Crookes' mill-wheel tube, 1879-1888.
© Science Museum / Science & Society Picture Library
This tube was produced between 1879 and 1888 in the laboratory of Sir William Crookes (1832-1919). It is one of a set of tubes that was donated to the Science Museum by Crookes for demonstrating the properties of cathode rays. When the tube is connected to a very high voltage electricity supply cathode rays are emitted from the negative electrode (the small rod on the right-hand side). When they hit the mill, it spins.