Crookes radiometer, 1879-1888.
© Science Museum / Science & Society Picture Library
One of a series of electrical radiometers used to illustrate Crookes' researches on electric discharge in high vacua, donated to the Science Museum in 1888 by Sir William Crookes (1832-1919). A radiometer is a device that responds to electromagnetic radiation. Crookes' radiometer consists of a glas bulb a few centimeters in diameter, evacuated to a presure of about 50m Torr, containing a fly of four vanes pivoted on the tip of a needle, which rotates when illuminated by visible or infrared radiation.