Ferrite Toroid Core: Nickel-Zinc Ferrite, Then 3 Days at 950 °C in Oxygen (Snoek 1948, Ex. III)

Snoek's nickel-zinc ferrite ring core (Example III), then heated for three full days at 950 °C in oxygen. The long heat lowered the initial permeability from 3000 to 800 and the Curie point from about 80 °C to about 60 °C. Nickel oxide dust is a cancer hazard: wear a respirator when milling. Source: J. L. Snoek, US 2,452,530 (Philips, 1948), Example III. Picture: the patent's Fig. 4, the ring core.