Cathode ray

Last updated November 20, 2024

Cathode rays or electron beams (e-beam) are streams of electrons observed in discharge tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, glass behind the positive electrode is observed to glow, due to electrons emitted from the cathode (the electrode connected to the negative terminal of the voltage supply). They were first observed in 1859 by German physicist Julius Plücker and Johann Wilhelm Hittorf,^[1] and were named in 1876 by Eugen Goldstein Kathodenstrahlen, or cathode rays.^[2]^[3] In 1897, British physicist J. J. Thomson showed that cathode rays were composed of a previously unknown negatively charged particle, which was later named the electron. Cathode-ray tubes (CRTs) use a focused beam of electrons deflected by electric or magnetic fields to render an image on a screen.

Description

Cathode rays are so named because they are emitted by the negative electrode, or cathode, in a vacuum tube. To release electrons into the tube, they first must be detached from the atoms of the cathode. In the early experimental cold cathode vacuum tubes in which cathode rays were discovered, called Crookes tubes, this was done by using a high electrical potential of thousands of volts between the anode and the cathode to ionize the residual gas atoms in the tube. The positive ions were accelerated by the electric field toward the cathode, and when they collided with it they knocked electrons out of its surface; these were the cathode rays. Modern vacuum tubes use thermionic emission, in which the cathode is made of a thin wire filament which is heated by a separate electric current passing through it. The increased random heat motion of the filament knocks electrons out of the surface of the filament, into the evacuated space of the tube.

Since the electrons have a negative charge, they are repelled by the negative cathode and attracted to the positive anode. They travel in parallel lines through the empty tube. The voltage applied between the electrodes accelerates these low mass particles to high velocities. Cathode rays are invisible, but their presence was first detected in these Crookes tubes when they struck the glass wall of the tube, exciting the atoms of the glass coating and causing them to emit light, a glow called fluorescence. Researchers noticed that objects placed in the tube in front of the cathode could cast a shadow on the glowing wall, and realized that something must be traveling in straight lines from the cathode. After the electrons strike the back of the tube they make their way to the anode, then travel through the anode wire through the power supply and back through the cathode wire to the cathode, so cathode rays carry electric current through the tube.

The current in a beam of cathode rays through a vacuum tube can be controlled by passing it through a metal screen of wires (a grid) between cathode and anode, to which a small negative voltage is applied. The electric field of the wires deflects some of the electrons, preventing them from reaching the anode. The amount of current that gets through to the anode depends on the voltage on the grid. Thus, a small voltage on the grid can be made to control a much larger voltage on the anode. This is the principle used in vacuum tubes to amplify electrical signals. The triode vacuum tube developed between 1907 and 1914 was the first electronic device that could amplify, and is still used in some applications such as radio transmitters. High speed beams of cathode rays can also be steered and manipulated by electric fields created by additional metal plates in the tube to which voltage is applied, or magnetic fields created by coils of wire (electromagnets). These are used in cathode-ray tubes, found in televisions and computer monitors, and in electron microscopes.

Crookes tube. The cathode (negative terminal) is on the right. The anode (positive terminal) is in the base of the tube at bottom.
Cathode rays travel from the cathode at the rear of the tube, striking the glass front, making it glow green by fluorescence. A metal cross in the tube casts a shadow, demonstrating that the rays travel in straight lines.
A magnet creates a horizontal magnetic field through the neck of the tube, bending the rays up, so the shadow of the cross is higher.
When the magnet is reversed, it bends the rays down, so the shadow is lower. The pink glow is caused by cathode rays striking residual gas atoms in the tube.

History

After the invention of the vacuum pump in 1654 by Otto von Guericke, physicists began to experiment with passing high voltage electricity through rarefied air. In 1705, it was noted that electrostatic generator sparks travel a longer distance through low pressure air than through atmospheric pressure air.

Gas discharge tubes

In 1838, Michael Faraday applied a high voltage between two metal electrodes at either end of a glass tube that had been partially evacuated of air, and noticed a strange light arc with its beginning at the cathode (negative electrode) and its end at the anode (positive electrode).^[4] In 1857, German physicist and glassblower Heinrich Geissler sucked even more air out with an improved pump, to a pressure of around 10⁻³ atm and found that, instead of an arc, a glow filled the tube. The voltage applied between the two electrodes of the tubes, generated by an induction coil, was anywhere between a few kilovolts and 100 kV. These were called Geissler tubes, similar to today's neon signs.

