A Crookes tube is an early experimental discharge tube, invented by British physicist William Crookes and others around 1875, in which cathode rays, that is electrons, were discovered. An evolution of the Geissler tube, it consists of a partially (but not completely) evacuated glass cylinder of various shapes, with two metal electrodes at either end. When a high voltage is applied between the electrodes, electrons travel in straight lines from the cathode to the anode. It was used by Crookes, Johann Hittorf, Juliusz Plücker, Eugen Goldstein, Heinrich Hertz, Philipp Lenard and others to discover the properties of cathode rays, culminating in J. J. Thompson's 1897 identification of cathode rays as the particles carrying the negative charge of atoms, which he named electrons. Crookes tubes are now used only for demonstrating cathode rays.
Wilhelm Röntgen discovered x-rays with the Crookes tube in 1895. The term 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 were cold cathode tubes, meaning they didn't have a heated filament in them to generate electrons like later electronic vacuum tubes. Instead the electrons were generated by ionization of the residual air by a high DC voltage of anywhere between a few kilovolts to 100 kV, applied between the electrodes, usually by an induction coil (Ruhmkorff coil). Therefore they require a small amount of air in them to function, from 10-6 to 5×10-8 atmosphere.
When high voltage is applied to the tube, it accelerates the small number of ions always present in the gas, created by natural processes like radioactivity. These collide with other gas molecules, knocking electrons off them and creating more positive ions in a chain reaction. All the positive ions are attracted to the cathode or negative electrode. When they strike it, they knock large numbers of electrons out of the surface of the metal, which in turn are repelled by the cathode and attracted to the anode or positive electrode. These are the cathode rays.
Enough of the air has been removed from the tube that most of the electrons can travel the length of the tube without striking a gas molecule. The high voltage accelerates these light particles to a high velocity. 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 fluorescence, 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 researchers painted the back wall of the tube inside with a fluorescent chemical such as zinc sulfide 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 above only describes the motion of the electrons. The full details of what goes on in an operating Crookes tube are complicated, because it contains a nonequilibrium plasma of positively charged ions, electrons, and neutral atoms which are constantly interacting. This creates different colored glowing regions in the gas, depending on the pressure in the tube. The details were not fully understood until the development of plasma physics in the mid 20th century.
Crookes tubes evolved from the earlier Geissler tubes, experimental tubes which are similar to modern neon lights. Geissler tubes had only a low vacuum, 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 didn't create beams of cathode rays, only a pretty glow discharge that filled the tube as the electrons struck the gas molecules and excited them, producing light.
Crookes was able to evacuate his tubes to a lower pressure, 10-6 to 5x10-8 atm, using an improved Sprengel mercury vacuum pump made 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, called the 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, 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 didn't lose energy to collisions, and also because Crookes tubes required 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 metal objects in the tube, such as the anode, cast a shadow on the tube wall. They realized that something must be travelling down the tube from cathode to anode, to cast the shadow. In 1896, Eugen Goldstein proved that they came from the cathode, and named them cathode rays.
At the time, atoms were the smallest particles known, the electron was unknown, and what carried electric currents was a mystery. Many ingenious types of Crookes tubes were built to determine the properties of cathode rays (see below). The high energy beams of pure electrons in the tubes revealed their properties much better than electrons flowing in wires. 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.
In 1895, Wilhelm Röntgen discovered x-rays emanating from Crookes tubes. The medical use of x-rays for taking pictures of the inside of the body was immediately apparent, the first practical application for Crookes tubes.
Crookes tubes were unreliable and tempramental. Both the energy and the quantity of cathode rays produced depended on the pressure of residual gas in the tube. Over time the gas was absorbed by the walls of the tube, reducing the pressure in the tube. This reduced the amount of cathode rays produced and caused the voltage across the tube to increase, creating 'harder' more energetic cathode rays. Soon the pressure got so low the tube stopped working entirely.
The electronic vacuum tubes invented later around 1906 operate at a still lower pressure, at which there are so few gas molecules that they don't conduct by ionization. Instead, they use a more reliable and controllable source of electrons, thermionic emission by a heated filament.
The technology of manipulating electron beams pioneered in Crookes tubes was applied practically in the design of vacuum tubes, and particularly in the invention of the cathode ray tube by Ferdinand Braun in 1897.
When the voltage applied to a Crookes tube is high enough, around 5,000 volts or greater, it can accelerate the electrons to a fast enough velocity to create x-rays when they hit the anode or the glass wall of the tube. In fact, the fluorescence of the tube's wall which revealed cathode rays may be partly caused by low energy x-rays created in the glass. Many early Crookes tubes undoubtedly generated x-rays, and 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 a nearby fluorescent screen faintly glowing. 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 paper on x-rays. He received the first Nobel Prize in physics for his discovery.
The medical 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.
Crookes put a tiny vaned turbine or paddlewheel in the path of the cathode rays, and found that it rotated when the rays hit it. The paddlewheel turned in a direction away from the cathode side of the tube, confirming that the rays were coming from the cathode. Crookes concluded at the time that this showed that cathode rays had momentum, so the rays were likely matter particles. But in 1903 J. J. Thompson proved that the paddlewheel wasn't turned by the force of the cathode rays hitting it, but by the radiometric effect. When the rays hit a paddle, they heated the side they hit. The air next to that side of the paddle expanded, pushing the paddle away. All this experiment really showed was that cathode rays could heat objects.