After considering Electricity, it is not impossible to approach the study of Magnetism, and even to construct a mariner’s compass. We shall find the method of doing so by borrowing an interesting passage from the “Magasin Pittoresque.” Let us take a small cork and pass through it an ordinary knitting-needle (fig. 203), which we have already magnetized by placing it N.S., rubbing it gently, and always in the same direction, with one of those little iron magnets with which children amuse themselves. After the needle has been passed through the cork, we also fix into it a sewing-needle, or rather a pin, the point of which rests in one of the little holes in the upper part of the thimble. In order to balance the magnetised needle, we thrust a match into both sides of the cork, as shown in the illustration, and fasten to the ends of each a ball of wax. Thus the needle, the balls, and the pin are all balanced at once, so that the contrivance has the appearance of the illustration.
As it is very important that with such a sensitive instrument any agitation of the air should be avoided, the thimble must be placed at the bottom of a common earthen pan, B D T, which should be covered over with a piece of glass, V. To graduate the compass a circle is described on a piece of paper. On this dial we trace the divisions sufficiently close only at the north extremity of the needle, and the paper is fixed underneath, as in fig. 203. Then we fix a piece of wax at the end of the match pointing N., opposite the northern extremity of the needle inside the basin. In this way we have a very useful and inexpensive compass.
We may also magnetize a fine sewing-needle, and grease it by rubbing it with a little suet. It is then capable of floating on the surface of water running in the direction of the north pole. We might go on multiplying indefinitely examples of physical experiments without apparatus, but we have probably already given a sufficient number to aid our readers in imagining others.
We have now in a simple manner shown how we can easily produce electricity. We may understand that electrical phenomena are produced—(1) friction between different bodies; (2) by placing bodies which differ in contact; (3) by the transition of bodies from one condition to another; (4) by chemical changes; (5) by animals. The two first, and the fourth, are the most usual causes.
We know that certain substances when rubbed with silk or wool acquire the property of attracting other substances. But in the case of a rod of glass or stick of wax, the attraction will only be perceived when the rubbing has been applied. But metal will behave differently. Any part of the metal rod will continue to attract. So metals are Conductors of electricity; while glass, wax, silk, amber, sulphur, etc., are bad, or Non-Conductors. Metals are the best conductors we have, but trees, plants, liquids, and the bodies of animals, including men, are all good conductors of electricity. Dry air is a bad conductor.
Fig. 204.—Attraction and repulsion.
There are two kinds of electricity, known as positive and negative (plus or minus), vitreous or resinous. We saw in fig. 200 that we can attract a small ball of pith or cork by a piece of sealing-wax rubbed with flannel. If we then present a glass rod rubbed with silk to it, it will be equally attracted, but will be at once repelled; and after being so repelled, if we put the wax to it, it will be attracted to the sealing-wax again. So wax at first attracts then repels the ball, and so does glass, but either will attract the ball if presented alternately (fig. 204). The reason for this is as follows :—
When we have rubbed the glass with silk, we charge it with positive electricity, and when the rod touches the ball, the latter imbibes that electricity, and flies away from the glass rod. The sealing-wax imparts negative electricity in the same way.
The law is, that bodies charged with the same kind of electricity repel each other, and those containing the opposite kinds attract each other. Positive repels positive; negative repels negative. But positive attracts negative, and negative attracts positive. Opposite electricities unite, and so neutralize each other that no effect is perceived; but it must be borne in mind that all bodies possess both electricities in some quantity, greater or less. By rubbing we separate these electricities, the rubber becoming negative, the rubbee positive. The friction of glass supplies positive electricity, and sealing-wax supplies negative electricity, or we can obtain the same effect by rubbing either with certain material.