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A Crookes tube also Crookes—Hittorf tube [1] is an early experimental electrical discharge tubewith partial vacuum, invented by English physicist William Crookes [2] and others around[3] in which cathode raysstreams of electronswere discovered. Developed from the earlier Geissler tubethe Crookes tube consists of a partially evacuated glass bulb of various shapes, with two metal electrodesthe cathode and the anodeone at either end.

When a high voltage is applied between the electrodes, cathode rays electrons are projected in straight lines from the cathode. Thomson ‘s identification of cathode rays as negatively charged particles, which were later named electrons. Crookes tubes are now used only for demonstrating cathode rays. The term Crookes tube is also used for the first generation, cold cathode X-ray tubes[5] which evolved from the experimental Crookes tubes and were used until about 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 kilovolts applied between the cathode and anode electrodes in the tube, usually by an induction coil a “Ruhmkorff coil”.

When high voltage is applied to the tube, the electric field accelerates the small number of electrically charged ions and free electrons always present in the gas, created by natural processes like photoionization and radioactivity. The electrons collide with other gas moleculesknocking electrons off them and creating more positive ions.

The electrons go on to create more ions and electrons in a chain reaction called a Townsend discharge. 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 ampolw the electrons can travel the length of the tube without striking a gas molecule. 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 ampila end wall of the tube. When they strike ampooa 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 fluorescencecauses the glass to glow, usually yellow-green.

Ampola de Crookes

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 phosphora fluorescent chemical such as zinc sulfidein 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 above only describes the motion of the electrons. The full details of the action in a Crookes tube are complicated, because it contains a nonequilibrium plasma of positively charged ionselectronsand neutral atoms which are constantly interacting. The details were not fully understood until the development of plasma physics in the early 20th century.

Crookes tubes evolved from the earlier Geissler tubesexperimental tubes which are similar to modern neon tube lights. 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. He found that as he pumped crpokes 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 Crookes dark spacespread 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 cgookes walls of the tube they excited the atoms in the glass, making them give off light or fluoresceusually yellow-green.

william crookes

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 that something must be travelling in straight lines from the cathode to cast the shadow. 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 amopla 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 ampooa 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.

The many uses for X-rays were immediately apparent, the first practical application for Crookes tubes. Medical manufacturers began to produce specialized Crookes tubes to generate X-rays, the first X-ray tubes. Crookes tubes were unreliable and temperamental. Both the energy and the quantity of cathode rays produced depended on the pressure of residual gas in the tube. Eventually the pressure got so cfookes the tube stopped working entirely.

The electronic vacuum tubes invented later around superseded the Crookes dee. Instead, they use a more reliable and controllable source of electrons, a heated filament or hot cathode which dr electrons by thermionic emission. The ionization method of creating cathode rays used in Crookes tubes is today only used in a few specialized gas discharge tubes such as thyratrons. 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 When the voltage applied to a Crookes tube is high enough, around 5, volts or greater, [14] 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 nucleusa process called bremsstrahlungampooa they knock an atom’s inner electrons into a higher energy leveland these in turn emit X-rays as they ampla 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.

He found that they could pass through books and papers on his desk. 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 platinumwhich 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. These cold cathode type X-ray tubes were used until aboutwhen they were superseded by the hot cathode Coolidge X-ray tube. During the last quarter of the 19th century Crookes tubes were used in dozens of historic experiments to try to find out what cathode rays were.

British scientists Crookes and Cromwell Varley believed they were particles of ‘radiant matter’, that is, electrically charged atoms. Wiedemann, Heinrich Hertzand Eugen Goldstein believed they were ‘ aether vibrations’, some new form ampolla electromagnetic wavesand were separate from what carried the current through the tube. Thomson measured their mass, proving they were a previously unknown negatively charged particle, the first subatomic particlewhich he called a ‘corpuscle’ but was later renamed the ‘electron’.


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.

This fluorescence was used as an argument that cathode rays were electromagnetic waves, since the only thing known to cause fluorescence at the time was ultraviolet light.

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 more strongly than the area around it.

Eugen Goldstein in found [20] that cathode rays were always emitted perpendicular to the cathode’s surface.

Category:Crookes tube

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 amola 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 temperature. 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 particlestheir path should be bent by the electric field created when a voltage was applied to the plates, causing the spot of light where the ampolw hit to move sideways. He did not 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 Arthur 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 therefore not electromagnetic waves. 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 effect now called the Lorentz force was similar to the behaviour of electric currents in an electric motor and showed that the cathode rays obeyed Faraday’s law of induction like currents in wires.

Both electric and magnetic deflection drookes evidence for the particle theory, because electric and magnetic fields have no effect on a beam of light amoola. 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, suggesting that the force of the cathode rays striking the paddles was causing the rotation.

Crookes tube – Wikipedia

Crookes concluded at the time that this showed that cathode rays had momentumso the rays were likely matter particles. However, later it was concluded that the paddle wheel turned not due to the momentum of the particles or electrons hitting the paddle wheel but due to the radiometric effect. When the rays hit the paddle surface they heated it, and the heat caused the gas next to it to expand, pushing the paddle.

This was proven in by J. Thomson who calculated that the momentum of the electrons hitting the paddle wheel would only be sufficient to turn the wheel one revolution per minute. All this experiment really showed was that cathode rays were able to heat surfaces. Jean-Baptiste Perrin wanted to determine whether the cathode rays actually carried negative chargeor whether they just accompanied the charge carriers, as the Germans thought.

In 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 its charge. The electroscope showed a negative charge, proving that cathode rays really carry negative electricity. Goldstein found in that if the cathode is made with small holes in it, streams of a faint luminous glow will be seen issuing from the holes on the back side of the cathode, facing away from the anode.

These were the positive ions which were attracted to the cathode, and created the cathode rays. They were named canal rays Kanalstrahlen by Goldstein.