In 1904, Sir John Ambrose Fleming invented the first vacuum tube called vacuum diode. It is also called Fleming valve or thermionic tu
be.Vacuum diode is an electronic device that allows the electronic current in one direction(cathode to anode) and blocks the electronic current in another direction (anode to cathode).
Two electrodes of vacuum diode Vacuum diode is the simplest form of vacuum tube. It consists of two electrodes, a cathode, and an anode or plate. The cathode emits the free electrons. Hence, it is called ass emitter. The anode collects the free electrons.Hence it is called as collector. The cathode and anode are enclosed in an empty glass envelope. The anode is hollow cylinder made of molybdenum or nickel and cathode is a nickel cylinder coated with strontium and barium diode. The anode surrounds the cathode. In between the cathode and anode an empty spaces present, through which the free electrons or electric current flow. What is an electrode? electrode is a conductor through which free electrons or electric current leaves or enters.In vacuum tube, cathode is an electrode or conductor from which the free electrons are emitted into the vacuum. On the other hand,anode is an electrode that collects the free electrons emitted by the cathode.In other words, free electrons leave the cathode and enter into anode.
Electron emission depends on the amount of heat applied and the work function.
The number of free electrons emitted by the cathode will depend on the wo factors: amount of heat applied and the work function. if more amount of heat is applied, more number of free electrons are emitted.similarly , if less amount of heat is applied, less number of free electrons is emitted. The minimum amount of energy required to remove the free electrons from the metal is called work function. metals with low work function will require less amount of heat energy to emit the free electrons. On the other hand, metals with high work function will require large amount of energy to emit the free electrons. Hence choosing a good material will increase the electron emission efficiency. Most commonly used thermionic emitters include oxide coated cathode, tungsten and thoriated tungsten. Directly and indirectly heated cathode When cathode is indirectly or directly heated, free electrons are emitted from it.
In the directly heated cathode , the heat energy is supplied directly to the cathode. hence,a small amount of heat energy is enough to emit the free electrons from the cathode. When the heat energy is directly supplied to the cathode, large number of free electrons gain sufficent energy and breaks the bonding with the cathode. the free electrons that break the bonding with the cathode are emitted into the vaccum. These emitted free electrons are attracted towards the anode.
In the indirectly heated cathode,no electrical connection is present between the cathode and the heater.Hence , the cathode is not heated directly. The heat enegy is supplied to the heater and the heater will transfer its heat enegy to the cathode. When the heat energy applied to the cathode is increased to a desired level, the free electrons in the cathode gain suffient energy and break the bonding with the cathode .The free electronics that break the bonding with the cathode are emitted into the vaccum.Thesa emitted free electronics are attracted towards the anode.
Vacuum diode with forward voltage
When the heat is supplied to the heater, it gains heat energy.This heat energy is transferred to the cathode.When the free electrons in the cathode gains sufficient energy,they breaks the bonding with the cathode and jumps into vacuum. The free electrons in the vaccum need sufficient kinetic energy to reach the anode. If voltage is applied to the vacuum diode, in such a way that anode is connected to a positive teminal and the cathode is connected to the negative terminal, the free electrons in the vacuum gains enough kinetic energy to reach the anode. We know that , if two opposite charged particles are placed close to each other they get attracted.In this case, anode is positively charged and free electrons emitted from the cathod are negatively charged . Hence the free electrons that gain enough kinetic energy will move or attracted towards the anode.These free electrons carry the electric current while moving from cathode to anode. If the positively voltage applied to plate or anode is increased, the number of free electrons attracted towards the anode is also increased. Thus, the electric current in the vacum diode increases with the increase in the anode or plate voltage. Vaccum diode with reverse voltage If voltage is applied to the vacuum diode, in such way that anode is connected to the negative terminal and the cathode is connected to the positive terminal , the free electrrons in the vaccum gains enogh kinetic energy to reach the anode. However anode repels the free electrons that try to move towards it. We know that if two likely charged particles are placed close to each other they get repelled.In this case , anode is negatively charged and the free electrons emitted from the cathode are also negatively charged and the free electrons emitted from the cathode are also negatively charged.Hence the anode repels the free electrons that are emitted by the cathode.Therefore ,no electric current flows in the vacuum diode. Vacuum diode with zero voltage If no voltage is applied to the vacuum diode,anode or plate acts as neutral.It neither attracts nor repels the free electrons emitted from the cathode do not remove attracted towards the anode. Therefore no electric current occurs in the vaccum diode. BY concluding we came to know that vacuum diode allows electric current from cathode to anode but not from anode to cathode.This one way direction of electric current it can be acts as a switch.
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