A gas-filled tube is essentially a vacuum tube having a small amount of some inert gas
at low pressure.
The gas pressure in a gas-filled tube usually ranges from 10 mm of Hg to 50 mm.
The construction of gas-filled tubes is similar to that of vacuum tubes, except that the
cathodes, grids and anodes are usually larger in order to carry heavier current.
However, the characteristic properties of the two are markedly different. Firstly, a gas
filled tube can conduct much
*more current than the equivalent vacuum tube. It is
because the electrons flowing from cathode to anode collide with gas molecules and ionize them i.e. knock out electrons from them.
The additional electrons flow to the
anode together with the original electrons, resulting in the increase in plate current.
Secondly, a gas filled tube has far less control on the electrons in the tube than that of
Once the ionization starts, the control of gas-filled tube is tremendously
Classification. Gas-filled tubes are usually classified according to the type of electron
emission employed. On this basis, they may be classified into two types namely; cold-
cathode type and hot-eath-ode type.
In this type of gas-filled tubes, the cathode is not heated as in a
vacuurn tube. The ionization of the gas is caused by the energy available from natural
sources such as cosmic rays, sun rays or radioactive particles in air.
sources are the underlying reasons for the start of conduction in cold-cathode gas
tubes. Most cold-cathode tubes are used as diodes.
Fig.1 shows the schematic symbol for a cold cathode gas diode, known as glow tube.
The dot within the circle indicates the presence of gas.
Fig.1 shows the schematic
symbol of cold-cathode gas triode, known as grid glow tube.
Hot-cathode type. In this type of gas-filled tubes, the cathode is heated
just as in an ordinary vacuum tube. The electrons flowing from cathode to plate
cause lionization of the gas molecules. Such tubes are used as diodes, triodes and
The ability of a gas-filled tube to carry large current is, of course, no
recommendation in itself. A copper wire will do the same thing and with better efficiency.
But a gas filled tube has one special ability which the wire does not possess; the ability
carry current in one direction.
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