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 radiation, which travels to the anode. If the inner glass walls behind the anode are coated with a phosphorescent material the incident electrons induce a glow. The prescence of cathode rays was first postulated in early studies in vacuum tubes by placing metal shapes between the electrodes, thereby casting a shadow on the phosphorescent coating. This suggested that the cause of the light emission was due to rays emitted by the cathode and hitting the coating. They travel towards the anode in straight lines and continue past it for some distance.
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. 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.
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 in 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 in many industries to perform precision electron beam welding, rapid curing of thermosetting plastics, and cross-linking of thermoplastics to improve their physical properties. Relativistic electron beams can also serve as a gain medium for free electron lasers.
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.