CHAPTER XVI.

MAGNETISM.

397. =Loadstones.=--It was discovered many centuries ago that a certain ore of iron has the property of attracting pieces of common iron or of steel. The fact was probably considered at first as a mere curiosity, and the world were slow to find out its value. It is not till quite recently that it has been discovered that in magnetism we have one of the great forces of the earth; and even now we know but little probably of the real extent and variety of its action. New and important discoveries are yet undoubtedly to be made in regard to the agency and the laws of this mysterious power, and its connections with the other grand forces of nature. The terms magnet and magnetism come from the fact that the loadstone was first found near Magnesia, an ancient city in Asia Minor. This ore appears in considerable masses in the iron mines of Sweden and Norway, and also in different parts of Arabia, China, and Siam. It has occasionally been found in small quantities in England and in this country.

398. =Attraction of Magnetism.=--The attraction of the magnet and iron for each other is exhibited in many different ways. If a magnet be brought near to a heap of iron-filings or needles, it will have a quantity of them adhering to it as you raise it up. In the toy fishes of children there is fastened in the head a bit of iron, which occasions the following of the fishes after the magnet. In this case you can see very plainly that the nearer the magnet and the iron are to each other the stronger is the attraction. Indeed, the attractive influence is governed by the same law in regard to distance as the common attraction of matter is, viz., it is inversely as the square of the distance. The attraction also is mutual here, the iron attracting the magnet as much as the magnet does the iron.

399. =Poles of the Magnet.=--Every magnet has two poles. It is about these poles where the chief power resides. For this reason, if a magnet be rolled in iron-filings, these are collected about the ends, as represented in Fig. 279. There is a diminution of attraction from the ends to the middle line, which is called the _neutral line_. These poles are called north and south poles, because if a magnet be suspended, or be supported upon a pivot, so that it can revolve, it will take a north and south direction, one of its ends invariably pointing toward the north. In Fig. 280 is represented a magnet supported upon a pivot, C.

400. =Magnetism by Induction.=--The magnet in exerting its attraction really temporarily makes a magnet of what it attracts. Actual contact is not necessary to this result. Thus if a large key be only brought very near to a powerful magnet it will support small keys, as represented in Fig. 281. When the key is removed away from the magnet the keys attached to it fall. You see the analogy to the induction of electricity noticed in § 379. As in the induction of electricity, so here the two ends of the body in which the influence is induced are in opposite states. If the end of the magnet, to which the first key is near or attached, be the north pole, the end of the key next to the magnet is the south pole, and its farther end is the north pole. The same is the case with the small key attached to the end of the large one. And so if a nail should hang from the small key, and a needle from that, both of these would have the same polarities. But all this would be reversed if the large key were attached to the south pole of the magnet. In this case the upper end of each of these articles would be the north pole, and its lower end the south pole.

401. =Attraction and Repulsion in Magnets.=--You have seen in induction that in magnets _like poles repel while unlike attract_. But this law can be more strikingly illustrated. If a magnet be placed on a pivot, as in Fig. 280, and another magnet be brought near it, attraction or repulsion will be manifested according to the mode of presentation. If a north pole be presented to a north pole, or a south to a south, repulsion will be the result. But if a north pole be presented to a south, or a south to a north, then attraction will be manifested.

402. =Magnetic Curves.=--The polarity of magnetism causes a very singular arrangement of iron filings when gently agitated upon a sheet of paper over a magnet, as represented in Figure 282. The curves which you see have been supposed by some to be occasioned by the escape of some fluid or influence from the magnet in these particular directions. But they are owing entirely to the fact that each bit of filing is polarized by the bit next preceding it in the row reckoning from the magnet outward, the nearest one in each row deriving its magnetic state from the magnet itself. This being so, as the chief power resides in the ends of the magnet, it is easy to see how such a disposition of the lines of magnetic filings is effected. These curves may be beautifully and curiously varied by having several magnets variously arranged under the paper.

