HISTORY

History of Crookes Radiometer

The radiometer is made of a glass bulb from which most of the air is removed, therefore it creates a partial vacuum and a rotor that is mounted on a vertical support inside the glass bulb. In 1873, while investigating infrared radiation and the element thallium, the eminent Victorian experimenter Sir William Crookes developed a special kind of radiometer, an instrument for measuring radiant energy of heat and light.

It was invented in 1873 by a chemist named Sir William Crookes, after some time he noticed the weighs were moved by the sunlight shown on the balance. The first radiometer was constructed to settle the controversy regarding whether light exerts a force. The idea was that a reflecting surface would experience a greater force from the light than an absorbing one.

This instrument was made in the now familiar form. Unexpectedly, the opposite effect was observed. The black vane retreated from the light source and moved forward. (Encyclopedia Britannica)

In this mechanism, the hotter gas molecules near the dark surfaces of the vane travels around the edges of those surfaces to the other, cooler sides, imparting some force to the edges. Cooler molecules traveling in the reverse direction possess less energy and so the force they impart in the opposite direction is considerably less.

The Editors of Encyclopedia Britannica. "Crookes Radiometer (instrument)." Encyclopedia Britannica Online. Encyclopedia Britannica, n.d. Web. 10 Feb. 2014.

http://math.ucr.edu/home/baez/physics/General/LightMill/light-mill.html