The rotating magnetic field induces an AC voltage in the stator windings. The SRM has wound field coils as in a DC motor for the stator windings. A bifilar coil is an electromagnetic coil that contains two closely spaced, parallel windings. The resistor represents the small residual resistance of the inductor's wire windings. Shellac was used for fixing inductor, motor, generator and transformer windings. Leakage inductance depends on the geometry of the core and the windings. Isolation transformers are designed with attention to capacitive coupling between the two windings. The magnetic amplifiers were based on tiny magnetic cores with two wire windings. This creates a small magnetic field within the ballast windings. This appears as a voltage drop across the windings as long as the current increases. For small transformers, resistance heating by injection of current into the windings is used. Thus, every brushed DC motor has AC flowing through its rotating windings. Oil-filled transformers use highly refined mineral oil to insulate and cool the windings and core. In practice, some flux traverses paths that take it outside the windings. Hz supply; with equivalent outputs on four secondary windings.