IN a paper entitled “Contact Non-Linearity, with reference to the Metal Rectifier and the Carborundum Ceramic Non-Linear Resistor” (j. Inst. Elec. Eng., 89, Pt. 1, No. 23 ; November 1942), A. Fairweather gives an account of an experimental analysis designed to correlate and formulate the behaviour of various structures possessing contact non-linearity. New data are presented concerning representative contact combinations such as various carborundum combinations, the carborundum ceramic non-linear resistor, the cuprous oxide and selenium rectifiers, and the diamond. Measurements have been made of the variation of the characteristic parameters, resistance and capacitance, with each of the four significant variables-current, mechanical pressure, thickness of semi-conductor and temperature. The ranges of current and voltage far exceed those which have been employed hitherto. Examination of the results obtained leads to various conclusions regarding the mechanism of contact non-linearity, but not, however, in conflict with the familiar 'barrier layer' hypothesis, which is fully supported by the work described.