403. =Artificial Magnets.=--The power residing in the loadstone can be communicated readily, as you have seen, to iron and steel. Though soft iron takes the magnetic influence more readily than steel, it does not retain it as steel does, and the latter is therefore used in making artificial magnets. When a magnet imparts its magnetic influence it loses none of its own power, whether it be an original loadstone or an artificial magnet. There, are many ways of imparting magnetism permanently to steel, but I will notice only two of them. If you wish to magnetize a bar or needle pass one pole of a magnet from one end of it to the other a considerable number of times, always in the same direction. A more effectual way is to take two magnets, and, placing the south pole of one and the north pole of the other in contact over the middle of the bar or needle, draw them slowly and steadily apart toward the opposite ends. This process must be repeated several times.

404. =Horseshoe Magnets.=--One of the most common forms of the magnet is the horseshoe magnet, Fig. 283. There is a piece of soft iron attached to the end of this, held there by attraction. This is called the _armature_. So long as it is suffered to remain there it is a magnet having its _two_ poles, the north pole + being attached to the south pole - of the magnet which holds it, while the reverse is the case with its south pole. The object of the armature is to preserve the power of the instrument. Indeed it is found that the exertion of the magnet's power not only preserves but actually increases it. If you attach, therefore, to a magnet an armature having a hook, as seen in Fig. 284, you can add to the weight gradually from day to day, and so considerably augment the power of the magnet.

405. =Magnetic Needle.=--The magnetic needle is a very small magnet fixed upon a pivot. As it points north and south it is of great use to the mariner. The mariner's compass is a round box with such a needle balanced in it, and having a card on which is drawn a circle divided into thirty-two parts, as seen in Fig. 285. The original compass was a rude affair, consisting of a slip of loadstone laid upon a piece of cork floating in water. The date and place of its first use are unknown.

406. =Declination of the Needle.=--The declination of the needle is its deviation from a north and south line. It is in comparatively few parts of the earth's surface that there is no deviation from this line to the east or the west. "True as needle to the pole" has become a proverb, and when it was first uttered it was supposed to be founded in strict truth; but modern investigation has shown not only that the needle varies in its pointing in different localities, but that it varies to some little degree in its variations. The declination of the needle was first observed by Columbus in his first voyage of discovery, and it occasioned great alarm among the sailors, who, as Irving states, "thought the laws of nature were changing, and that the compass was about to lose its mysterious power." Notwithstanding these and other observations of a similar character, no great account was made of the declination of the needle till the middle of the seventeenth century. But since that time extensive records of its declinations at different localities have been made, and tables and charts have been constructed exhibiting them. These declinations are not constant, but vary somewhat every day, from the influence, it is supposed, of the sun upon the earth.

407. =Dip of the Needle.=--It is found that in most parts of the earth, if a needle be balanced before it is magnetized, and then be suspended from the same point, it will not be balanced, but one end will dip downward. This fact was discovered by Norman, a London optician, in 1576. He found that the dip at London was toward the north at an angle of 72°. In pursuing the investigation of this phenomenon it was found that going from the north toward the equator the dip constantly lessened, until a point was reached where the needle was horizontal. Then, on going south of this, a reverse dip occurred, that of the south pole, and the farther south the needle was carried the greater was the dip. In the north, Captain Ross in 1832 came to a locality north of Hudson's Bay, in lat. 70° 5′ N., long. 96° 45′ W., where the magnetic needle, freely suspended, was in a vertical line. No such locality has yet been discovered toward the south pole.

408. =The Earth a Magnet.=--You can readily see, from all that has been stated in regard to the magnetic needle, that the earth is a magnet, or has that covered up in it which in some way acts as such. The dip of the needle shows that the two poles of this magnet are somewhere near the north and south poles of the earth. The locality which Captain Ross found must be near the north pole of the magnet in that quarter of the world. The vertical position of the needle there is analogous to the straight lines of iron filings which you see in Fig. 282, near the poles of the magnet; and it is easy also to trace the analogy between the dip of the needle at different distances from what is called the magnetic equator of the earth, where the needle is horizontal, and the curves which you see extending from pole to pole. The different declinations of the needle and the different intensities of the magnetic force in different localities corresponding in latitude show that the magnet in the earth, if there be one, is irregular in shape, or in some way has its power varied much in differed parts of the earth's crust.

