Crookes radiometer

The Crookes radiometer was invented by the chemist Sir William Crookes as the byproduct of some chemical research. In the course 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 devised the device named after him, still manufactured and sold to this day as a curiosity item.

The radiometer consists of 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, are several (usually four) lightweight metal vanes. Each vane is polished on one side, and blackened on the other. In sunlight, or exposed to a source of infrared radiation (even the heat of a hand nearby can be enough), the vanes turn with no apparent motive power.

One misconception (often seen in explanatory leaflets packaged with the device) is that the radiometer is demonstrating the pressure of light, but this is not the case. If this explanation held, the better the vacuum in the bulb, the less air resistance to movement, and the faster the vanes should spin: in fact, the radiometer only works when there is low pressure gas in the bulb, and the vanes stay motionless in a `hard' vacuum - the actual pressure exerted by light, though it exists, and can be measured with devices such as the Nichols radiometer, is far too small to move these vanes.

The actual explanation is in temperature differentials between the two sides of the vanes. The blackened side, absorbing radiation, is slightly hotter than the silvered side. Gas molecules hitting the warmer side of the vane will pick up some of the heat i.e., will bounce off the vane with increased velocity. 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.