Crookes tube

Crookes tube
	Experimental Crookes tube belonging to Wilhelm Rontgen, c. 1890, at Gedächtnisstätte Würzburg (Rontgen Memorial), Würzburg, Germany
Uses	Observing and studying cathode rays
Notable experiments	Discovery of X-rays
Inventor	William Crookes
Related items	X-ray tube; Cathode-ray tube

Power off

Without magnet, rays travel straight

With magnet, rays are bent up

With magnet reversed, rays are bent down

A Crookes tube demonstrating magnetic deflection. With a magnet held at the neck of the tube (right) the rays are bent upward or downward, perpendicular to the horizontal magnetic field, so the green fluorescent patch appears higher or lower. Residual air in the tube glows pink when it is struck by electrons.

A Crookes tube (also Crookes–Hittorf tube) is an early experimental discharge tube with partial vacuum invented by English physicist William Crookes and others around 1869–1875, in which cathode rays, streams of electrons, were discovered.

Developed from the earlier Geissler tube, the Crookes tube consists of a partially evacuated glass bulb of various shapes, with two metal electrodes, the cathode and the anode, one at either end. When a high voltage is applied between the electrodes, cathode rays (electrons) are projected in straight lines from the cathode. It was used by Crookes, Johann Hittorf, Julius Plücker, Eugen Goldstein, Heinrich Hertz, Philipp Lenard, Kristian Birkeland and others to discover the properties of cathode rays, culminating in J. J. Thomson's 1897 identification of cathode rays as negatively charged particles, which were later named electrons. Crookes tubes are now used only for demonstrating cathode rays.

Wilhelm Röntgen discovered X-rays using the Crookes tube in 1895. The term Crookes tube is also used for the first generation, cold cathode X-ray tubes, which evolved from the experimental Crookes tubes and were used until about 1920.