The sphere gap as a means of measuring high voltage

A gap method of measuring high voltage is desirable in certain tests because a gap measures the maximum point of the voltage wave. The needle gap is unreliable, and is subject to large corrections due to humidity, etc. The sphere gap is free from complicated corrections and the curve is subject to calculation. Wherever voltage is measured by a gap, correction must be made for air density — that is, altitude or barometric pressure, and temperature. The laws for this correction have been deduced for the sphere gap, from tests, over an air density range equivalent to that from sea level to 18,000 ft. elevation. An equation is given for calculating sphere spark-over curves for various spacings, radii, air density, etc. Standard measured curves for convenient sizes of spheres at sea level are given for practical use, as well as a table for applying these curves to any altitude. The altitude correction may be made very accurately to the standard curve, either by applying the calculated correction constant or by applying the correction factor given in the tables. The complete standard curves should not be calculated when reliable measured curves are available. The effect of high frequency and impulse voltages are discussed. In making measurements certain precautions are necessary for accuracy, as well as for protection of apparatus. These precautions are discussed and test results are given.