Cathode Ray Oscilloscope: Thermionic Emission

Electrons are attracted to the nucleus of an atom. There is a strong attractive force between the electrons and the positive charges of the nucleus. In order to move these electrons farther from the atom, energy is needed. This can be done by heating a metal using electric current. We can look at a vacuum diode and see how electrons move between the cathode and anode.

Thermionic emission

A vacuum diode consists of a glass bulb containing two electrodes. One electrode is called the anode and the other is the cathode. The cathode is made up of tungsten filament. The cathode can be heated by a small current connected to the filament. This filament when heated will release electrons from its surface. These electrons can be attracted to the anode when there is a high ptential difference applied between the anode and the cathode.


The filament is connected to a 6 volt external battery (usually). When it is heated, a large number of electrons are free to move. As a result, a cloud of electrons is found outside the metal surface of the filament. Many of these electrons are held back by the attractive force of the atomic nucleus. Some of the electrons gained enough energy and escape from its surface. This effect is called thermionic emission.

Thus, Thermionic emission can be defined as the escape of high energy electrons from the surface of a tungsten filament.

Thermionic emission can be used to produce a continuous flow of electrons in a cathode ray tube. When the cathode is connected to the anode by an extra high tension (EHT) voltage supply, a narrow beam of fast electrons will move to the anode. The beam of electrons moving from the cathode to the anode is called cathode rays.

Thus, Cathode ray is the beam of electrons moving from the cathode to the anode in a cathode ray tube.

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