This Matter of Contact Potential

The term "contact potential," when used in connection with thermionic vacuum tubes, has come to include certain combinations of spurious voltages which affect the operation of the tube. The combination which is effective depends upon the method of measurement used or upon the application to which the tube is put. The combinations which are effective are grouped under four heads: (1) floating element potential, (2) floating, shunted element potential, (3) effective current cutoff, and (4) calculated correction potential. The first three are points upon the current-voltage characteristic of the element while the fourth is the correction term appearing in the equation giving plate current as a function of the applied voltages. The true nature of contact potential is also pointed out.