The multiple chopping gap serves simultaneously as a load capacitor for the impulse generator and allows excellent reproducibility of the chopping time and does not distort the wave shape until the chopping point. Rod gaps and sphere gaps (above 1000 kV) draw a substantial pre-discharge current prior to the voltage break-down, causing a voltage drop. This affects the comparison tests when testing power transformers, hence particular attention must be paid to the way the circuit is built. The sphere distance is automatically adjusted by the Haefely Impulse Generator controls In the automatic mode, the gap distance is set automatically as a function of the charging voltage. The gap distance is displayed on the controls. The chopping is initiated by the first stage, having a triggering spark plug.