The Wehnelt is biased to a negative voltage ( typically " 200V to " 300V ) relative to the emitter, which is usually a tungsten filament or Lanthanum hexaboride ( LaB 6 ) hot cathode.
32.
The first cathode ray tube to use a hot cathode was developed by John B . Johnson ( who gave his name to the term Johnson noise ) and Harry Weiner Weinhart of Western Electric, and became a commercial product in 1922.
33.
The classical example of thermionic emission is the emission of electrons from a hot cathode into a vacuum ( also known as "'thermal electron emission "'or the "'Edison effect "') in a vacuum tube.
34.
For decades, electron-tube designers tried to augment amplifying tubes with electron multipliers in order to increase gain, but these suffered from short life because the material used for the dynodes " poisoned " the tube's hot cathode . ( For instance, the interesting RCA 1630 secondary-emission tube was marketed, but did not last . ) However, eventually, Philips of the Netherlands developed the EFP60 tube that had a satisfactory lifetime, and was used in at least one product, a laboratory pulse generator.
