Bifilar coil

Some bifilars have adjacent coils in which the convolutions are arranged so that the potential difference is magnified (i.e., the current flows in same parallel direction).

[1] A different type of bifilar coil is used in some relay windings and transformers used for a switched-mode power supply to suppress back-emf.

When the current through the primary is interrupted, as happens when the relay is switched off, most of the magnetic energy is intercepted by the secondary coil which converts it to heat in its internal resistance.

This is only one of several methods of absorbing the energy from the primary coil before it can damage the device (usually a vulnerable semiconductor) that drives the relay.

If this clamping coil were not used, the stray magnetic flux would attempt to force a current to flow through the primary wire.

[3] Large examples were used in inventor Daniel McFarland Cook's 1871 "Electro-Magnetic Battery"[4] and Nikola Tesla's high frequency power experiments at the end of the 1800s.