Thermionic Emission

Author: E.E. Kimberly

If the plates of the condenser of Fig. 5-14 be maintained at normal room temperature, say 22 C, and be separated by air at normal pressure, the attraction of the electrons of the charged negative plate toward the positive charge of the other plate will result in no migration between plates unless the potential difference be made so great as to cause breakdown of the resistance of the intervening air and permit a spark discharge. If the plates be sealed in a highly evacuated cell, the same phenomenon would be observed. If, however, the negative plate (or electrode) be heated to incandescence, the movements of the electrons therein will be greatly accelerated and some of the electrons themselves will leave the electrode and cluster in an invisible cloud around it. This cloud of electrons is said to constitute a "space charge" and the phenomenon is called "thermionic emission" Some of these electrons in space, having been freed from the negative electrode by high temperature, will migrate to the positive plate or electrode under the urge of the applied potential. This migration of electrons and their passage around the circuit constitutes an electric current, and its strength may be measured by a sensitive ammeter. A simple circuit carrying such a current is shown in Fig. 27-1. If no potential were applied, the cloud would form nevertheless; but no migration would occur.

Fig. 27-1 Diode and Simple Circuit

It should be noted that the direction of flow of electrons in the circuit is opposite to that in which electricity is conventionally said to flow. This contradiction occurs because the convention was established before the facts were known and it cannot now conveniently be reversed.