Crookes tube

thumb|A Crookes tube: light and dark. Electrons (cathode rays) travel in straight lines from the [[cathode (left), as shown by the shadow cast by the metal Maltese cross on the fluorescence of the righthand glass wall of the tube. The anode is the electrode at the bottom]]

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.

History

Invention

thumb|left|[[William Crookes and his glowing tubes gained renown, as shown by this 1902 caricature in Vanity Fair. The caption read "ubi Crookes ibi lux", which in Latin means roughly, "Where there is Crookes, there is light"]]

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 5 × 10⁻⁸ 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.

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. Over time the gas was absorbed by the walls of the tube, reducing the pressure. it can accelerate the electrons to a high enough velocity to create X-rays when they hit the anode or the glass wall of the tube. The fast electrons emit X-rays when their path is bent sharply as they pass near the high electric charge of an atom's nucleus, a process called bremsstrahlung, or they knock an atom's inner electrons into a higher energy level, and these in turn emit X-rays as they return to their former energy level, a process called X-ray fluorescence. Many early Crookes tubes undoubtedly generated X-rays, because early researchers such as Ivan Pulyui had noticed that they could make foggy marks on nearby unexposed photographic plates.

On November 8, 1895, Wilhelm Röntgen was operating a Crookes tube covered with black cardboard when he noticed that a nearby fluorescent screen glowed faintly. He realized that some unknown invisible rays from the tube were able to pass through the cardboard and make the screen fluoresce. He found that they could pass through books and papers on his desk. Röntgen began to investigate the rays full-time, and on December 28, 1895, published the first scientific research paper on X-rays. Röntgen

was awarded the first Nobel Prize in Physics (in 1901) for his discoveries.

The many applications of X-rays created the first practical use for Crookes tubes, and workshops began manufacturing specialized Crookes tubes to generate X-rays, the first X-ray tubes. The anode was made of a heavy metal, usually platinum, which generated more X-rays, and was tilted at an angle to the cathode, so the X-rays would radiate through the side of the tube. The cathode had a concave spherical surface which focused the electrons into a small spot around 1 mm in diameter on the anode, in order to approximate a point source of X-rays, which gave the sharpest radiographs. These cold cathode type X-ray tubes were used until about 1920, when they were superseded by the hot cathode Coolidge X-ray tube.

Operation

thumb|Diagram showing a Crookes tube circuit

Crookes tubes are cold cathode tubes, meaning that they do not have a heated filament in them that releases electrons as the later electronic vacuum tubes usually do. Instead, electrons are generated by the ionization of the residual air by a high DC voltage (from a few kilovolts to about 100 kilovolts) applied between the cathode and anode electrodes in the tube, usually by an induction coil (a "Ruhmkorff coil").). When they get to the anode end of the tube, they have so much momentum that, although they are attracted to the anode, many fly past it and strike the end wall of the tube. When they strike atoms in the glass, they knock their orbital electrons into a higher energy level. When the electrons fall back to their original energy level, they emit light. This process, called cathodoluminescence, causes the glass to glow, usually yellow-green. The electrons themselves are invisible, but the glow reveals where the beam of electrons strikes the glass. Later on, researchers painted the inside back wall of the tube with a phosphor, a fluorescent chemical such as zinc sulfide, in order to make the glow more visible. After striking the wall, the electrons eventually make their way to the anode, flow through the anode wire, the power supply, and back to the cathode.

The full details of the action in a Crookes tube are complicated, There were two theories: British scientists Crookes and Cromwell Varley believed they were particles of 'radiant matter', that is, electrically charged atoms. German researchers E. Wiedemann, Heinrich Hertz, and Eugen Goldstein believed they were 'aether vibrations', some new form of electromagnetic waves, and were separate from what carried the current through the tube. If the cathode was a flat plate, the rays were shot out in straight lines perpendicular to the plane of the plate. This was evidence that they were particles, because a luminous object, like a red hot metal plate, emits light in all directions, while a charged particle will be repelled by the cathode in a perpendicular direction. Cathode rays heat matter which they strike.

Spectral shift

Eugen Goldstein thought he had figured out a method of measuring the speed of cathode rays. If the glow discharge seen in the gas of Crookes tubes was produced by the moving cathode rays, the light radiated from them in the direction they were moving, down the tube, would be shifted in frequency due to the Doppler effect.