Ferrite Toroid Core: Copper-Cadmium Ferrite, 200-Hour Grind, 1000 °C in Oxygen (Snoek 1948, Ex. I)

A low-loss ferrite ring core for radio coils: copper and cadmium ferrite made, ground for about 200 hours, pressed into a ring and heated 6 hours at 1000 °C in oxygen. Snoek's core had an initial permeability of 100 and a loss factor of about 0.01 at 1000 kHz. Cadmium oxide is poisonous. Source: J. L. Snoek, US 2,452,529 (Philips, 1948), Example I. Picture: our diagram.