explanation was incorrect. The theory given here was put forward, and, wary of making another mistake Maxwell checked it and found it to be incorrect also. This is easy to see: Imagine the glass to be shrunk so that it is only just big enough to allow the vanes to move, with no gap between vanes and glass (and a frictionless surface). Hold the vanes from moving. Shine light on the vanes. Hotter at black side, cooler on white side. Pressure equalises between the vanes (or the gas would move). After allowing the device to reach equilibrium there is no overall force. Let go: nothing happens. The correct solution was discovered by Osbourne Reynolds and is called thermal transpiration and is due to assymetric effects as molecules bounce off the *edge* of the vanes in the presence of the temperature gradient. This effect could be tested by cutting holes in the vanes. According to the incorrect theory, this should produce less heating and less rotation, thermal transpiration says it will increase the rotation.