radiometer

[Latin radius =a ray, and Eng. meter (q. v.).] 1. An instrument for taking the altitudes of the celestial bodies. 2. An instrument invented by Crookes for measuring the mechanical effect of radiant energy, and exhibited by him at the Royal Society, April 7, 1875. It resembles a miniature anemometer, and is made to revolve by the action of light. The cups of the anemometer are replaced by discs, colored white on one side and black on the other, and the instrument is inclosed in a gla globe from which air has been exhausted, so that no heat is transmitted. When the discs are exposed to light, revolution begins immediately, and its speed is governed by the intensity of the light. Two candles produce twice the effect of one, and the flame of magnesium wire Radio me makes the discs spin round with great rapidity. ter.