Electricity

It is fairly certain that the Chinese knew of the lodestone long before Greek and Roman times, and according to ancient Chinese records this knowledge extends as far back as 2600 B.C. Humboldt, in his Cosmos, states that a miniature figure of a man which always turned to the south was used by the Chinese to guide their caravans across the plains of Tartary as early as 1000 B.C. The ancient Greek and Roman writers frequently refer to the lodestone. Thales, of whom we spoke in Chapter I., believed that its mysterious power was due to the possession of a soul, and the Roman poet Claudian imagined that iron was a food for which the lodestone was hungry. Our limited space will not allow of an account of the many curious speculations to which the lodestone has given rise, but the following suggestion of one Famianus Strada, quoted from Houston’s Electricity in Every-Day Life, is really too good to be omitted.

“Let there be two needles provided of an equal Length and Bigness, being both of them touched by the same lodestone; let the Letters of the Alphabet be placed on the Circles on which they are moved, as the Points of the Compass under the needle of the Mariner’s Chart. Let the Friend that is to travel take one of these with him, first agreeing upon the Days and Hours wherein they should confer together; at which times, if one of them move the Needle, the other Needle, by Sympathy, will move unto the same letter in the other instantly, though they are never so far distant; and thus, by several Motions of the Needle to the Letters, they may easily make up any Words or Sense which they have a mind to express.” This is wireless telegraphy in good earnest!

The lodestone is a natural magnet. If we rub a piece of steel with a lodestone we find that it acquires the same properties as the latter, and in this way we are able to make any number of magnets, for the lodestone does not lose any of its own magnetism in the process. Such magnets are called artificial magnets. Iron is easier to magnetize than steel, but it soon loses its magnetism, whereas steel retains it; and the harder the steel the better it keeps its magnetism. Artificial magnets, therefore, are made of specially hardened steel. In this chapter we shall refer only to steel magnets, as they are much more convenient to use than the lodestone, but it should be remembered that both act in exactly the same way. We will suppose that we have a pair of bar magnets, and a horse-shoe magnet, as shown in Fig. 13.

If we roll a bar magnet amongst iron filings we find that the filings remain clinging to it in two tufts, one at each end, and that few or none adhere to the middle. These two points towards which the filings are attracted are called the poles of the magnet. Each pole attracts filings or ordinary needles, and one or two experiments will show that the attraction becomes evident while the magnet is still some little distance away. If, however, we test our magnet with other substances, such as wood, glass, paper, brass, etc., we see that there is no attraction whatever.

If one of our bar magnets is suspended in a sort of stirrup of copper wire attached to a thread, it comes to rest in a north and south direction, and it will be noticed that the end which points to the north is marked, either with a letter N or in some other way. This is the north pole of the magnet, and of course the other is the south pole. If now we take our other magnet and bring its north pole near each pole of the suspended magnet in turn, we find that it repels the other north pole, but attracts the south pole. Similarly, if we present the south pole, it repels the other south pole, but attracts the north pole. From these experiments we learn that both poles of a magnet attract filings or needles, and that in the case of two magnets unlike poles attract, but similar poles repel one another. It will be noticed that this corresponds closely with the results of our experiments in Chapter I., which showed that an electrified body attracts unelectrified bodies, such as bits of paper or pith balls, and that unlike charges attract, and similar charges repel each other. So far as we have seen, however, a magnet attracts only iron or steel, whereas an electrified body attracts any light substance. As a matter of fact, certain other substances, such as nickel and cobalt, are attracted by a magnet, but not so readily as iron and steel; while bismuth, antimony, phosphorus, and a few other substances are feebly repelled.

The simplest method of magnetizing a piece of steel by means of one of our bar magnets is the following: Lay the steel on the table, and draw one pole of the magnet along it from end to end; lift the magnet clear of the steel, and repeat the process several times, always starting at the same end and treating each surface of the steel in turn. A thin, flat bar of steel is the best for the purpose, but steel knitting needles may be made in this way into useful experimental magnets.

We have seen that a magnet has two poles or points where the magnetism is strongest. It might be thought that by breaking a bar magnet in the middle we should get two small bars each with a single pole, but this is not the case, for the two poles are inseparable. However many pieces we break a magnet into, each piece is a perfect magnet having a north and south pole. Thus while we can isolate a positive or a negative charge of electricity, we cannot isolate north or south magnetism.

