See NEEDLE, Magnetical.

Magnetism.

INTRODUCTION.

General Principles.

IF the mineral body called magnet or loadstone (an ore of iron which will be described under MINERALOGY) is brought within a moderate distance from a piece of iron or steel, or other ferruginous body, such as a small key, a sewing needle, or the like, the ferruginous body will approach the magnet; and if no obstacle intervene, will come in contact with it, and the two bodies will adhere together, so as to require an evident force to separate them from each other.

Again, if a magnet be freely balanced, so that it be left at liberty to assume any direction, as if it be suspended by a thread, or made to float on the surface of water by placing it on a piece of cork or wood, it will soon settle itself in one particular direction, so as to turn one part of its surface towards the northern point of the horizon, and the opposite part of course towards the southern point. These two parts of the surface of the magnet are called its north and south poles; this property of the magnet, of assuming this particular direction, is called its polarity, or its directive power; and when a magnet is placed so as to arrange itself in such a direction, it is said to traverse.

The direction in which a suspended magnet finally settles is called the magnetic meridian, and it is different in different places, and at different times. It is generally, however, very different from the real meridian line, so that the north pole of a magnet declines a little to the east or west, and the south pole to the west or east. The difference of the magnetic from the astronomical meridian, is called the declination, or variation of the magnet; and the declination is said to be east or west, according as the north pole of the magnet verges to the one or the other of these points.

If an oblong magnet be suspended on a pivot by its centre of gravity, it does not settle in a perfectly horizontal position, but one of its poles is depressed below the magnet, the horizontal line, and the other elevated as far above it, making an angle with the horizon that is also different on different parts of the earth's surface. This depression of one of the poles is called the dipping of the magnet.

If two magnets that are each freely suspended, be brought within a moderate distance from each other, so that the north pole of the one is opposed to the south pole of the other, they will attract each other; and if no obstacle intervene, will rush together: but if the two north poles, or the two south poles, be mutually opposed, the magnets will repel each other.

Such are the leading properties of what is called the natural magnet; but what is of more importance, as we shall see hereafter, any piece of iron or steel may, by being rubbed with a natural magnet, or by some other processes to be afterwards explained, be made to acquire the same properties, and thus in every useful respect serve the same purposes as the natural magnet. These pieces of iron or steel thus magnetised, are called artificial magnets; and when they are of a slender, oblong form, they are termed magnetic needles. When afterwards we speak of the polarity, the declination, or the dipping of the magnetic needle, we would be understood as alluding to these slender, oblong, artificial magnets.

A straight line joining the two poles of a magnet is called its axis, and a line drawn transversely on the face of the magnet, perpendicular to the axis, is called a magnet's equator.

The properties of natural and artificial magnets above enumerated, are attributed to the agency of some unknown force or power, either inherent in the magnet, or imparted to it by the processes to which it is subjected. This force is sometimes called magnetism, but we shall for the present denominate it the magnetic power. power, restricting the term magnetism to the science that illustrates and attempts to explain the phenomena.

The most important property of the magnet is its polarity, as it is by means of this that the mariner is enabled to find his way along the trackless ocean, where, before the discovery of this important property, he had no other guide but the stars, and could therefore seldom venture far from the coast. It is by this property too, that the miner is enabled to pursue a direct course through the bowels of the earth, or the traveller direct his steps through immense forests, or over sandy deserts. The uses of the magnet are therefore obvious and important, and the science which places these uses in the best point of view, and thus enables us to turn them to the greatest advantage, is well deserving our attention. Many of the facts to be related under this article are highly curious, and form a pleasing addition to those scientific amusements which are so well calculated to excite the attention of beginners in the study of experimental philosophy.

It is unnecessary for us to attempt giving here a history of the origin and progress of our knowledge in magnetism. To a general reader, it would be uninteresting, and to such as are better informed, superfluous. We shall only mention the most important works that have appeared on the subject.

