Ayrton-Perry Winding

Named after physicists William Ayrton and John Perry who developed it in 1888 for precision instruments, Ayrton-Perry winding places two layers of fine resistance wire on the same ceramic core, the second layer wound in the opposite helical direction to the first. The two layers are connected in parallel from end to end. Current divides between the layers, and because the helices are mirror-images the magnetic fields generated by the two halves of the current cancel along the entire length, not just turn-by-turn as in a bifilar.

Residual inductance values are typically 5 – 10× lower than a comparable bifilar — under 50 nH for 100 Ω parts and below 20 nH for 10 Ω current-sense shunts. Capacitance is also better controlled because the two wires alternate positions over the surface rather than running adjacent at constant potential. The combined effect is the flattest impedance-versus-frequency curve of any wirewound construction.

Ayrton-Perry parts are essential where wirewound stability is needed at high frequency: precision current shunts in switching regulators, high-voltage divider arms in HV measurement, and damping resistors in pulse power. The two-layer construction roughly doubles the wire length per ohm, increasing cost. Long-time supplier qualification and tight assembly process control are required to maintain layer-to-layer alignment.