Gas-Filled Triodes

Author: E.E. Kimberly

The gas in a gas-filled diode will not ionize until the plate potential exceeds a certain minimum value because, in order that a migrating electron may dislodge another electron from a gas atom by collision, it must have attained a considerable velocity before it collides with the atom.

The velocity attained by a free body is a direct function of the force applied and depends also on the duration of application of the force. Hence, if the gas pressure is high (there are many atoms per unit volume), an electron leaving the filament cloud will not travel far toward the plate before it collides with a gas atom. Therefore, to accelerate the electron to an ionizing velocity in such a short distance, a higher plate potential (force) must be used than if the gas were less dense.

A grid placed near the cathode and between the cathode and the plate may be charged and used to control the electron velocity and, hence, the plate voltage necessary for ionization of the gas. The ionization potential having been reached, the tube becomes highly conducting, and the current flow is limited almost entirely by the resistance of the external plate circuit. The grid potential, having started the current flow, is powerless to stop it, even though the grid be made highly negative to the cathode. This is called a "trigger" action. The grid voltage which just fails to produce ionization is called the critical voltage. If the plate current be interrupted, the grid will regain control. Thus, even though the gas-filled triode is similar in construction to the vacuum-type triode, its characteristics are much different. The cold-cathode tube of this type is called a grid-glow tube, and the hot-cathode type is a thyratron.