409. =The Earth as a Magnetizer.=--As the earth is really a magnet, it might be expected to impart magnetism by induction as other magnets do. And this is found to be the fact. If you hold a bar of soft iron in the direction of the dip of the needle it becomes a magnet, its lower end being the north pole, and its upper the south. That this is so can be ascertained by bringing a small magnetic needle near each end. No effect of this kind is produced when the bar is held horizontally east and west. Lightning-rods, pokers, upright iron bars in fences, etc., are often found to be magnetized because they have continued so long nearly in the required position for magnetization. When a bar of iron has been magnetized in the manner indicated, its magnetism may sometimes be fixed by giving it a stroke with a hammer. It is a curious but inexplicable fact that this vibration of the particles of the iron should have this effect. But though such vibration helps to impart magnetism, it is not at all favorable to its retention, for magnets are always injured by blows or falls, or indeed any rude treatment. For this reason care is requisite in removing an armature from a magnet. If pulled off abruptly the power of the magnet is lessened.

410. =Magnetism in Other Substances besides Iron.=--It was formerly supposed that magnetism was confined to ferruginous substances, but this has been found not to be true. Various minerals are magnetic, especially when they have been heated, also some of the precious stones, and even silica, which enters so largely into some of the rocks of the earth. And it is supposed by some that future investigations will show that the influence of magnetism is as extensive in the earth as that of electricity.

411. =In what Magnetism is Like Electricity.=--Magnetism is like electricity in several particulars: 1. Its power is on the surface of bodies. 2. It is of two kinds, north and south, or boreal and austral, comparing with the positive and negative electricities. 3. The same rule of attraction and repulsion applies to both; viz., like repel and unlike attract. 4. As electricity can be communicated by induction, so can magnetism.

412. =In what Magnetism is Unlike Electricity.=--The circumstances in which magnetism is unlike electricity are chiefly these: 1. The obvious manifestations of magnetism are to a great extent confined to one class of substances, the ferruginous, and to but a portion of them; while electricity makes its manifestations in connection with all kinds of substances. 2. Magnetism is never transferred as electricity is from one body to another, but a body gains rather than loses in imparting magnetic power to other bodies. 3. The two magnetisms, the boreal and austral, can not be obtained separately as the two electricities can. If a magnet be broken in two, each piece will have in it the two magnetisms and the two poles as the whole did. This is in entire contrast with the electrical experiment noticed in the last of § 379. 4. There are no non-conductors to interrupt magnetic influence. If in the experiments in § 379 a plate of glass or resin were interposed between A and B, the influence would cease, but it would have no effect on the induction of magnetism if interposed between a magnet and a bit of steel or iron.

413. =Electro-Magnetism.=--Though electricity and magnetism differ so much from each other, yet they have intimate relations, and it is now the general opinion among scientific men that they are merely different modes of the same power. Magnetism can produce electricity, and electricity can produce magnetism. The first discovery of facts revealing this connection was made by Professor Oersted of Copenhagen in 1819. Since that time electro-magnetism, or the production of magnetism by electricity, has been a prominent subject of observation and experiment. Oersted's first observation was that a current of electricity passing over a wire near a magnetic needle affected the position of the needle. He found also that iron filings would adhere to a wire over which a current of electricity is passing, just as they do to a magnet, dropping off, however, as soon as the current ceases to pass. Such facts led to a great variety of investigations and arrangements of apparatus by Oersted and others.

414. =Electro-Magnets.=--The most powerful electro-magnets are made by bending a thick cylindrical bar of soft iron into the form of a horseshoe, A B, Fig. 286, and coiling around it a copper wire. The wire must be insulated by being wound with some non-conducting material, as silk, so that the electric current may pass through the whole length of the wire. With the instrument thus prepared, if the two ends of the wire be connected with the poles of a voltaic battery which is in action, the bar will be magnetized, and will hold up a heavy weight so long as the electric current is passing through the wire. Whenever the current is cut off by disconnecting the wires the weight will fall.

Electro-magnets have been made in this way having such power as to sustain a weight of four thousand pounds. In Fig. 287 we have represented an apparatus which exhibits electro-magnetism very prettily. The soft iron, you see, is in two pieces which when put together form a ring, _d_, and each piece has a handle. If the pieces be put together with the coil, _c_, in the position represented, on connecting the wires P and N with a battery in action, the adhesion is so strong as to resist a great force; but as soon as the connection is broken the pieces come apart at once.

