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Cathode ray
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Cathode rays (also called an electron beam or e-beam) are streams of electrons observed in vacuum tubes, i.e. evacuated glass tubes that are equipped with at least two metal electrodes to which a voltage is applied, a cathode or negative electrode and an anode or positive electrode.
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Cathode rays (also called an electron beam or e-beam) are streams of electrons observed in vacuum tubes, i.e. evacuated glass tubes that are equipped with at least two metal electrodes to which a voltage is applied, a cathode or negative electrode and an anode or positive electrode. They were discovered by German scientist Johann Hittorf in 1869 and in 1876 named by Eugen Goldstein kathodenstrahlen (cathode rays). Electrons were discovered in cathode rays. In 1897 British physicist J. J. Thompson showed they were composed of a previously unknown negatively charged particle, which he named electron.
Cathode rays are so named because they are emitted by the cathode electrode. To release electrons into the tube, they first must be detached from the atoms of the cathode. In modern vacuum tubes, this is done by making the cathode a thin wire filament and passing an electric current through it. The current heats the filament red hot. The increased random heat motion of the filament atoms knocks electrons out of the atoms at the surface of the filament, into the evacuated space of the tube. This process is called thermionic emission.
Since the electrons have a negative charge, they are repelled by the cathode and attracted to the anode. They travel in straight lines through the empty tube. The voltage applied between the electrodes accelerates these light particles to high velocities. Cathode rays are invisible, but their presence was first detected in early vacuum tubes when they struck the glass wall of the tube, exciting the atoms of the glass 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 travelling in straight lines from the cathode. After the electrons reach the anode, they travel through the anode wire to the power supply and back to the cathode, so cathode rays carry electric current through the tube.
History
After the 1650 invention of the vacuum pump by Otto von Guericke, physicists began to experiment with mixtures of rarefied air and electricity. In 1705, it was noted that electrostatic generator sparks travel a longer distance in rarefied air than in standard air. The scientists of the day did not think this could happen. In 1838, Michael Faraday passed current through a rarefied air filled glass tube and noticed a strange light arc with its beginning at the cathode (negative electrode) and its end almost at the anode (positive electrode). The only place where there was no luminescence was just in front of the cathode, which came to be called the "cathode dark space", "Faraday dark space" or "Crookes dark space". Hence, it became known that whenever a voltage is applied to rarefied air, light is produced.
Scientists began traveling from town-to-town delighting audiences by making light glow in glass tubes. They did this by first taking an air-filled glass tube of which they would pump the air out. Next, wires would be attached at the opposite ends of the tube, and then the voltage would be turned up. This would make the tube glow in lovely patterns. In 1857, German physicist and Glass blower Heinrich Geissler sucked even more air out with an improved pump and noticed a fluorescent glow, thus inventing the Geissler tube. While Geissler tubes are intended to cause an enclosed low pressure gas to glow, observers noticed that certain glasses used in the tube envelope (enclosure) would glow, but only at the end connected to the positive side of the power supply. Special tubes were developed for the study of these rays by William Crookes and are called Crookes tubes.
Toward the end of the 19th century, this phenomenon was studied in great detail by physicists, yielding a Nobel Prize, for example, to Philipp von Lenard. It was soon understood that cathode rays consist of the actual carriers of electricity which are now known as electrons. The fact that the cathode emits the rays showed that electrons have negative charge.
Properties of Cathode Rays
Like a wave:
- they travelled in straight lines
- Produced a shadow when obstructed by objects
- could pass through thin metal foils without disturbing them (Tested by New Zealander Ernest Rutherford using gold foil.)
These conflicting properties caused disruptions when trying to classify it as a wave or particle. Crookes insisted it was a particle, whilst 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 evidence was strong because scientists knew it was impossible to deflect electromagnetic waves with an electric field.
See also
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