The CATHODE is the electron-emitting element of a tube. Cathodes are usually composed of
special materials that are heated either directly or indirectly.
DIODE OPERATION depends upon current flow through the tube. Because the cathode is the only
electron-emitting element in the tube, current can only flow in one direction, from the cathode to the
plate. For current to flow, the plate must be positive relative to the cathode. When the plate is negative
relative to the cathode, current cannot flow within the tube.