Control of Gaseous Conduction

In electric circuits of all sorts, power is transmitted principally by metallic conductors. In order to control the flow of power, or to convert it from one form to another, the constants of the circuits are varied. The connections of the metallic conductors may be altered for this purpose; but on high potentials this operation is attended with difficulty. The rapidity and ease of control are also limited. For this reason there has been a tendency, in recent years, toward the increased use of control devices employing non-metallic conduction and these are now being rapidly brought to the fore. As examples we have the very versatile thermionic tube and the mercury arc rectifier. The former has received particular attention of late. Devices employing conduction by gaseous ions have been handicapped in this general evolution by three main disabilities: the first being the difficulty of placing the discharge where wanted, the second, the tendency of the working gas to disappear, and the third, decidedly erratic action. These disabilities have recently been to a large extent removed by the advent of a principle called the ``short path principle´´, by which discharge can be prevented except where wanted. This has also led to long life, for by placing the discharge in proximity only to certain porous materials, gaseous clean-up and disintegration are both prevented. With this change, uniformity of action also appears. By utilizing this principle, gaseous conduction devices may be designed, for example, as rectifiers.