Since one joule is nearly equal to three-quarters of a foot-pound, the
energy storage in foot-pounds F is roughly given by the rule F =
(3/8)CV^{2}/10^{6}. For spark lengths of the order of five to fifteen
millimetres, the disruptive voltage in air of ordinary pressure is at
the rate of 3,000 volts per millimetre. Hence, if S stands for the
spark length in millimetres, and C for the aerial capacity in
microfarads, it is easy to see that the energy storage in foot-pound
is