The explanation of these effects was that the high voltage accelerated free electrons and electrically charged atoms (ions) naturally present in the air of the tube.^{[ citation needed ]} At low pressure, there was enough space between the gas atoms that the electrons could accelerate to high enough speeds that when they struck an atom they knocked electrons off of it, creating more positive ions and free electrons, which went on to create more ions and electrons in a chain reaction,^{[ citation needed ]} known as a glow discharge. The positive ions were attracted to the cathode and when they struck it knocked more electrons out of it, which were attracted toward the anode. Thus the ionized air was electrically conductive and an electric current flowed through the tube.

Geissler tubes had enough air in them that the electrons could only travel a tiny distance before colliding with an atom. The electrons in these tubes moved in a slow diffusion process, never gaining much speed, so these tubes didn't produce cathode rays. Instead, they produced a colorful glow discharge (as in a modern neon light), caused when the electrons struck gas atoms, exciting their orbital electrons to higher energy levels. The electrons released this energy as light. This process is called fluorescence.

Cathode rays

By the 1870s, British physicist William Crookes and others were able to evacuate tubes to a lower pressure, below 10⁻⁶ atm. These were called Crookes tubes. Faraday had been the first to notice a dark space just in front of the cathode, where there was no luminescence. This came to be called the "cathode dark space", "Faraday dark space" or "Crookes dark space". Crookes found that as he pumped more air out of the tubes, the Faraday dark space spread down the tube from the cathode toward the anode, until the tube was totally dark. But at the anode (positive) end of the tube, the glass of the tube itself began to glow.

What was happening was that as more air was pumped from the tube, the electrons knocked out of the cathode when positive ions struck it could travel farther, on average, before they struck a gas atom. By the time the tube was dark, most of the electrons could travel in straight lines from the cathode to the anode end of the tube without a collision. With no obstructions, these low mass particles were accelerated to high velocities by the voltage between the electrodes. These were the cathode rays.

When they reached the anode end of the tube, they were traveling so fast that, although they were attracted to it, they often flew past the anode and struck the back wall of the tube. When they struck atoms in the glass wall, they excited their orbital electrons to higher energy levels. When the electrons returned to their original energy level, they released the energy as light, causing the glass to fluoresce, usually a greenish or bluish color. Later researchers painted the inside back wall with fluorescent chemicals such as zinc sulfide, to make the glow more visible.

Cathode rays themselves are invisible, but this accidental fluorescence allowed researchers to notice that objects in the tube in front of the cathode, such as the anode, cast sharp-edged shadows on the glowing back wall. In 1869, German physicist Johann Hittorf was first to realize that something must be traveling in straight lines from the cathode to cast the shadows. Eugen Goldstein named them cathode rays (German Kathodenstrahlen).

Discovery of the electron

At this time, atoms were the smallest particles known, and were believed to be indivisible. What carried electric currents was a mystery. During the last quarter of the 19th century, many historic experiments were done with Crookes tubes to determine what cathode rays were. There were two theories. Crookes and Arthur Schuster believed they were particles of "radiant matter," that is, electrically charged atoms. German scientists Eilhard Wiedemann, Heinrich Hertz and Goldstein believed they were "aether waves", some new form of electromagnetic radiation, and were separate from what carried the electric current through the tube.

The debate was resolved in 1897 when J. J. Thomson measured the mass of 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 he originally called "corpuscle" but was later named electron, after particles postulated by George Johnstone Stoney in 1874. He also showed they were identical with particles given off by photoelectric and radioactive materials.^[5] It was quickly recognized that they are the particles that carry electric currents in metal wires, and carry the negative electric charge of the atom.

Thomson was given the 1906 Nobel Prize in Physics for this work. Philipp Lenard also contributed a great deal to cathode-ray theory, winning the Nobel Prize in 1905 for his research on cathode rays and their properties.

Vacuum tubes

The gas ionization (or cold cathode) method of producing cathode rays used in Crookes tubes was unreliable, because it depended on the pressure of the residual air in the tube. Over time, the air was absorbed by the walls of the tube, and it stopped working.

A more reliable and controllable method of producing cathode rays was investigated by Hittorf and Goldstein,^{[ citation needed ]} and rediscovered by Thomas Edison in 1880. A cathode made of a wire filament heated red hot by a separate current passing through it would release electrons into the tube by a process called thermionic emission. The first true electronic vacuum tubes, invented in 1904 by John Ambrose Fleming, used this hot cathode technique, and they superseded Crookes tubes. These tubes didn't need gas in them to work, so they were evacuated to a lower pressure, around 10⁻⁹ atm (10⁻⁴ Pa). The ionization method of creating cathode rays used in Crookes tubes is today only used in a few specialized gas discharge tubes such as krytrons.

