Sir William Crookes was a British chemist and a British physicist. He discovered the chemical element thallium and determined its atomic weight. He also made many devices that were widely used in scientific research for many years.

During the 1870's, Crookes constructed a vacuum tube for the study of a form of radiation that came to be called later cathode rays. The vacuum tube which became known as the Crookes tube, contributed to the construction of the cathode-ray tube which is used in electronic equipment. One type of cathode-ray tube is a television picture tube. Crookes also made the radiometer, which is a device that measures the intensity of radiant energy.

William Crookes was born in London and graduated from the Royal College of Chemistry in 1851. In 1859, he founded the journal of Chemical News. He was knighted in 1897 that is why people call him Sir William Crookes.