Sir William Crookes. Portrait of English physicist Sir William Crookes (1832-1919), discoverer of the element Thallium. Crookes is best known for his invention of the radiometer, a device consisting of a set of pivoted vanes in a vacuum. One side of each vane is blackened, the other shiny, and in the presence of sunlight the vanes turn steadily. Although never more than a toy, Crookes' radio- meter did provide evidence for the kinetic theory of gases. Crookes developed methods for producing better vacuums, worked on radioactivity, and also studied cathode rays, showing that they were deflected by a magnet. He was the first to realise that they were beams of charged particles.