Figs. 1, 2, 3.—Diagrams illustrating the Attraction and Repulsion of Magnets.
In [Fig. 1] the lower outline, M, represents a magnet fixed in position, and the upper bar represents another magnet arranged to swing freely around the pivot a. A magnet, as is generally known, will arrange itself in a north-to-south position if suspended from its center, like a scale beam, and allowed to swing freely, and the same end will always point toward the north. On this account the ends of a magnet are called its poles, and the one that will point toward the north is designated the north pole, while the other one is the south pole. The terms north and south poles were applied to magnets centuries ago, but at the present time the ends are more commonly designated as positive and negative. In [Fig. 1] it will be noticed that the stationary magnet has its positive end upward, and this attracts the negative end of the swinging magnet. If the order of the poles is reversed, so that the positive of the swinging magnet will come opposite the positive of the stationary one, then there will be a repulsive action instead of an attraction, as is shown in [Fig. 2]. If the two negative ends were placed opposite, the effect would be the same. From this we see that to obtain an attraction we must place the magnets so that opposite poles come together, and that by reversing the order we obtain a repulsive action.
If the swinging magnet is replaced by a bar of iron, as is shown in [Fig. 3], there will be an attraction, no matter what end of the magnet may be uppermost, thus showing that either end of a magnet will attract a bar of iron. The explanation of these different actions is that when two magnets are brought into proximity to each other each one exerts its force without any regard to the other, and if the two are set to act together they will attract one another, but if set to act in opposition they will repel. When one of the bars is not a magnet, but simply a piece of iron or steel, this bar, having no attractive or repulsive force of its own, can only obey the attractive action of the other, which is the only one that exerts a force.
Figs 4, 5.—Diagrams illustrating the Method of obtaining Rotary Motion with Magnets.
In [Fig. 4] M is a magnet bent into the form of a U, commonly called a horseshoe magnet. The short bar set between the upper ends is also a magnet, and is arranged so as to revolve around the shaft s. From what has just been explained in connection with [Figs. 1 and 2] it will be understood that, with the poles as indicated by the letters, there will be an attractive force set up between the top end of the straight bar and the P end of the horseshoe, and thus rotation will be produced in the direction of the arrow. The rotation, however, will necessarily stop when the bar reaches the position shown in [Fig. 5], for then the attraction between the poles will resist further movement. If the straight bar were not a magnet, but simply a piece of iron or steel, it is evident that when in the position of [Fig. 4] the attraction would be just as much toward the right as toward the left, and if the bar were placed accurately in the central position it would not swing in either direction. It would be in the condition called, in mechanics, unstable equilibrium. In practice this condition could not be very well realized, as it would be difficult to set and retain the bar in a position where the attraction from both sides would be the same, therefore the rotation would be in one direction or the other; but whichever way the bar might move, it would only swing through one quarter of a revolution, into the horizontal position of [Fig. 5].
If we reflect upon these actions we can see that if we could destroy the magnetism of both parts before the straight bar reaches the position of [Fig. 5] it would be possible to obtain rotation through a greater distance than one quarter of a turn, for then the headway acquired by the rotating part would cause it to continue its motion. If, after the completion of one half of a revolution, we could remagnetize both parts, we would then set up an attraction between the lower end of the straight bar and the left side of the horseshoe, for then the polarity of the former would be the reverse of that shown in [Fig. 4]—that is, the lower end would be negative. By means of this second attraction we would cause the bar to rotate through the third quarter of the revolution, and if, just before completing this last quarter, we were to remove all the magnetism again, the headway would keep up the motion through the final quarter of the revolution, thus completing one full turn. From this it will be realized that if we could magnetize and demagnetize the two parts twice in each revolution a continuous rotation could be obtained.
If the magnetizing and demagnetizing action were only applied to the rotating part we would fail to keep up a continuous rotation, for, as was shown in connection with [Fig. 3], the action when the straight bar reached the position of [Fig. 5] would be the same as if it were magnetized, owing to the fact that a magnet always exerts an attraction upon a mass of iron. Suppose, however, that we were to reverse the polarity of the rotating part just as it reaches the position of [Fig. 5], then there would be two poles of the same polarity opposite each other, and, as shown in [Fig. 2], the force acting between them would be repulsive, and would push the bar around in the direction of rotation. Not only would the right-side pole of the horseshoe force the end of the bar away from it, but the negative pole, on the left side, would attract this same end, and thus a force would be exerted by the two poles of M to keep up the rotation through the next half of a circle. On reaching this last position the rotation would stop if the polarity of the revolving bar were left unchanged, for then the poles facing each other would be of opposite polarity. If, however, we again reversed the polarity, a repulsion would be set up between the poles facing each other, and thus a force would be exerted to continue the rotation. Thus we see that if the polarity of the horseshoe magnet is not disturbed it is necessary to reverse that of the rotating part to obtain a continuous motion, but if we change the magnetic conditions of both parts, then it is only necessary to magnetize and demagnetize them alternately.
From the foregoing it is seen that there are two ways in which the force of magnetism could be utilized to keep up a continuous rotation, and the question now is, Can either of them be made available in practice? To this we answer that, by the aid of the relations existing between electricity and magnetism, both can be and are made available, as will be shown in the following paragraphs:
