Conductivity of Insulating Materials up to Breakdown Voltages.—(a) In the method of measurement used, the recording instruments were protected from injury in case of puncture, by placing an insulated hot cathode rectifier in series with the sample being tested. This enabled the current through the sample to be limited and also controlled by controlling the temperature of the cathode of the rectifier. When with a given applied voltage the current did not reach a constant maximum value upon raising the temperature of the cathode, the voltage had reached the breakdown point. (b) These classes of insulating materials were found whose conductivity, respectively, (1) obeys Ohm's law throughout; (2) obeys Ohm's law up to near breakdown and then increases at an accelerated rate; and (3) increases over the whole range of voltage at an accelerated rate to breakdown. In general, material in class 3 have the greatest dielectric strength; these in class 1, the least. Results are given only for these materials, untreated cement paper, paraffined fish paper, and black treated cloth, which were selected as representative, respectively, of the three classes.