Jochen's High Voltage Page

Measuring high voltages by spark length

The simplest way of measuring high voltages is by their maximum spark length. The maximum spark length is determined by applying the high voltage to a pair of electrodes and bringing the electrodes closer to each other until a spark jumps over. Of course, this distance depends on shape and dimensions of the electrodes, as well as on the pressure of the surrounding gas, it's composition and many other factors. For AC voltages, their frequency and general time dependence also play a role. The following table and graph gives maximum spark length for DC or low frequency sinusoidal voltages, between spherical electrodes of the specified diameter and in dry air at normal pressure (760 Torr = 1013 hPa) and temperature (25 °C). Spark lengths between needle points are also given, but they are less reliable as they imply a hv source capable of delivering infinite current.

For homogenous fields or nearly homogenous fields (e.g. between sperical electrodes with a spacing much less than their diameter), the following formula applies approximately:

U[kV] = 30 d[cm]

The data for the following table and graph was taken from The CRC Handbook of Chemistry and Physics (62nd edition) (left column) and A. Bouwers: Elektrische Höchstspannungen, Springer, Berlin 1939 (right column).

Print this graph: PostScript PDF

Print this graph: PostScript PDF

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