Cathode ray
Description
Description
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[thumb\|A diagram showing a [Crookes tube](/wiki/Crookes_tube "Crookes tube") connected to a high voltage supply. The Maltese cross has no external electrical connection.](/wiki/File:Crookes_tube2_diagram.svg "Crookes tube2 diagram.svg")

Cathode rays are so named because they are emitted by the negative electrode, or [cathode](/wiki/Cathode "Cathode"), in a vacuum tube. To release electrons into the tube, they first must be detached from the [atoms](/wiki/Atom "Atom") of the cathode. In the early experimental [cold cathode](/wiki/Cold_cathode "Cold cathode") vacuum tubes in which cathode rays were discovered, called [Crookes tubes](/wiki/Crookes_tube "Crookes tube"), this was done by using a high electrical potential of thousands of [volts](/wiki/Volt "Volt") between the anode and the cathode to [ionize](/wiki/Ionization "Ionization") the residual gas atoms in the tube. The positive ions were accelerated by the [electric field](/wiki/Electric_field "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](/wiki/Thermionic_emission "Thermionic emission"), in which the cathode is made of a thin wire [filament](/wiki/Electrical_filament "Electrical filament") which is heated by a separate [electric current](/wiki/Electric_current "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](/wiki/Fluorescence "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](/wiki/Control_grid "Control 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](/wiki/Vacuum_tube "Vacuum tube") to [amplify](/wiki/Amplifier "Amplifier") electrical signals. The [triode](/wiki/Triode "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](/wiki/Radio_transmitter "Radio transmitter"). High speed beams of cathode rays can also be steered and manipulated by [electric fields](/wiki/Electric_field "Electric field") created by additional metal plates in the tube to which voltage is applied, or [magnetic fields](/wiki/Magnetic_field "Magnetic field") created by coils of wire ([electromagnets](/wiki/Electromagnet "Electromagnet")). These are used in [cathode\-ray tubes](/wiki/Cathode-ray_tube "Cathode-ray tube"), found in televisions and computer monitors, and in [electron microscopes](/wiki/Electron_microscope "Electron microscope").

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 File:Katódsugarak mágneses mezőben(1).jpg|Crookes tube.  The cathode (negative terminal) is on the right.  The anode (positive terminal) is in the base of the tube at bottom. 
 File:Katódsugarak mágneses mezőben(2).jpg|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.
 File:Katódsugarak mágneses mezőben(3).jpg|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.
 File:Katódsugarak mágneses mezőben(4).jpg|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.

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What is cathode rays?