For a transformer with a separate primary and secondary winding, each winding is a portion. This is a step-up transformer, which produces a high voltage from the secondary winding. The bottom of the driver's secondary winding is connected to ground. The current through the secondary winding is zero at the balanced condition. An Epstein frame comprises a primary and a secondary winding. A wire wound coil is the secondary winding. There is the resistance of secondary winding, the cables, and the welding electrodes. The arrangement sequentially directs the output of the secondary winding to the appropriate spark plugs. The supply transformer "(T)" secondary winding is connected across the primary tuned circuit. The collapsing magnetic field then induces an electric field in the secondary winding. This induces a current through the secondary winding, which is connected to the sensing circuit. The secondary winding has a burden resistor to set the current scale.