The Triode

Author: N.H. Crowhurst

Whether electrons come away from the filament or stay close to it depends on the electric field at the filament surface. This force can be made negative, even though the plate is positive, by putting a negatively charged wire mesh, or grid, between the plate and the filament. If the grid is sufficiently negative, it will keep all the electrons close to the filament; if it is less negative, the positive force from the plate will overcome it to some extent, and some of the electrons will get through. Thus, the number of electrons that pass from the filament to the plate depends on how negative the grid is and how positive the plate is.

Construction and action of a triode

Beside its modern counterpart, the early triode was crude. The filament had to have current flowing through it to keep it hot, so that electrons would be thrown off (emitted). The only essential thing, however, is to have a hot metal emitter, or cathode. By using a separate heater wire, to which current is supplied to provide the heat, the cathode does not need to have this supply connected to it. This makes the tube much more versatile. Improvement in materials and better design has enabled smaller tubes to be made, and now space is further saved by putting two or more "tubes," or complete electrode assemblies, into one evacuated "envelope." (Strictly speaking, the envelope - made of glass in most cases - is the tube, but as each electrode assembly in one envelope can be used separately if desired, it has become the practice to call each assembly a "tube" because it acts as one.)