In 1906, Lee De Forest found that a small voltage on a grid of metal wires between the cathode and anode could control a current in a beam of cathode rays passing through a vacuum tube. His invention, called the triode, was the first device that could amplify electric signals, and revolutionized electrical technology, creating the new field of electronics . Vacuum tubes made radio and television broadcasting possible, as well as radar, talking movies, audio recording, and long-distance telephone service, and were the foundation of consumer electronic devices until the 1960s, when the transistor brought the era of vacuum tubes to a close.

Cathode rays are now usually called electron beams. The technology of manipulating electron beams pioneered in these early tubes was applied practically in the design of vacuum tubes, particularly in the invention of the cathode-ray tube (CRT) by Ferdinand Braun in 1897, which was used in television sets and oscilloscopes. Today, electron beams are employed in sophisticated devices such as electron microscopes, electron beam lithography and particle accelerators.

Properties

Like a wave, cathode rays travel in straight lines, and produce a shadow when obstructed by objects. Ernest Rutherford demonstrated that rays could pass through thin metal foils, behavior expected of a particle. These conflicting properties caused disruptions when trying to classify it as a wave or particle. Crookes insisted it was a particle, while Hertz maintained it was a wave. The debate was resolved when an electric field was used to deflect the rays by J. J. Thomson. This was evidence that the beams were composed of particles because scientists knew it was impossible to deflect electromagnetic waves with an electric field. These can also create mechanical effects, fluorescence, etc.

Louis de Broglie later (1924) suggested in his doctoral dissertation that electrons are like photons and can act as waves. The wave-like behaviour of cathode rays was later directly demonstrated using reflection from a nickel surface by Davisson and Germer,^[6] and transmission through celluloid thin films and later metal films by George Paget Thomson and Alexander Reid^[7] in 1927. (Alexander Reid, who was Thomson's graduate student, performed the first experiments but he died soon after in a motorcycle accident^[8] and is rarely mentioned.)

Related Research Articles

An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ACID, for "anode current into device". The direction of conventional current in a circuit is opposite to the direction of electron flow, so electrons flow from the anode of a galvanic cell, into an outside or external circuit connected to the cell. For example, the end of a household battery marked with a "+" is the cathode.

An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge carriers, which may be one of several types of particles, depending on the conductor. In electric circuits the charge carriers are often electrons moving through a wire. In semiconductors they can be electrons or holes. In an electrolyte the charge carriers are ions, while in plasma, an ionized gas, they are ions and electrons.

A cathode is the electrode from which a conventional current leaves a polarized electrical device such as a lead-acid battery. This definition can be recalled by using the mnemonic CCD for Cathode Current Departs. A conventional current describes the direction in which positive charges move. Electrons have a negative electrical charge, so the movement of electrons is opposite to that of the conventional current flow. Consequently, the mnemonic cathode current departs also means that electrons flow into the device's cathode from the external circuit. For example, the end of a household battery marked with a + (plus) is the cathode.

A triode is an electronic amplifying vacuum tube consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 1906 Audion, a partial vacuum tube that added a grid electrode to the thermionic diode, the triode was the first practical electronic amplifier and the ancestor of other types of vacuum tubes such as the tetrode and pentode. Its invention helped make amplified radio technology and long-distance telephony possible. Triodes were widely used in consumer electronics devices such as radios and televisions until the 1970s, when transistors replaced them. Today, their main remaining use is in high-power RF amplifiers in radio transmitters and industrial RF heating devices. In recent years there has been a resurgence in demand for low power triodes due to renewed interest in tube-type audio systems by audiophiles who prefer the sound of tube-based electronics.

<span class="mw-page-title-main">Vacuum tube</span> Device that controls current between electrodes

A vacuum tube, electron tube, valve, or tube is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

<span class="mw-page-title-main">Nixie tube</span> Electronic numeric display device

A Nixie tube, or cold cathode display, is an electronic device used for displaying numerals or other information using glow discharge.

Sir Joseph John Thomson was a British physicist and Nobel Laureate in Physics, credited with the discovery of the electron, the first subatomic particle to be found.

<span class="mw-page-title-main">Cold cathode</span> Type of electrode and part of cold cathode fluorescent lamp.

