Cathode ray
From Wikipedia, the free encyclopedia
Cathode rays are streams of electrons observed in vacuum tubes, i.e. evacuated glass tubes that are equipped with at least two electrodes, a cathode (negative electrode) and an anode (positive electrode) in a configuration known as a diode.
When the cathode is heated, it emits some radiation which travels to the anode. If the inner glass walls behind the anode are coated with a phosphorescent material, they glow. A metal shape placed between the electrodes casts a shadow on the glowing coating. This suggested that the cause of the light emission was comprised of rays emitted by the cathode and hitting the coating. They travel towards the anode in straight lines, and continue past it for some distance.
This phenomenon was studied in great detail by physicists toward the end of the 19th century, yielding a Nobel Prize for Philipp von Lenard. Cathode rays were first produced by Geissler tubes. 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. It was soon understood that cathode rays consist of the actual carriers of electricity which are now known as electrons. The fact that the rays are emitted by the cathode, i.e. the negative electrode, showed that electrons have negative charge.
Cathode rays propagate in a straight line in the absence of external influences, but are deflected by electric or magnetic fields (which can be produced by placing high-voltage electrodes or magnets outside the vacuum tube - this explains the effect of magnets on a TV screen). The refinement of this idea is the cathode ray tube (CRT), also known as Braun's tube (because it was invented 1897 by Ferdinand Braun). The CRT is key to television sets (though alternative display technologies are making inroads), oscilloscopes, and vidicon television cameras.
In addition to their use within cathode ray tubes, higher energy beams of relativistic electrons (generated by various types of electron beam accelerators) are used extensively within many industries to perform precision electron beam welding, rapid curing of thermosetting plastics, and cross-linking of thermoplastics to improve their physical properties.
Recent developments in electron beam accelerator technology include compact modular KeV accelerators which are being adopted by consumer packaging, medical device sterilization, and air treatment applications. These devices produce far less x-ray radiation than MeV accelerators with housings that look like early microwave ovens as opposed to lead lined concrete bunkers.
