Cathode and Cathode Rays
A cathode is the negative electrode of a battery or electrolytic capacitor. A cathode is also the reduction electrode in an electrochemical cell. Cathode rays are beams of electrons that emanate from acathode and are most commonly produced by heating a filament in a vacuum tube or by bombarding a target with positive ions.
Metals are excellent conductors of electricity because the outer orbiting electrons (negative charges) in the metal atom are very loosely bound to the nucleus. A traditional and still useful conceptual model, depicts a metal as containing a cloud of electrons with the ability to migrate throughout the metal. Accordingly, any electrical force (voltage) applied to a metal will cause this cloud of electrons to move in response to the force. In contrast, insulators are materials (e.g., rubber and plastic) in which there are few free electrons. Without these responsive electrons, electrical force has little or no effect. Electrons, usually do not actually leave the metal. If the metal is heated, however, some of the electrons become energetic enough to be expelled from the metal. The hotter the metal the greater the number of electrons exuded from the surface. If the metal is placed in a vacuum the stream of electrons meets little resistance (e.g., from air molecules) and the stream or cloud of exuded electrons is free to move in response to electrical and gravitational fields. Because electrical forces are vastly stronger than gravitational forces, gravitational effects can usually be ignored and the drift of the electron cloud or direction of movement of the electron stream is controlled solely by the application of electromagnetic fields.
The cloud of electrons causes a green glow to appear on the glass tube wall near the cathode. The glass fluoresces because of the bombardment by the electrons. When first discovered, researchers thought that the bombarding rays must have been emitted from the cathode, and termed these cathode rays. Within a year or two, researchers managed to pull the electrons in a constant stream from the cathode and across a space in a glass vacuum tube. The cloud of electrons can be induced to move away from the hot electron exuding metal by placing a positively charged object (anode) nearby. The electrons will cross the vacuum with velocity directed toward the anode with a speed directly proportional to the strength of the positive charge on the anode.
The first to realize that the cathode rays were, indeed, streams of charged negative particles (electrons) was the English physicist William Crookes (1832-1919) who demonstrated that cathode rays could be deflected by a magnet. In 1897, English physicist J. J. Thomson (1856-1940) demonstrated that the cathode rays were attracted to positively charged bodies. These two experiments proved beyond doubt that cathode rays were negatively charged particles, and in 1906 Thomson received the Nobel Prize for establishing the existence of the electron.
Cathode rays (electron streams) are utilized in many electrical devices including television screens, computer monitors and x-ray machines. Before the advent of transistors and integrated circuits (chips) electrical devices relied upon glass vacuum bottles called vacuum tubes.
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