The Relation Between Frequency and Spark-Over Voltage in a Sphere-Gap Voltmeter

The standard instrument for measuring crest values of high alternating voltages at 60 cycles is the sphere-gap voltmeter, which measures a voltage by the distance which it will flash between spheres. In much of the high-voltage research, however, very high frequencies are used. For measuring the voltages used in these high-frequency tests, the sphere-gap voltmeter is used, the assumption being made that its calibration at high frequency is but little, if any, different from that at 60 cycles. In the endeavor to make the sphere-gap a standard for measuring peak values of voltage at high frequencies, as it is at present a standard at commercial frequencies, experimental data were obtained from which calibration curves for the sphere-gap voltmeter were plotted for frequencies ranging from 28,000 to 425,000 cycles per sec. for standard conditions of temperature and pressure. These curves cover a voltage range from about 10,000 to 50,000 volts, the source of the high-frequency voltage being a Poulsen arc with variable inductance and capacity in, its a-c. circuit. The results show no appreciable change in voltage required to flash across a given gap as the frequency increases until a frequency of about 20,000 cycles is reached, then a gradual decrease inl required voltage as the frequency increases from 20,000 to 60,000 cycles, after which a single curve holds for all frequencies at least up to 425,000 cycles per sec., the highest frequency tested.