Gas Filled Tubes | Conduction in a Gas

Gas filled tubes


In the vacuum tubes, the electrons pass from the cathode to anode in a vacuum. In that type of tubes, extreme care is taken to generate as perfect a vacuum as possible to prevent ionization of gases and the resulting large uncontrolled currents. It may be mentioned here that the logic of the triode is the fine control of free electrons within the valve through the electrostatic fields of the grid and anode. If gas is present even in a little amount, the electrons pass from cathode to anode will be the reason for ionization of the gas. The ionized molecules would interact with the fine control and it makes the device useless amplifier. In some applications, fine control of electrons within the valve is of less importance than the efficient handling and turning HIGH LOW OR on-off of huge currents. In such a condition, some inert gases (e.g argon, neon, helium) at small pressures are purposely introduced into the valve envelope. Such tubes are called as gas-filled tubes. The gas-filled tubes are able of doing many jobs that vacuum tubes never perform and which are especially useful in industrial systems and control circuits. In this chapter, we shall focus our attention on the most important types of gas-filled tubes having special reference to their characteristic properties.

Gas-Filled Tubes

Gas filled tubes 1

A gas-filled tube is essentially a vacuum tube with a small amount of inert gas at low pressure. The gas pressure in a gas-filled tube normally ranges from 10 mm of Hg to 50 mm. The development of gas-filled tubes is similar to that of vacuum tubes, but one thing is different which is the cathodes, grids, and anodes are normally larger in order to carry huge current. However, the characteristic properties of both are markedly different. Firstly, a gas-filled tube can conduct or pass much * more current than the equivalent vacuum tube. It is due to the electrons passing from the cathode to anode collide having gas molecules and ionize them i.e. knock out electrons from them. The additional electrons pass to the anode together with the original electrons, the resulting plate current will increase. Secondly, a gas-filled tube has very small control over the electrons in the tube as compared to that of a vacuum tube. Once the ionization initiates, the control of the gas-filled tube is continuously reduced.


Gas-filled tubes are normally classified according to the type of electron emission employed. On that basis, they may be classified into two main types namely; cold-cathode type and hot-cathode type.

Cold-cathode type.

In this type of gas-filled tubes, the cathode is will never be heated as in a vacuum tube. The ionization of the gas is due to the energy available from natural sources like, cosmic rays, sun rays, or radioactive particles in the air. These natural sources are the underlying reasons for the initiate of flow current in cold-cathode gas tubes. majority of cold-cathode tubes are used as diodes.

cold cathode type

Fig. 3.1 shows the schematic symbol of a cold-cathode gas diode, called a glow tube. The dot within the circle shows the presence of gas. Fig. 3.2 shows the schematic symbol of cold-cathode gas triode called a grid glow tube. Hot-cathode type. In this type of gas-filled tubes, the cathode is heated just as in a normal vacuum tube do. The electrons passing from cathode to plate cause ionization of the gas molecules. that types of tubes are used as diodes, triodes, and tetrodes.

cold cathode type

Fig. 3.3 shows the schematic symbol of a hot-cathode gas diode, called as phanotron whereas
Fig. 3.4 shows the symbol of hot-cathode gas triode, called as thyratron.

Conduction in a Gas

Conduction in a Gas

A gas under normal pressure is a perfect insulator and never conducts current. However, if the gas pressure is small, it is possible to generate a huge number of free electrons in the gas by the process of ionization and thus the gas to become a conductor. This is exactly what happens in gas-filled tubes. The current passes in gas at very low pressure can be Awesomely illustrated by referring to the hot-cathode gas diode shown in Fig. 3.5. The space between cathode and anode of the tube has gas molecules. When the cathode is heated, it emits a huge number of electrons. These electrons make a cloud of electrons near the cathode, known as space charges. If the anode is made positive with respect to the cathode, the electrons (magenta dots) from the space charge speed towards the anode and collide with gas molecules (cyan circles) in the tube. ions slowly drift or moves towards the cathode and neutralize the space charge. Consequently, the tube resistance decreases, resulting in a huge plate current. Hence, it is because of the neutralization of space charge by the positive gas ions that plate current in a gas tube is too much increased.

The following points may be noted regarding the passing current in a gas at low pressure :
(i) At small anode-cathode voltage, the ionization of the gas did not happen and the plate current is about the same as for a vacuum tube at the same anode voltage.
(ii) At some anode-cathode voltage, known as ionization voltage, ionization of the gas takes place. The plate current increases dramatically to a huge value due to the neutralization of space charge by the positive gas ions. The ionization voltage depends upon the type of gas and pressure of the gas in the tube.
(iii) Once ionization has initiated, it is maintained at an anode-cathode voltage much smaller than ionization voltage. However, minimum anode-cathode voltage, known as deionizing voltage, exists below which ionization can’t be maintained. Under such a situation, the positive gas ions combine with electrons to make neutral gas molecules and current passes stop. due to switching action, a gas-filled tube can be used as an electronic switch.

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Reference: Principles Of Electronics By V K Mehta And Rohit Mehta

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