In the 1870s, British physicist William Crookes and others were able to evacuate rarefied tubes to a pressure below 10 −6 atm. In 1838, Michael Faraday passed a 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). 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. 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. The voltage applied between the electrodes accelerates these low mass particles to high velocities.Ĭathode 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. They travel in straight lines through the empty tube. Since the electrons have a negative charge, they are repelled by the cathode and attracted to the anode. The increased random heat motion of the filament atoms knocks electrons out of the atoms at the surface of the filament and into the evacuated space of the tube. Modern vacuum tubes use thermionic emission, in which the cathode is made of a thin wire filament that is heated by a separate electric current passing through it. The electric field accelerated the ions and the ions released electrons from the cathode when they collided with the cathode. The early cold cathode vacuum tubes, called Crookes tubes, used a high electrical potential between the anode and the cathode to ionize the residual gas in the tube. To release electrons into the tube, they must first be detached from the atoms of the cathode. The image in a classic television set is created by focused beam of electrons deflected by electric or magnetic fields in cathode ray tubes (CRTs).Ĭathode rays are so named because they are emitted by the negative electrode, or cathode, in a vacuum tube. Electrons were first discovered as the constituents of cathode rays. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, the glass opposite the negative electrode is observed to glow from electrons emitted from the cathode. crookes tube: An early experimental electrical discharge tube, invented by English physicist William Crookes and others around 1869-1875, in which cathode rays, streams of electrons, were discoveredĬathode rays (also called electron beams or an e-beams) are streams of electrons observed in vacuum tubes.
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