This Radiometer is made from a glass bulb from which much of the air has been removed. Inside the bulb, on a low-friction spindle, is a rotor with 4 vanes. Each vane has one blackened side and one silvered side; one black vane features a white dot in order to count rotations easily. When exposed to sunlight or artificial light, the vanes turn with no apparent motive power. This effect is due to differential heating of the black and silvered vanes. Dimensions 6" x 4" x 4".