A bifilar coil is wound with two wires laid side by side, going through each turn together. Depending on how you connect the four wire ends (two starts, two finishes), you get different electrical behaviors:
- Series-aiding: Connect end-of-wire-A to start-of-wire-B. The magnetic fields add. This is Tesla's original configuration (US Patent 512,340, 1894).
- Series-opposing: Connect end-of-wire-A to end-of-wire-B. The magnetic fields cancel, leaving primarily the inter-winding capacitance.
- Parallel: Connect both starts together and both ends together. Halves the inductance.
Why it's interesting
The key difference between a bifilar coil and a conventional coil is inter-winding capacitance. Because the two wires run parallel and very close together for the entire length of the winding, there's significant capacitance between them. This shifts the coil's self-resonant frequency — sometimes dramatically.
The first experiment tutorial walks through winding and measuring both a conventional and a bifilar coil on the same core to see this effect directly.
Historical context
Tesla's 1894 patent describes using the bifilar configuration specifically to increase the coil's energy storage capacity. Whether his claims hold up under modern measurement is exactly what the Bifilar Coil Resonance Characterization experiment tests.