415. =Electric Telegraph.=--The most remarkable and useful application of electro-magnetism we have in the electric telegraph. As before stated, voltaic electricity is used. This is generated at the place from which the message is sent, and passes over the wire to the place where the message is received. There it acts upon soft iron by passing through coiled wire, producing the modified power called electro-magnetism. I will make all this plain to you by describing the machine used in Morse's Telegraph, Fig. 288. W W are the wires which connect with the station from which the message is to be received, and these connect with the copper wire coiled round the horseshoe of soft iron, _m m_. Above the magnet is a lever, _a l_, which works on a fulcrum at _d_. One end of this lever has a steel point, _s_, attached to it. At _c_ is an arrangement of wheel-work, the object of which is to pass along regularly a slip of paper, _p_, in the direction of the arrows. Observe now how the apparatus works. When the electric current passes through the coiled copper wire it makes a magnet of the iron, _m m_. The lever, _a l_, is therefore attracted at the end, _a_, downward. Of course the end, _l_, moves upward, bringing the steel point, _s_, against the paper, where it makes a mark. The length of this mark depends upon the length of time the electricity is allowed to pass along the coiled wire, for the moment that it is shut off _m m_ ceases to be magnetic, the "keeper," _a_, being no longer attracted, moves upward, and the other end, _l_, of the lever moves downward, taking the point, _s_, from the paper.

In order to make the marks on the paper of different lengths, there is a contrivance for regulating the length of time that the current shall pass through the coiled wire. This contrivance, called the _signal key_ is represented in Fig. 289. N and P are two strips of brass connected with the two wires R and M, of which M comes from the battery. The end of the strip N is raised a little above the end of P. So long as they do not touch the circuit is not complete, and no electricity passes. But if the operator press N down upon P, the circuit is established, and the electricity passes to the station with which he is in communication, and there acts upon the apparatus seen in Fig. 287. Now the longer the finger presses down N upon P the longer will be the mark on the paper at the distant station. An operator then at New York, for example, controls by this key the length of the marks made on the paper in New Haven or any other place with which he is communicating.

You can see then very readily how a telegraphic alphabet can be constructed by combinations of marks of different lengths agreed upon to represent different letters and numerals. I give the alphabet used in connection with Morse's Telegraph:

A -- ---- B ---- -- -- -- C -- -- -- D ---- -- -- E -- F -- ---- -- G ---- ---- -- H -- -- -- -- I -- -- J ---- -- ---- -- K ---- -- ---- L -------- M ---- ---- N ---- -- O -- -- P -- -- -- -- -- Q -- -- ---- -- R -- -- -- S -- -- -- T ---- U -- -- ---- V -- -- -- ---- W -- ---- ---- X -- ---- -- -- Y -- -- -- -- Z -- -- -- --

Numerals.

1 -- ---- ---- -- 2 -- -- ---- -- -- 3 -- -- -- ---- -- 4 -- -- -- -- ---- 5 ---- ---- ---- 6 -- -- -- -- -- -- 7 ---- ---- -- -- 8 ---- -- -- -- -- 9 ---- -- -- ---- 0 ---------------

One of the most singular and interesting things in the operation of the telegraph remains to be noticed. In order to have the electricity work it is necessary to have the connection between the poles of the battery at the point where the effect is to be produced. You see this in the experiments represented in Figs. 286 and 287. The same is true of the electro-magnet of the telegraph. This being so, it was thought at first that it was necessary to have two wires connecting two communicating stations; but it was found that only one wire was needed, the earth itself answering the same purpose as another wire. To make the communication through the earth effectual, there is at each station a plate of metal, having a surface of several square feet, buried in the ground, with a wire running up to the machine.

QUESTIONS.

[Teachers differ much in their plans of conducting recitations. Some are very minute in their questions; while others go to the other extreme, and merely name the topics, the pupils being expected to give in full what is said upon them. Neither of these plans should be adopted exclusively, but the mode of recitation should be much varied from time to time. This variety is somewhat aimed at in the questions which I have prepared, though in no case are the questions as minute as they should occasionally be made by the teacher. The numbers refer to the pages.

It would be well to have the pupils draw many of the figures upon the blackboard, and then recite from them. By drawing the simplest figures first sufficient skill may be acquired to enable the pupil to draw those which are quite difficult.]