Magnetism

The Compass.—The property of orientation of a magnet, by which it tends to turn approximately north and south, was appar ently not known in classical antiquity; the suggestion that a passage in Homer (Odyssey, viii.) : "In wondrous ships, self mov'd, instinct with mind; No helm secures their course, no pilot guides; Like man intelligent, they plough the tides." (Pope's translation) —indicates a use of the compass being hardly convincing. In China, according to tradition, some form of compass was used at a very early period. Hoang-ti is said to have constructed a chariot on which was a female figure indicating the four cardinal points at a date which can be fixed as 2637 B.C, A "chariot of the south" was given to some envoys to direct them on their way about III° B.C., while in the following century, according to later history, there were Chinese cars which held a floating needle. The first really explicit reference to polarity is found in a diction ary of A.D. I21, where it is stated that the south-pointing property may be imparted to iron by blows, or by means of the lodestone. In the third century, it is mentioned that ships were directed to the south by a needle, but a description of a water compass is not found in any Chinese work until A.D. I I I I–I 7, This is very little earlier than Guyot de Provins (c. 20o), a minstrel at the court of Frederick I. (Barbarossa), refers to the use by sailors of a floating needle which was rubbed on an ugly brown stone. About the same time (1207) Neckam of St. Albans mentions the pivoted needle, which showed mariners their course. The Chinese, then, possibly had some knowledge of the polar properties of natural lodestone, and of iron rubbed on lodestone in very early times, but there was no continuous systematic application of the knowledge. The use of directing needles may have waxed and waned with the course of time, and it was not until about the twelfth century that the compass came into wide-spread use. It is then referred to in Arabic writings, and it seems probable that the knowledge of the directing needle passed from the Chinese to the Arabs, and from the Arabs to the Franks at the time of the first Crusades; but the possibility that the Western invention was independent of the Eastern cannot be definitely ruled out. The early compasses con sisted simply of a magnetic needle in a splinter of wood floating in water. The pivoted needle seems to be of Western origin. (See COMPASS.) Peter Peregrinus.—The earliest systematic investigations on magnets were made by Peter Peregrinus of Maricourt, who at one time received instruction from Roger Bacon. Bacon had a great admiration for him. "What others strive to see dimly and blindly, like bats in twilight, he gazes at in full light of day, because he is a master of experiment. Through experiment, he gains knowl edge of things natural, medical and chemical; indeed of everything in the heavens or earth." The discoveries in magnetism are set forth in a letter to a soldier friend sent from camp at Lucera, besieged by Charles of Anjou (Aug. 1269). This letter—Epistola Petri Peregrini de Maricourt ad Sygerum de Foucaucourt militern de magnete—is the first treatise on magnetism. Peregrinus dis cusses the requirements in an investigator, and among them he specially stresses assiduity in handiwork for experimental research. He clearly realized the importance of experiment, the recognition of which is so characteristic of modern science. Peregrinus fash ioned a lodestone into the form of a globe, on which he drew lines indicating the direction in which a magnetic needle tended to set itself. From the similarity of these to meridians, he was led to the invention, by analogy, of the term magnetic "poles," these being the regions in which the magnetic power was concentrated. He distinguished the north and south poles by placing the stone in a wooden skiff, which was floated in water, and noting which end pointed to the north. "If this pole were then turned away a thou sand times, a thousand times would it return to its place by the will of God." By bringing an already marked magnet near the floating stone, he found that like poles repelled, unlike poles attracted each other. He observed that when an iron needle was magnetized, the end which had touched the south pole of the stone turned to the north, and that a strong magnet could reverse the polarity of a weaker one. An important discovery was that the fragments of a broken magnet behaved as magnets, isolated poles not occurring. Peregrinus thought that a magnet turned towards

the pole of the sky, deriving its power, in some way, from the whole of the heavens. In the second part of the letter methods for constructing a floating compass and also a better instrument, a pivoted compass, are described.

The range of Peregrinus' magnetic investigations was remark able, and it is not till some three hundred years later that any material advance was made. In 1581 Robert Norman published the New Attractive, which gives a clear statement of the funda mental laws of attraction and repulsion between poles, and con tains an account of Norman's discovery and measurement of magnetic dip (1571). The downward tendency of the north pole of a pivoted needle had been noticed previously by Hartmann of Nuremberg in 1544, but was not placed on record till much later.

William Gilbert.

In 1600 William Gilbert of Colchester (154o-16o3) published his book De Magnete, Magnetisque Corporibus, et de magno magnete tellure; Physiologia nova, plurimis et argumentis et experimentis demonstrate (on the mag net, magnetic bodies also, and on the great magnet the earth; a new physiology, demonstrated by many arguments and experi ments). Gilbert, who has been called the "father of modern elec tricity" and the "Galileo of magnetism," and to whom Peregrinus may be regarded as a worthy forerunner, laid the foundations of the modern sciences of electricity and magnetism. After a career at Cambridge, Gilbert travelled on the continent, and on his return practised as a doctor in London. He was later appointed court physician to Queen Elizabeth, and his house near St. Paul's be came a meeting place for scientists. Gilbert dedicates his epoch making book "to you alone, true philosophers, ingenious minds, who not only in books, but in things themselves look for knowl edge," and he does not conceal his scorn of those whose opinions are based on hearsay, tradition and speculation. Again and again he stresses the importance of experiment : "In the discovery of secrets and in the investigation of the hidden causes of things, clear proofs are afforded by trustworthy experiments rather than by probable guesses and opinions of ordinary professors and philosophers." It was by experiment that he was able to refute the unfounded speculations and idle superstitions that had grown up round the magnet, and to make positive advances in laying the foundations of the science of magnetism. Gilbert was, however, truly appreciative of the work of sincere investigators. The first part of De Magnete is occupied by a comprehensive résumé of previous writings. To separate the true from the false, many painstaking experiments were carried out, and in the course of these such investigations as those of Peregrinus were refined and extended.

Though Gilbert's claim to honour rests largely on the scientific method which he practised, his actual discoveries were of the utmost importance. In his investigations he made use of a globu lar piece of lodestone, a "terrella," as Peregrinus had done ; for experiments on electric action he devised a "versorium," an early form of electroscope, consisting of a light pivoted needle of any metal. He clearly discriminated between magnetic and electric actions. He made a systematic study of amber, which attracted light bodies, but only when rubbed, and he showed that this property was common to a large number of substances, such as glass, sulphur and diamond, which he called "electrics." Whereas all bodies could be attracted by electrics, lodestone (which Gilbert proved to be an iron ore) attracted only magnetizable substances, and required no frictional stimulus. It had been stated that iron rubbed by diamonds became magnetized, and turned to the north. "We made the experiment ourselves," says Gilbert, "with seventy five diamonds in presence of many witnesses, employing a num ber of iron bars and pieces of wire, manipulating them with the greatest care while they floated on water, supported by corks; but never was it granted to me to see the. effect mentioned by Porta." Gilbert found that the lodestone exhibited no electric at tractive power; that magnetic power was exerted through screens which cut off the electric action entirely; and that there was no tendency for an electrified body to orientate itself in a definite direction, like a magnetized piece of iron.