Radiometer
an instrument that is used for taking the altitudes of the celestial bodies. Also 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 revolves by the action of light. The cups of the anemometer are replaced by disks, colored white on one side and black on the other, and the RADIOPHONE instrument is inclosed in a glass globe from which the air has been exhausted, so that no heat is transmitted. When the disks are exposed to light, revolution begins 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 makes the disks spin with great rapidity.