If we place the north pole of a bar magnet near to, but not touching, a bar of soft iron, as in Plate II.a, we find that the latter becomes a magnet, as shown by its ability to support filings; and that as soon as the magnet is removed the filings drop off, showing that the iron has lost its magnetism. If the iron is tested while the magnet is in position it is found to have a south pole at the end nearer the magnet, and a north pole at the end farther away; and if the magnet is reversed, so as to bring its south pole nearer the iron, the poles of the latter are found to reverse also. The iron has gained its new properties by magnetic induction, and we cannot fail to notice the similarity between this experiment and that in Fig. 2, Chapter II., which showed electro-static induction. A positively or a negatively electrified body induces an opposite charge at the nearer end, and a similar charge at the further end of a conductor, and a north or a south pole of a magnet induces opposite polarity at the nearer end, and a similar polarity at the further end of a bar of iron. In Chapter II. we showed that the attraction of a pith ball by an electrified body was due to induction, and from what we have just learnt about magnetic induction the reader will have no difficulty in understanding why a magnet attracts filings or needles.

Any one who experiments with magnets must be struck with the distance at which one magnet can influence filings or another magnet. If a layer of iron filings is spread on a sheet of paper, and a magnet brought gradually nearer from above, the filings soon begin to move about restlessly, and when the magnet comes close enough they fly up to it as if pulled by invisible strings. A still more striking experiment consists in spreading filings thinly over a sheet of cardboard and moving a magnet to and fro underneath the sheet. The result is most amusing. The filings seem to stand up on their hind legs, and they march about like regiments of soldiers. Here again invisible strings are suggested, and we might wonder whether there really is anything of the kind. Yes, there is. To put the matter in the simplest way, the magnet acts by means of strings or lines of force, which emerge from it in definite directions, and in a most interesting way we can see some of these lines of force actually at work.

Place a magnet, or any arrangement of magnets, underneath a sheet of glass, and sprinkle iron filings from a muslin bag thinly and evenly all over the glass. Then tap the glass gently with a pencil, and the filings at once arrange themselves in a most remarkable manner. All the filings become magnetized by induction, and when the tap sets them free for an instant from the friction of the glass they take up definite positions under the influence of the force acting upon them. In this way we get a map of the general direction of the magnetic lines of force, which are our invisible strings.

Many different maps may be made in this way, but we have space for only two. Plate III.a shows the lines of two opposite poles. Notice how they appear to stream across from one pole to the other. It is believed that there is a tension along the lines of force not unlike that in stretched elastic bands, and if this is so it is easy to see from the figure why opposite poles attract each other.

Plate III.b shows the lines of force of two similar poles. In this case they do not stream from pole to pole, but turn aside as if repelling one another, and from this figure we see why there is repulsion between two similar poles. It can be shown, although in a much less simple manner, that lines of electric force proceed from electrified bodies, and in electric attraction and repulsion between two charged bodies the lines of force take paths which closely resemble those in our two figures. A space filled with lines of magnetic force is called a magnetic field, and one filled with lines of electric force is called an electric field.

A horse-shoe magnet, which is simply a bar of steel bent into the shape of a horse-shoe before being magnetized, gradually loses its magnetism if left with its poles unprotected, but this loss is prevented if the poles are connected by a piece of soft iron. The same loss occurs with a bar magnet, but as the two poles cannot be connected in this way it is customary to keep two bar magnets side by side, separated by a strip of wood; with opposite poles together and a piece of soft iron across the ends. Such pieces of iron are called keepers, and Fig. 13 shows a horse-shoe magnet and a pair of bar magnets with their keepers. It may be remarked that a magnet never should be knocked or allowed to fall, as rough usage of this kind causes it to lose a considerable amount of its magnetism. A magnet is injured also by allowing the keeper to slam on to it; but pulling the keeper off vigorously does good instead of harm.

If a magnetized needle is suspended so that it is free to swing either horizontally or vertically, it not only comes to rest in a north and south direction, but also it tilts with its north-pointing end downwards. If the needle were taken to a place south of the equator it would still tilt, but the south-pointing end would be downwards. In both cases the angle the needle makes with the horizontal is called the magnetic dip.

It is evident that a suspended magnetized needle would not invariably come to rest pointing north and south unless it were compelled to do so, and a little consideration shows that the needle acts as if it were under the influence of a magnet. Dr. Gilbert of Colchester, of whom we spoke in Chapter I., gave a great deal of time to the study of magnetic phenomena, and in 1600 he announced what may be regarded as his greatest discovery: The terrestrial globe itself is a great magnet. Here, then, is the explanation of the behaviour of the magnetized needle. The Earth itself is a great magnet, having its poles near to the geographical north and south poles. But a question at once suggests itself: “Since similar poles repel one another, how is it that the north pole of a magnet turns towards the north magnetic pole of the earth?” This apparent difficulty is caused by a confusion in terms. If the Earth’s north magnetic pole really has north magnetism, then the north-pointing end of a magnet must be a south pole; and on the other hand, if the north-pointing end of a magnet has north magnetism, then the Earth’s north magnetic pole must be really a south pole. It is a troublesome matter to settle, but it is now customary to regard the Earth’s north magnetic pole as possessing south magnetism, and the south magnetic pole as possessing north magnetism. In this way the north-pointing pole of a magnet may be looked upon as a true north pole, and the south-pointing pole as a true south pole.

