Measurements of maximum values in high voltage testing

The breakdown strength of insulating materials is measured by the maximum voltage which they will withstand under given conditions. Tests are ordinarily made on alternating voltages, and the measurement of the maximum voltage is effected by indirect rather than direct means; that is, by measuring the virtual value of the voltage on either the primary or the secondary of the transformer and arriving at the maximum value by using a measured or assumed value of the peak-factor^∗ of the wave. It is unfortunately true that this peak-factor is more often assumed than measured, and no doubt many tests of dielectric strength are inaccurate because of unsuspected changes in the wave-form produced by conditions of the test. Only in the spark-gap has an apparatus been available whereby a measure of the actual maximum value of the high voltage could be obtained. The spark-gap is however, a most uncertain piece of apparatus, the vagaries in the behavior of which have by no means been accounted for. Installed according to the Standardization Rules of the Institute, a spark-gap for measuring 250,000 volts requires a space such that no “extraneous” body comes nearer than 4 ft. 3 in. (129.4 cm.); that is, a clear space or room 8½ by 8½ by 8½ feet (259 cm.) is required. In view of this requirement, the spark-gap is a very large and cumbersome, as well as uncertain and unsatisfactory apparatus.