Text: Materials are classified according to whether or not they offer a good path for the magnetic lines of force. Substances of comparatively low reluctance (q.v.) are termed magnetic. They are also often referred to as f erromagnetic. They include iron, steel, nickel, cobalt manganese, chromium, magnetite, certain alloys of copper, manganese and aluminum (called Heusler Alloys ), and certain other oxides. The magnetic laws governing these substances can best be set forth by means of magnetization curves. Substances having high reluctance are termed nonmagnetic. For commercial purposes, air is the most important non-magnetic substance. However, practically all substances, with the exception of the ferromagnetic ones, follow the same magnetic laws as air. While the reluctance of a non-magnetic substance is constant, the reluctance of magnetic substances varies, as the amount of f fox (q.v.) in the substance changes. Although air and all other substances, with the exception of the ferro-magnetie ones are classed as non-magnetic substances, and are poor conductors of magnetie flux, when compared to iron, etc., nevertheless they are by no means magnetic insulators. There are no substances which will absolutely prevent the passage of magnetic flux.

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