Magnetic dip also is seen to be a natural result of the Earth’s magnetic influence. Here in England, for instance, the north magnetic pole is much nearer than the south magnetic pole, and consequently its influence is the stronger. Therefore a magnetized needle, if free to do so, dips downwards towards the north. At any place where the south magnetic pole is the nearer the direction of the dip of course is reversed. If placed immediately over either magnetic pole the needle would take up a vertical position, and at the magnetic equator it would not dip at all, for the influence of the two magnetic poles would be equal. A little study of Fig. 14, which represents a dipping needle at different parts of the earth, will make this matter clearer. N and S represent the Earth’s north and south magnetic poles, and the arrow heads are the north poles of the needles.

Since the Earth is a magnet, we should expect it to be able to induce magnetism in a bar of iron, just as our artificial magnets do, and we can show that this is actually the case. If a steel poker is held pointing to and dipping down towards the north, and struck sharply with a piece of wood while in this position, it acquires magnetic properties which can be tested by means of a small compass needle. It is an interesting fact that iron pillars and railings which have been standing for a long time in one position are found to be magnetized. In the northern hemisphere the bases of upright iron pillars are north poles, and their upper ends south poles, and in the southern hemisphere the polarity is reversed.

The most valuable application of the magnetic needle is in the compass. An ordinary pocket compass for inland use consists simply of a single magnetized needle pivoted so as to swing freely over a card on which are marked the thirty-two points of the compass. Ships’ compasses are much more elaborate. As a rule a compound needle is used, consisting of eight slender strips of steel, magnetized separately, and suspended side by side. A compound needle of this kind is very much more reliable than a single needle. The material of which the card is made depends upon whether the illumination for night work is to come from above or below. If the latter, the card must be transparent, and it is often made of thin sheet mica; but if the light comes from above, the card is made of some opaque material, such as very stout paper. The needle and card are contained in a sort of bowl made of copper. In order to keep this bowl in a horizontal position, however the ship may be pitching and rolling, it is supported on gimbals, which are two concentric rings attached to horizontal pivots, and moving in axes at right angles to one another. Further stability may be obtained by weighting the bottom of the bowl with lead. There are also liquid compasses, in which the card is floated on the surface of dilute alcohol, and many modern ships’ compasses have their movements regulated by a gyrostat.

The large amount of iron and steel used in the construction of modern vessels has a considerable effect upon the compass needle, and unless the compass is protected from this influence its readings are liable to serious errors. The most satisfactory way of giving this protection is by placing on each side of the compass a large globe of soft iron, twelve or more inches in diameter.

On account of the fact that the magnetic poles of the Earth do not coincide with the geographical north and south poles, a compass needle seldom points exactly north and south, and the angle between the magnetic meridian and the geographical meridian is called the declination. The discovery that the declination varies in different parts of the world was made by Columbus in 1492. For purposes of navigation it is obviously very important that the declination at all points of the Earth’s surface should be known, and special magnetic maps are prepared in which all places having the same declination are joined by a line.

It is an interesting fact that the Earth’s magnetism is subject to variation. The declination and the dip slowly change through long periods of years, and there are also slight annual and even daily variations.

At one time magnets were credited with extraordinary effects upon the human body. Small doses of lodestone, ground to powder and mixed with water, were supposed to prolong life, and Paracelsus, a famous alchemist and physician, born in Switzerland in 1493, believed in the potency of lodestone ointment for wounds made with steel weapons. Baron Reichenbach, 1788–1860, believed that he had discovered the existence of a peculiar physical force closely connected with magnetism, and he gave this force the name Od. It was supposed to exist everywhere, and, like magnetism, to have two poles, positive and negative; the left side of the body being od-positive, and the right side od-negative. Certain individuals, known as “sensitives,” were said to be specially open to its influence. These people stated that they saw strange flickering lights at the poles of magnets, and that they experienced peculiar sensations when a magnet was passed over them. Some of them indeed were unable to sleep on the left side, because the north pole of the Earth, being od-negative, had a bad effect on the od-negative left side. The pretended revelations of these “sensitives” created a great stir at the time, but now nobody believes in the existence of Od.

Professor Tyndall was once invited to a seance, with the object of convincing him of the genuineness of spiritualism. He sat beside a young lady who claimed to have spiritualistic powers, and his record of his conversation with her is amusing. The Reichenbach craze was in full swing at the time, and Tyndall asked if the lady could see any of the weird lights supposed to be visible to “sensitives.”

Tyndall adds, “Our host here deprecated discussion as it ‘exhausted the medium.’”