there is a limit to the length of a magnet was early commented upon. It is impossible to maintain the poles of a magnet ten feet from each other on a steel bar; intermediate poles will generally form, which are called consequent poles. My attention, a few years since, was called to a magnetic motor which well illustrates the formation of consequent poles. An inventor claimed Ո N N S N FIG. 3. that he had discovered a neutral line in the space near the poles of a horseshoe magnet. His motor consisted of an armature of soft iron (N S, Fig. 3) on the end of a balance arm. The other end of the arm vibrated be tween two stops. It was claimed that when the proper rate of vibration was reached the armature would move across the neutral line to and fro, the neutral line acting as a species of cut-off. The existence of the neutral line was shown by presenting small tacks to a bar of iron which was moved in front of the poles of the magnet. As the bar was moved away from the poles the tacks dropped off; but in continuing the motion away from the magnet another position was found in which the tacks again were attracted to the bar. The bar apparently had moved through a neutral line. The phenomenon, however, was caused by the shifting of the consequent poles, due to the changing intensity of the field, and had no real existence. It is needless to say that the motor did not run from this cause. For many years there was no substantial change in the form of the ship's compass. One or more comparatively heavy bar magnets were affixed to a card bearing the points of the compass, and the card was pivoted so that its graduations should pass fixed points on the compass box. Lord Kelvin made a great improvement in the old form by fixing a number of very light steel magnets on a light disk, getting in this way strong magnetism, so to speak, or magnetic moment combined. with extreme lightness and steadiness. The employment of iron ships has made it necessary to compensate the attraction of the vessel upon the compasses by placing steel magnets in proper positions on the deck near the compass, or by placing a compass high above the deck, and by means of its indications correcting the lower compasses. It is necessary also to swing the ship occasionally—that is, to take the bearings of some point on the shore near a harbour while the ship is turned completely around. In this way the attraction of the mass of the ship on the compass when sailing on different courses is ascertained. The force between two attracting pith balls can be represented by F= mm where m and m, are the charges on the pith balls and r is the distance between them. When there is a plate of glass between the charged balls the force is very much diminished. If k is a factor depending on the insulating power of the glass, the force is F-3 = m m that is, the greater the insulating power the smaller the force. In a similar manner we can express the force of attraction between two magnetic poles as F: = m m in which m and m, 2 represent the strength of the poles and is the distance between them. If, however, the poles are immersed in a solution of iron the attraction between them would be expressed by F- m m, in which μ is a quantity which = μ depends upon the medium. If μ is large, the force of attraction is small. The attraction, for instance, between two magnets placed in a vessel containing a salt of iron would be less than in air. It is to such considerations of the nature of the surrounding medium that we owe the advances in our knowledge of magnetism. Previous to the year 1800, I have said, no account was taken of the surrounding media, and one magnetic pole was considered to act upon another as if it were an action at a distance and not from point to point in the medium between the attracting poles. It is well to consider these two points of view carefully, for in the present study of electricity and magnetism we are chiefly occupied with a study of what goes on in the medium in which the attracting bodies are immersed. The early workers in magnetism apparently formed no mental picture of the state of the field in which the attracting bodies were placed. They had not formed the conception of lines of force, although they were familiar with the arrangement of iron filings around the poles of a magnet. The defenders of the theory of action at a distance did not carefully examine the disturbance of a magnetic field by the introduction of a magnetic pole, whereas in the modern theory of action from point to point in the medium this disturbance is fully considered. All space around the earth is filled with lines of force, which crowd into the north and south pole. In the room in which the reader of this book is seated these lines are generally parallel and the space of the room is filled with them. If there is an iron article in the room they tend to crowd through the iron, finding it a better conductor than the air. We say, therefore, that there is a flow of magnetic induction through the iron. We can think of magnetic induction as the accumulation of the lines of force in the space occupied by the iron. To obtain a space anywhere on the earth's surface free from lines of force we must employ a thick-walled hollow sphere of iron. In the space inside the sphere there will be no lines of force. The space has been, so to speak, swept free of lines of force, and there is a flow of induction through the walls of iron. A compass inside such a sphere would no longer be influenced by the poles of the earth. An examination of the terminology of a subject often will show the trend of the subject. We speak to-day of the flow or flux of magnetic induction, and we picture to ourselves this flow as taking place from one pole of a magnet to the other through the air. We also speak of closed magnetic circuits, which we obtain to a great extent when we put an armature on a horseshoe magnet. The flow of induction takes place then through the armature, and not through the air. A compass is not affected by a closed magnetic circuit, for no lines of force escape to flow through its steel. We also speak of the resistance of the air to the flow of magnetic induction. It is harder to force the flow of magnetic induction through air than through iron or steel. If, for instance, we should cut a horseshoe magnet in two at its bend and then put the ends together again, it can not be made as strong as it was originally; although the joint may be made very perfect, the thin layer of air opposes a resistance to the flow of induction. We also speak of the magneto-mo tive force which establishes the flow of induction, much as we speak of the electro-motive force which establishes a current of electricity. In a certain sense, therefore, we can regard a magnet as a little battery. It has a magneto-motive force. It has a circuit with a certain resistance. It has a flow analogous to a current. Although this terminology shows that we are studying the action from point to point in the substance of a magnet and in the space around it, we must not conclude that we have evidence of a flow of a magnetic fluid or the flow of the ether of space around and through the substance of a magnet. We shall see, also, that we have no evidence of an actual flow of electricity in the case of an electrical current. It is often stated that certain persons have detected a peculiar effect when the poles of a magnet are passed over their bodies, and that they have seen lambent flames proceeding in the dark from the poles of a magnet. The reader curious in this matter will find many observations of these alleged phenomena in a book entitled The Odic Force, by Baron Reichenbach.* It has been found, however, that wooden bars painted to resemble magnets produce the same effect as the actual magnets. The effect of very powerful electro-magnets on the human body has been tried recently. With the head placed between the poles of an electro-magnet sufficiently strong to lift tons of weight one can carry on mental calculations with perfect ease, and a genius undisturbed by the milieu doubtless could write a sonnet to magnetism. The human nervous system does not appear to be sensitive to magnetic force. *Edited by John Ashburner, M. D. Partridge & Brittan, New York, 1855. |