Cathode fall of potential, the potential difference between the cathode of an electric discharge in a gas and the plasma column. Most often the cathode fall is due to an excess of positive ions near the cathode, which forms a positive space charge that screens the cathode. In some kinds of non-self-sustained electric current in a gas, however, intense electron emission from the cathode gives rise to a cathode fall created by a negative space charge (an excess of electrons); such a cathode fall limits the emission and prevents any further increase of the space charge.
In the region of the cathode fall the primary generation of electrons takes place that makes the flow of electric current in a gas possible: a distinction is drawn between emission from the surface of the cathode (field emission, thermionic emission, explosive electron emission and the like), the formation of the layer, the acceleration of the electrons, ionisation and so on. The energy needed for these processes is drawn from the cathode fall, which varies with the conditions of the discharge from a few V to 1 kV. The differences between the various forms of gas discharge are due first of all to the features and the differences of these near-cathode processes. The particular value of the cathode fall depends on the kind of gas, on the material and shape of the cathode and on the state of its surface. The cathode fall does not depend on the distance between the electrodes, nor on the magnitude of the discharge current over a wide range of values of the latter.
In the region of the cathode fall the primary generation of electrons takes place that makes the flow of electric current in a gas possible: a distinction is drawn between emission from the surface of the cathode (field emission, thermionic emission, explosive electron emission and the like), the formation of the layer, the acceleration of the electrons, ionisation and so on. The energy needed for these processes is drawn from the cathode fall, which varies with the conditions of the discharge from a few V to 1 kV. The differences between the various forms of gas discharge are due first of all to the features and the differences of these near-cathode processes. The particular value of the cathode fall depends on the kind of gas, on the material and shape of the cathode and on the state of its surface. The cathode fall does not depend on the distance between the electrodes, nor on the magnitude of the discharge current over a wide range of values of the latter.