Description
The radiometer, also known as crookes radiometer, the light-mill or the solar motor, is an instrument for measuring small amounts of radiant energy. It demonstrates this by transforming radiant energy into mechanical movement.

William Crookes, who was later knighted for his contribution to science, invented the radiometer in 1873.

The radiometer consists of a sealed bulb, which contains a partial vacuum and a rotor with four vanes. Each vane has one side polished and the other side blackened.

The first theory stated that as radiant energy hits each vane it would be reflected off the polished side and attracted by the blackened side, causing the rotor to turn with the blackened side leading. However, it did not turn as expected, and it was proposed that the blackened sides reflected rather than absorbed the energy. This led to much debate during the late 1800’s. Eventually it was decided that gasses passing around the edge of each vane, from the colder polished side to the warmer blackened side caused the rotation. This explained the unusual movement of the rotor and solved the mystery of the radiometer.

The Radiometer has never been considered an accurate way of measuring radiant energy and was always considered more of a demonstration tool than anything else.


Created by: Boris Jardine