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Crookes tube

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.

How it works

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.

History

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.

Discovery of x-rays

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.

Experiments with Crookes tubes

Crookes tubes were used in dozens of experiments to try to find out what cathode rays were. There were two theories: Crookes and Cromwell Varley believed they were 'corpuscles' or '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. The debate continued until J. J. Thompson measured their mass, proving they were a previously unknown negatively charged particle, which he named electron.

Maltese cross

Juliusz Plücker in 1869 built an anode shaped like a Maltese Cross in the tube. It was hinged, so it could fold down against the floor of the tube. When the tube was turned on, it cast a sharp cross-shaped shadow on the fluorescence on the back face of the tube, showing that the rays moved in straight lines. After a while the fluorescence would get 'tired' and decrease. If the cross was folded down out of the path of the rays, it no longer cast a shadow, and the previously shadowed area would fluoresce stronger than the area around it.

Deflection by electric fields

Heinrich Hertz built a tube with a second pair of metal plates to either side of the cathode ray beam, a crude CRT. If the cathode rays were charged particles, their path should be bent by the electric field created when a voltage was applied to the plates, causing the spot of light where the rays hit to move sideways. He didn't find any bending, but it was later determined that his tube was insufficiently evacuated, causing accumulations of surface charge which masked the electric field. Later Artur Shuster repeated the experiment with a higher vacuum. He found that the rays were attracted toward a positively charged plate and repelled by a negative one, bending the beam. This was evidence they were negatively charged, and so not electromagnetic waves.

Deflection by magnetic fields

Crookes put a magnet across the neck of the tube, so that the North pole was on one side of the beam and the South pole was on the other, and the beam travelled through the magnetic field between them. The beam was bent down, perpendicular to the magnetic field. This was similar to the behavior of electric currents in an electric generator and showed that the cathode rays obeyed Faraday's law like currents in wires.

Paddlewheel

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.

Charge

Jean-Baptiste Perrin wanted to determine whether the cathode rays actually carried negative charge, or whether they just accompanied the charge carriers, as the Germans thought. In 1895 he constructed a tube with a 'catcher', a closed aluminum cylinder with a small hole in the end facing the cathode, to collect the cathode rays. The catcher was attached to an electroscope to measure it's charge. The electroscope showed a negative charge, proving that cathode rays really carry negative electricity.

Perpendicular emission

Eugen Goldstein found that cathode rays were always emitted perpendicular to the cathode's surface. 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. If the electrode was made in the form of a concave spherical dish, the cathode rays would be focused to a spot in front of the dish. This could be used to heat samples to a high heat.

Doppler 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. This could be detected with a spectroscope because the emission line spectrum would be shifted. He built a tube shaped like an 'L', with a spectroscope pointed through the glass of the elbow down one of the arms. He measured the spectrum of the glow when the spectroscope was pointed toward the cathode end, then switched the power supply connections so the cathode became the anode and the electrons were moving in the other direction, and again observed the spectrum looking for a shift. He didn't find one, which he calculated meant that the rays were travelling very slowly. It is now recognized that the glow in Crookes tubes is emitted from gas atoms hit by the electrons, not the electrons themselves. Since the atoms are thousands of times more massive than the electrons, they move much slower, accounting for the lack of doppler shift.

Lenard window

Philipp Lenard wanted to see if cathode rays could pass out of the Crookes tube into the air. He built a tube with a 'window' in the glass envelope made of aluminum foil just thick enough to hold the atmospheric pressure out (later called a Lenard window) facing the cathode so the cathode rays would hit it. He found that something did come through. Holding a fluorescent screen up to the window caused it to flouresce, even though no light reached it. A photographic plate held up to it would be darkened, even though it wasn't exposed to light. The effect had a very short range of about 2 inches. He measured the ability of cathode rays to penetrate sheets of material, and found they could penetrate much farther than moving atoms could. Since atoms were the smallest particles known at the time, this was first taken as evidence that cathode rays were waves. Later it was realized that electrons were much smaller than atoms, accounting for their greater penetration ability. Lenard received the 1905 Nobel Prize in physics for this work.