Calibration of uniform-field spark-gaps for high-voltage measurement at power frequencies

A calibration is given for spark-gap electrodes designed to ensure that the spark path lies in a uniform field, and it is shown that for all three sizes of electrodes investigated, covering a range of 9 to 315 kV (peak), the sparkover voltages are within 0.2% of the values given by the empirical law V=24.22S+6.082¿S kV (peak) at 20°C and atmospheric pressure of 760 mm Hg, where S is the electrode spacing in centimetres. All observations were made with vertical gaps having the lower electrode connected to earth, and comparative data were obtained for a sphere gap with electrodes 25 cm in diameter. The effects of surrounding objects and of faulty alignment of the electrodes were determined experimentally, and on the basis of these results a number of recommendations are made, giving the limits within which the spark-over voltage is defined by the above equation.