Radiometer. Radiometer invented by British physicist Sir William Crookes (1832-1919). The instrument was used to measure solar radiation. It consists of a glass bulb containing a partial vacuum and four vanes pivoted on an axis at right angles to each other. The vanes are dark on one side and light on the other. The dark sides absorb solar radiation more readily than the light sides. As gas molecules in the tube strike one of the dark sides they absorb some of this heat which makes them move faster. This means that they exert more force on the dark sides as they bounce off. This causes the vanes to rotate at up to 2500 revolutions a minute.