A cold cathode is a cathode that is not electrically heated by a filament. A cathode may be considered "cold" if it emits more electrons than can be supplied by thermionic emission alone. It is used in gas-discharge lamps, such as neon lamps, discharge tubes, and some types of vacuum tube. The other type of cathode is a hot cathode, which is heated by electric current passing through a filament. A cold cathode does not necessarily operate at a low temperature: it is often heated to its operating temperature by other methods, such as the current passing from the cathode into the gas.

An electron gun is an electrical component in some vacuum tubes that produces a narrow, collimated electron beam that has a precise kinetic energy.

<span class="mw-page-title-main">Gas-filled tube</span> Assembly of electrodes at either end of an insulated tube filled with gas

A gas-filled tube, also commonly known as a discharge tube or formerly as a Plücker tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric discharge in gases, and operate by ionizing the gas with an applied voltage sufficient to cause electrical conduction by the underlying phenomena of the Townsend discharge. A gas-discharge lamp is an electric light using a gas-filled tube; these include fluorescent lamps, metal-halide lamps, sodium-vapor lamps, and neon lights. Specialized gas-filled tubes such as krytrons, thyratrons, and ignitrons are used as switching devices in electric devices.

A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the striking voltage, the gas ionization becomes self-sustaining, and the tube glows with a colored light. The color depends on the gas used.

An X-ray tube is a vacuum tube that converts electrical input power into X-rays. The availability of this controllable source of X-rays created the field of radiography, the imaging of partly opaque objects with penetrating radiation. In contrast to other sources of ionizing radiation, X-rays are only produced as long as the X-ray tube is energized. X-ray tubes are also used in CT scanners, airport luggage scanners, X-ray crystallography, material and structure analysis, and for industrial inspection.

A Geissler tube is a precursor to modern gas discharge tubes, demonstrating the principles of electrical glow discharge, akin to contemporary neon lights, and central to the discovery of the electron. This device was developed in 1857 by Heinrich Geissler, a German physicist and glassblower. A Geissler tube is composed of a sealed glass cylinder of various shapes, which is partially evacuated and equipped with a metal electrode at each end. It contains rarefied gases—such as neon or argon, air, mercury vapor, or other conductive substances, and sometimes ionizable minerals or metals like sodium. When a high voltage is applied between the electrodes, there is an electric current through the tube, causing gas molecules to ionize by shedding electrons. The free electrons reunite with the ions and the resulting energic atoms emit light via fluorescence, with the emitted color characteristic of the contained material.

An anode ray is a beam of positive ions that is created by certain types of gas-discharge tubes. They were first observed in Crookes tubes during experiments by the German scientist Eugen Goldstein, in 1886. Later work on anode rays by Wilhelm Wien and J. J. Thomson led to the development of mass spectrometry.

A vacuum arc can arise when the surfaces of metal electrodes in contact with a good vacuum begin to emit electrons either through heating or in an electric field that is sufficient to cause field electron emission. Once initiated, a vacuum arc can persist, since the freed particles gain kinetic energy from the electric field, heating the metal surfaces through high-speed particle collisions. This process can create an incandescent cathode spot, which frees more particles, thereby sustaining the arc. At sufficiently high currents an incandescent anode spot may also be formed.

A Crookes tube is an early experimental electrical 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.

Johann Wilhelm Hittorf was a German physicist who was born in Bonn and died in Münster, Germany.

The hot-filament ionization gauge, sometimes called a hot-filament gauge or hot-cathode gauge, is the most widely used low-pressure (vacuum) measuring device for the region from 10⁻³ to 10⁻¹⁰ Torr. It is a triode, with the filament being the cathode.

In electromagnetism, the Townsend discharge or Townsend avalanche is an ionisation process for gases where free electrons are accelerated by an electric field, collide with gas molecules, and consequently free additional electrons. Those electrons are in turn accelerated and free additional electrons. The result is an avalanche multiplication that permits significantly increased electrical conduction through the gas. The discharge requires a source of free electrons and a significant electric field; without both, the phenomenon does not occur.

Eugen Goldstein was a German physicist. He was an early investigator of discharge tubes, the discoverer of anode rays or canal rays, later identified as positive ions in the gas phase including the hydrogen ion. He was the great uncle of the violinists Mikhail Goldstein and Boris Goldstein.

