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 lawV=24.22S+6.082√SkV (peak) at 20°C and atmospheric pressure of 760 mm Hg, whereSis 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.