MAGNETISM, the name applied to a pe culiar force action first observed in connection with certain iron ores. This ore, often called lodestone, is supposed to have been discovered in Magnesia, a part of Asia Minor. It is not possible to state just when this discovery was made, hut certain passages in Lucretius show that something was known concerning it before the beginning of the Christian era. About the year 1200 we have the statement by Neckham that a lodestone free to turn takes up a definite position in space. Some further details were noted by Peregrinus (1269) and Ferrara (1629), but the greatest of the early works is that of Dr. Gilbert, a physician, who published his (De Magnet& in 1600. Those interested in the his tory of the subject may consult the (Intellectual Rise of Electricity> by Park Benjamin, in which an excellent historical sketch may be found.
The only direct evidence that a body is mag netic is its ability to exert a force on certain substances, which, by reason of their suscepti bility to this action, are called magnetic sub stances. A lodestone brought in contact with several small bodies will select those of iron or steel, if such be present, but show no apprecia ble force on copper, lead, wood or in fact on any except iron, nickel, cobalt, certain rare metals and certain alloys, discovered by Hensler, of relatively non-magnetic metals, and a few others to a lesser degree. Of even greater in terest and importance is the fact that the lode stone is able to endow steel or iron with the ability to exert this force. Soft iron loses its external magnetic qualities when removed from the immediate neighborhood of the exciting source, but hard steel or iron will retain this property for a long time. If a bar or rod of hard steel is drawn across a piece of lodestone or other permanent magnet and is then sus pended so as to be free to turn about a vertical axis it will take up a definite position, usually its line of greatest length will be approximately north and south. If it be plunged into a box of iron filings, little magnetic action will be mani fest near the middle of the bar, but near the ends considerable quantities of filings will be attracted and may he lifted against the action of gravity. These facts led to the naming of the regions of greatest external action the poles of the magnet and since the lines joining these regions would, in the case of a freely suspended magnet, lie in many places nearly north and south, the pole which seeks the north is often called the north-seeking or positive pole, the other which turns toward the south is corre spondingly named the south-seeking or negative pole. The entire subject was formerly studied with reference to the behavior of like and un like poles, and it was even supposed that these poles consisted of opposite sorts of magnetic matter. Later investigations have developed
methods less directly dependent upon the idea of poles, which are preferable for many purposes.
If we suppose two long magnets placed as shown in Fig. 1 a study of their mutual force action would indicate that each is exerting a push tending to increase the distance between them, and that the amount of this repulsion will vary with the distance between the magnets. If one of the bars be replaced by another whose magnetic quality is different the force action will be modified. If one of the magnets be re versed in position a corresponding force tending to reduce the distance between the bars would be observed. It is convenient to use as a pre liminary definition the statement that a unit pole is one which would exert unit force upon a precisely equal pole at a distance of one centi meter. The law of pole action can then be stated by saying that the force is equal to the product of the two poles strengths divided by the square of the distance between the poles.
If a freely suspended magnet is brought into the neighborhood of a large bar magnet as cated in Fig. 2 it will be observed to take up a position somewhat as indicated in the lower part of this diagram, as its point of suspension is moved along the line. The region where this directive force is noticeable is called the field of the magnet. (Gilbert's "orb of virtue"). If continuous lines are driwn, which at each point have the direction taken by the free magnet, these lines are called lines of magnetic force, and they offer a very convenient method for a general study of magnetic action. While these lines have no objective existence, it is, neverthe less, desirable to imagine that they are real and that they possess certain definite qualities. They should always be considered as being directed away from the north-seeking or positive pole. In the early conception of magnetic action these lines would have been regarded as the lines of flow of the magnetic material, and the word flux, still in use, bears evidence of this concep tion. It is convenient also to regard the lines of force as being tinder tension and capable of repelling each other. The number of actual lines of force which could be drawn about a magnet is infinite. For purposes of compari son, however, it is customary to represent the force action at a point upon the unit pole placed at that point by the number of lines drawn per square centimeter on a surface per pendicular to the field. A unit field is one in which a force action upon a unit pole is one dyne, about the weight of 1-1000 of a gramme.