Crookes tube

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.

Crookes tubes evolved from the earlier Geissler tubes invented by the German physicist and glassblower Heinrich Geissler in 1857, experimental tubes which are similar to modern neon tube lights. Geissler tubes had only a low vacuum, around 10⁻³ atm (100 Pa), and the electrons in them could only travel a short distance before hitting a gas molecule. So the current of electrons moved in a slow diffusion process, constantly colliding with gas molecules, never gaining much energy. These tubes did not create beams of cathode rays, only a colorful glow discharge that filled the tube as the electrons struck the gas molecules and excited them, producing light.

By the 1870s, William Crookes (among other researchers) was able to evacuate his tubes to a lower pressure, 10⁻⁶ to 5x10⁻⁸ atm, using an improved Sprengel mercury vacuum pump invented by his coworker Charles A. Gimingham. He found that as he pumped more air out of his tubes, a dark area in the glowing gas formed next to the cathode. As the pressure got lower, the dark area, now called the Faraday dark space or Crookes dark space, spread down the tube, until the inside of the tube was totally dark. However, the glass envelope of the tube began to glow at the anode end. What was happening was that as more air was pumped out of the tube, there were fewer gas molecules to obstruct the motion of the electrons from the cathode, so they could travel a longer distance, on average, before they struck one. By the time the inside of the tube became dark, they were able to travel in straight lines from the cathode to the anode, without a collision. They were accelerated to a high velocity by the electric field between the electrodes, both because they did not lose energy to collisions, and also because Crookes tubes were operated at a higher voltage. By the time they reached the anode end of the tube, they were going so fast that many flew past the anode and hit the glass wall. The electrons themselves were invisible, but when they hit the glass walls of the tube they excited the atoms in the glass, making them give off light or fluoresce, usually yellow-green. Later experimenters painted the back wall of Crookes tubes with fluorescent paint, to make the beams more visible.

This accidental fluorescence allowed researchers to notice that objects in the tube, such as the anode, cast a sharp-edged shadow on the tube wall. Johann Hittorf was first to recognise in 1869 that something must be travelling in straight lines from the cathode to cast the shadow. In 1876, Eugen Goldstein proved that they came from the cathode, and named them cathode rays (Kathodenstrahlen).

At the time, atoms were the smallest particles known and were believed to be indivisible, the electron was unknown, and what carried electric currents was a mystery. During the last quarter of the 19th century, many ingenious types of Crookes tubes were invented and used in historic experiments to determine what cathode rays were. There were two theories: Crookes believed they were 'radiant matter'; that is, electrically charged atoms, while German scientists Hertz and Goldstein believed they were 'aether vibrations'; some new form of electromagnetic waves. The debate was resolved in 1897 when J. J. Thomson measured the mass to charge ratio of the cathode rays, showing they were made of particles, but were around 1800 times lighter than the lightest atom, hydrogen. Therefore, they were not atoms, but a new particle, the first subatomic particle to be discovered, which was later named the electron. It was quickly realized that these particles were also responsible for electric currents in wires, and carried the negative charge in the atom.

The colorful glowing tubes were also popular in public lectures to demonstrate the mysteries of the new science of electricity. Decorative tubes were made with fluorescent minerals, or butterfly figures painted with fluorescent paint, sealed inside. When power was applied, the fluorescent materials lit up with many glowing colors.