radiometer (rā´dēŏm´ətər), instrument for detection or measurement of electromagnetic radiation; the term is applied in particular to devices used to measure infrared radiation. One of the earliest experiments in radiometry was performed c.1800 by W. Herschel, who observed the heating of a mercury thermometer by sunlight; he was also able to detect heat radiated from hot but not incandescent bodies. E. Becquerel was able (c.1843) to detect near-infrared radiation by photographic means. Radiometers that function by an increase in the temperature of the device, such as Herschel's thermometer, are called thermal detectors. Commonly used thermal detectors include the thermocouple, which produces a voltage when heated, and the bolometer, which changes in electrical resistance when heated. Devices that can, in principle, detect a single quantum of radiant energy, such as Becquerel's photographic plate, are called quantum detectors. Many current quantum detectors are based on the photoelectric cell. The term radiometer is often used to refer specifically to a type of thermal detector invented by Sir William Crookes (c.1874). Because his device was somewhat insensitive and not readily calibrated, it is rarely used today as a scientific instrument. A Crookes radiometer consists essentially of two parts. The first part is a glass bulb from which most of the air has been removed, creating a partial vacuum. The second part is a rotor that is mounted on a vertical support inside the bulb. The rotor consists of four light, horizontal arms mounted at right angles to one another on a central pivot; the rotor can turn freely in the horizontal plane. At the outer end of each arm is mounted a metal vane, placed vertically. Each vane has one side polished and the other blackened; the vanes are arranged so that the polished side of one faces the blackened side of the next. When radiant energy strikes the polished surfaces, most of it is reflected away, but when it strikes the blackened surfaces, most of it is absorbed, raising the temperature of the surfaces. The air near a blackened surface thus becomes hotter, exerts a greater pressure on the blackened surface, and causes the rotor to turn. The rate of rotation provides an indication of the intensity of the radiation.