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Sir William Crookes. Portrait of Sir William Crookes (1832-1919), British physicist and chemist. Crookes used vacuum tubes to investigate cathode rays in 1878. He discovered that the rays (which were made of electrons) made the sides of the glass tube fluoresce. He showed that the rays themselves travelled in straight lines and that they could be deflected by a magnetic field. Crookes invented the spinthariscope which measured alpha radiation using scintillation caused by these charged particles. He also discovered the element thallium. He was a great populariser of science though his spiritualism attracted criticism.
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