Crookes tube, c 1888.
© Science Museum / Science & Society Picture Library
This tube, made in William Crooke's (1832-1919) laboratory, was used for investigating the deflection of cathode rays by a magnet. It is similar to the one used by him a few years earlier to investigate the behaviour of cathode rays in a magnetic field. When the tube is connected to a very high voltage a narrow beam of cathode rays travels acros the tube, producing a glowing green line on the fluorescent screen in the tube. The line bends when a magnet is brought near the tube, showing that the cathode rays are deflected by a magnetic field.