The experimental conditions may get out of control, so that the experimenter will eventually have the wrong idea of the natural dispersion of spark parameters. Spark investigators have given much attention to the Fitch circuit capable of providing steadier pulses than the conventional Marx design. Suppose a shunt mounted on a sectioned electrode is used to register the conduction current at the base of a spark being formed. Current measurement by a tubular shunt produces a methodological error due to the skin effect. Spurious oscillations will then rapidly damp due to the coil resistance, which grows because of the skin effect at high oscillation frequency. The chapter presents mutual inductance determining the level of the voltage to be measured is not calculated but is found from the calibrated measuring circuit. Calibration is not difficult to make in laboratory conditions, but it presents a problem when one attempts to make remote measurements of lightning current by registering its electromagnetic field.