Some Theory The electrons in a conductor are relatively free moving. If a potential difference is placed across the conductor an electron current will flow. Electrons will flow through the conductor but they cannot escape the conductor to the space outside it. They need to have enough energy to overcome the induced positive charge from their absence. The energy needed for an electron to escape the induced positive charge is called the work function of the material. This energy is normally supplied in the form of heat. Sir Owen Williams Richardson coined the term “Thermionic Emission” and he received the Nobel Prize in Physics for his work. Thermionic Emission is the process of emitting charges from the surface of a heated metal. Sir Owen Williams Richardson Image Credit: www.nobelprize.org The equation below is the Emission Equation and it calculates J, the emission current density and is measured in amps per square meter. J=λRA0(T^2)*e^[(-w)/KT] J=emission current density T=absolute temperature of emitter W=work function of the material or binding energy K=Boltzmann Constant= 1.3806488 × 10-23 m2 kg s-2 K-1 λR=Material specific constant A0=(4πm(k^2)e)/(h^3)  For A0, m is the mass of an electron, e is the charge of an electron, and h is Plank’s constant = 6.62606957 × 10-34 m2 kg / s. In order to obtain a useful value for J, a high temperature must be applied to the emitter. Image depicting electrons failing to escape the attractive force from the induced positive charge of the metal. Image Credit: www.st-andrews.ac.uk Image depicting electrons escaping the heated cathode. Image Credit: www.st-andrews.ac.uk Electrons close to the nucleus need to absorb large amounts of energy to move up to higher orbital shells. When enough energy is added, the electron is at the point where it is too far away to be pulled back by the nucleus. It has been emitted from the atom because of the thermal energy supplied. Thermionic emitters in vacuum tubes are either directly heated or indirectly heated. In a directly heated tube, the heater doubles as the emitter. This can cause problems when connected directly to AC power which is why most tubes were indirectly heated. The hot cathode supplies the free charges to the vacuum. The cathode is placed at ground or 0 Volts and the anode is supplied with a high positive voltage. Electrons with enough energy leave the space just outside the cathode and travel to the anode.  When all emitted electrons travel to the anode the tube is in its saturation point. The electrons easily travel through the tube since it is a vacuum and there is nothing in the way to stop the flow. That is why trace gases are not needed. This device with two terminals (cathode and anode) is the diode. For more information on the theory of vacuum tubes check out http://www.tubebooks.org/ . They have loads of books on vacuum tube theory, operation, and design all in PDF format.

Intro What is a Vacuum Tube? Some History How they work Types and Applications  Bibliography