MAGNETISM. This article, which deals mainly with the magnetic properties of materials, is divided into the following sections : history ; fundamental phenomena and concepts ; magnetic measurements ; diamagnetism ; paramagnetism ; ferromagnetism; susceptibilities of the elements; magnetic deflection of atomic rays; magnetism and the structure of matter. An account is given of the experimental facts, the methods of investigation, and the theoretical interpretation of the results. A number of aspects of magnetism are only referred to incidentally here, as they are dealt with in other articles. (See ELECTRICITY, LIGHT, TERRES TRIAL MAGNETISM, ATOM, ELECTRON, QUANTUM THEORY, ZEE This survey of the development of magnetism deals mainly with the period up to the beginning of the nineteenth century. The welding of electricity and magnetism into a single wider science, which is considered elsewhere (see ELECTRICITY), and the later work, which forms the main subject matter of other sections of this article, are only briefly reviewed.
Early History.—The science of magnetism may be said to have grown from the observation that a certain mineral ore, lodestone, possesses the property of attracting iron. The native magnet seems to have been long known in every part of the world. Among the Greeks, Thales of Miletus (c. 630-55o B.c.) is credited with a knowledge of the attractive power, which he attributed to a soul, but it was probably familiar considerably earlier. It is mentioned by Plato, Aristotle, Theophrastus and others. In one of Plato's Dialogues (Ion), Socrates says that the stone "not only attracts iron rings, but also imparts to them a similar power of attracting other rings ; and sometimes you may see a number of pieces of iron and rings suspended from one an other so as to form quite a long chain," a phenomenon shown with the Samothracian rings by the workers at the iron mines on the island of Samothrace. Magnetization by induction had been ob served. Legend attributed the discovery of lodestone to a Cretan shepherd who was so strongly attracted to the earth by his iron tacked sandals, and iron-tipped crook, that he dug to ascertain the cause. Imagination was stimulated by the mysterious attrac tive power of the magnet, and later, many fables sprang up round the remarkable stone—of magnetic domes supporting vast ii on (and even bronze!) statues in mid-air, and of "mountains in the north of such great powers of attraction that ships are built with wooden pegs, lest the iron nails should be drawn from the timber."
Lucretius, the Roman poet, in De Rerum Nature (c. 6o B.c.), in the course of an account of extraordinary and paradoxical telluric phenomena, for which he gives non-supernatural explanations, dis cusses magnets at some length (vi. 906-1087) and states that the name magnes for lodestone is derived from Magnesia, the district in Asia Minor in which the ore occurred plentifully. The passage as a whole probably gives a good idea of the Romans' knowledge of magnetism. Lucretius in the following passage, as translated by W. E. Leonard—from whose rendering (1922) our other ex tracts from the De Rerum Natura are also taken—attributed the attractive properties of lodestone for iron to an exhalation of fine particles: . . Stream there must from off the lode-stone seeds Innumerable, a very tide, which smites By blows that air asunder lying betwixt The stone and the iron." The seeds swim through the pores of the iron, and the place be tween the iron and the stone becomes a void, when "the primal germs of iron, headlong slipping, fall conjoined into the vacuum" dragging the body of the iron with them, for "Naught there is That of its own primordial elements More thoroughly knit or tighter linked coheres Than nature and cold roughness of stout iron." The process is aided by the buffetings of the air molecules beyond the iron and in its own substance. A definitely new fact is recorded by Lucretius—repulsive phenomena have been observed: "I've seen Those Samothracian iron rings leap up And iron filings in the brazen bowls Seethe furiously, when underneath was set The magnet stone." Lucretius manages to devise an explanation of even this apparently unaccountable behaviour, in terms of exhalations from the brass. Lucretius' discussion of magnets is representative of much of the Greek and Roman scientific outlook. There was much speculation, frequently showing remarkable insight, and a desire for wide generalization ; but the idea of carrying out experiments to find a definite answer to a definite question had not arisen. Knowledge remained vague and qualitative, and the speculations, being un checked by experiment, were of little permanent scientific value.