Barium Ferrite Disc Magnet, Sintered at 1300 °C (Went, Philips 1956)

The dark grey ceramic magnet on fridges and in loudspeakers is barium ferrite. It is made from two cheap powders: one part barium carbonate to six of iron oxide, milled, fired, milled again, pressed and sintered. No cobalt or nickel is needed. This is Example 34 of the Philips patent. This iteration sinters at 1300 °C, where his rod held 1,850 gauss with a coercive force of 1,780 oersted. Source: J. J. Went, G. W. van Oosterhout and E. W. Gorter (Philips), US 2,762,777 (1956). They pressed rods; the claim covers "a body of a desired shape", and the disc size is ours. The patent does not say how to magnetise it; our way is stated. Picture: ferrite magnets, Omegatron, Wikimedia Commons, CC BY-SA 3.0.