References

↑ Martin, Andre (1986), "Cathode Ray Tubes for Industrial and Military Applications", in Hawkes, Peter (ed.), Advances in Electronics and Electron Physics, Volume 67, Academic Press, p. 183, ISBN 9780080577333, Evidence for the existence of "cathode-rays" was first found by Plücker and Hittorf ...
↑ E. Goldstein (May 4, 1876) "Vorläufige Mittheilungen über elektrische Entladungen in verdünnten Gasen" (Preliminary communications on electric discharges in rarefied gases), Monatsberichte der Königlich Preussischen Akademie der Wissenschaften zu Berlin (Monthly Reports of the Royal Prussian Academy of Science in Berlin), 279-295. From page 286: "13. Das durch die Kathodenstrahlen in der Wand hervorgerufene Phosphorescenzlicht ist höchst selten von gleichförmiger Intensität auf der von ihm bedeckten Fläche, und zeigt oft sehr barocke Muster." (13. The phosphorescent light that's produced in the wall by the cathode rays is very rarely of uniform intensity on the surface that it covers, and [it] often shows very baroque patterns.)
↑ Joseph F. Keithley The story of electrical and magnetic measurements: from 500 B.C. to the 1940s John Wiley and Sons, 1999 ISBN 0-7803-1193-0, page 205
↑ Michael Faraday (1838) "VIII. Experimental researches in electricity. — Thirteenth series.," Philosophical Transactions of the Royal Society of London, 128 : 125-168.
↑ Thomson, J. J. (August 1901). "On bodies smaller than atoms". The Popular Science Monthly. Bonnier Corp.: 323–335. Retrieved 2009-06-21.
↑ Davisson, C.; Germer, L. H. (1927). "Diffraction of Electrons by a Crystal of Nickel". Physical Review. 30 (6): 705–740. Bibcode:1927PhRv...30..705D. doi: 10.1103/PhysRev.30.705 .
↑ Thomson, G. P.; Reid, A. (1927). "Diffraction of Cathode Rays by a Thin Film". Nature. 119 (3007): 890. Bibcode:1927Natur.119Q.890T. doi: 10.1038/119890a0 . ISSN 1476-4687.
↑ Navarro, Jaume (2010). "Electron diffraction chez Thomson: early responses to quantum physics in Britain". The British Journal for the History of Science. 43 (2): 245–275. doi:10.1017/S0007087410000026. ISSN 0007-0874.

General Chemistry (structure and properties of matter) by Aruna Bandara (2010)

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[1] Martin, Andre (1986), "Cathode Ray Tubes for Industrial and Military Applications", in Hawkes, Peter (ed.), Advances in Electronics and Electron Physics, Volume 67, Academic Press, p. 183, ISBN 9780080577333, Evidence for the existence of "cathode-rays" was first found by Plücker and Hittorf ...

[2] E. Goldstein (May 4, 1876) "Vorläufige Mittheilungen über elektrische Entladungen in verdünnten Gasen" (Preliminary communications on electric discharges in rarefied gases), Monatsberichte der Königlich Preussischen Akademie der Wissenschaften zu Berlin (Monthly Reports of the Royal Prussian Academy of Science in Berlin), 279-295. From page 286: "13. Das durch die Kathodenstrahlen in der Wand hervorgerufene Phosphorescenzlicht ist höchst selten von gleichförmiger Intensität auf der von ihm bedeckten Fläche, und zeigt oft sehr barocke Muster." (13. The phosphorescent light that's produced in the wall by the cathode rays is very rarely of uniform intensity on the surface that it covers, and [it] often shows very baroque patterns.)

[3] Joseph F. Keithley The story of electrical and magnetic measurements: from 500 B.C. to the 1940s John Wiley and Sons, 1999 ISBN 0-7803-1193-0, page 205

[4] Michael Faraday (1838) "VIII. Experimental researches in electricity. — Thirteenth series.," Philosophical Transactions of the Royal Society of London, 128 : 125-168.

[Thomson-5] Thomson, J. J. (August 1901). "On bodies smaller than atoms". The Popular Science Monthly. Bonnier Corp.: 323–335. Retrieved 2009-06-21.

[6] Davisson, C.; Germer, L. H. (1927). "Diffraction of Electrons by a Crystal of Nickel". Physical Review. 30 (6): 705–740. Bibcode:1927PhRv...30..705D. doi: 10.1103/PhysRev.30.705 .

[7] Thomson, G. P.; Reid, A. (1927). "Diffraction of Cathode Rays by a Thin Film". Nature. 119 (3007): 890. Bibcode:1927Natur.119Q.890T. doi: 10.1038/119890a0 . ISSN 1476-4687.

[8] Navarro, Jaume (2010). "Electron diffraction chez Thomson: early responses to quantum physics in Britain". The British Journal for the History of Science. 43 (2): 245–275. doi:10.1017/S0007087410000026. ISSN 0007-0874.

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