Few treatises expressly on magnetism have appeared in this country. In the year 1660, Dr Gilbert, a physician of Colchester, and the friend of Lord Bacon, published an excellent work De Magnete et Corporibus Magneticis, which is still perhaps the most valuable that we possess. Mr Cavallo's Treatise on Magnetism, first published in 1787, contains a great variety of facts and experiments; and a neat compendium of it is given in the 3d volume of the same author's Elements of Natural and Experimental Philosophy. Mr Cavallo's Treatise, and Mr Adams's Essay on Magnetism, form the substance of most of the compilations on this subject that have lately appeared.

To those who wish to enter minutely on the study of magnetism, the following list of foreign publications recommended by the late Professor Robison of Edinburgh will be acceptable.

Æpinii Tentamen Theorice Magn. et Elecûr. Eberhard's Tentam. Theor. Magnetismi, 1720. Dissertations sur l'Aimant, par Dufay, 1728. Mutschelbroeck Dissert. Phylico-Experimentalis de Magnete.

Pieces qui ont emporté la prire de l'Acad. des Sciences à Paris sur la meilleure construction des Bouffées de declination. Recueil des pieces couronnées, tom. v. Euleri Opuscula, tom. iii. continens Theoriam Magnetis. Berlin, 1751. Æpinii Oratio Academica, 1758. Æpinii item Comment. Petrop. nov. tom. x. Anton. Brugmanni Tentamen. Phil. de Materia Magnetica. Franquiere, 1765. There is a German translation of this work by Eifenhach, with many valuable additions. Scarella de Magnete, 2 tom. fol. Van Swinden sur l'Analogie entre les phenomenes Electriques et Magnetiques, 3 tom. 8vo. Dissertation sur les Aimants Artificielles, par Nicholas Fuls, 1782.

Essai sur l'Origine des Forces Magnetiques, par M. Prevost. Sur les Aimants artificielles, par Rivoir. Paris, 1752. Differtatio de Magnetismo, par Sam. Klingenstein et Jo. Brander. Holm. 1752. Description des Courants Magnetiques. Strasbourg, 1753. Traité de l'Aimant, par Dalancé. Amst. 1687. Besides the above original works, there are several valuable dissertations on magnetism by Des Cartes, Bernoulli, Euler, Du Tour, Coulomb, &c. either published in the miscellaneous works of these authors, or in the journals and transactions of academies.

We shall divide this article into three chapters. In the first we shall briefly describe the principal instruments made use of in magnetical experiments; in the second we shall endeavour to arrange under distinct heads or propositions, the leading principles of magnetism, point out how these may be illustrated by experiment, and explain the construction and uses of the magnetical apparatus, as they are deducible from the principles laid down; and in the third we shall notice the more important theories of magnetism, and exemplify the illustration of some of the preceding facts by that theory which we shall feel most disposed to adopt.

CHAP. I. Of Magnetical Apparatus.

The principal instruments employed in magnetical experiments and observations, are reducible to three instruments: First, Magnets of various kinds and forms; Secondly, Magnetic needles and compasses; and, Thirdly, the Dipping needle. Of compasses we have nothing to say here, having fully treated of them under Compass.

Magnets, as we have said, are either natural or artificial. The natural magnet may be cut into various forms, according to the experiments that are to be made with it. The most usual shape is oblong, having the poles at the two most distant extremities. Dr Gilbert, whom we shall mention more at large hereafter, made his magnets of a spherical shape, so as to resemble the terrestrial globe. Magnets of this shape are called terrelle, or little earths, and have usually marked upon their surface the magnetic poles, meridian, and equator.

Natural magnets of an oblong shape are usually a piece of soft iron attached to each pole, called the con- of magnets, ducor; and another piece of soft iron placed so as to join two of the extremities of the former pieces, and usually furnished with a hook or hole in the middle. The magnet thus fitted up, as represented at fig. 1, is said to be armed, and the iron pieces CD, CD, are called the armature of the magnet AB. The magnet with its armature is commonly inclosed in a brass box, represented in the figure by the dotted lines DC, CC, CD: and to the upper part of the box is fixed a ring E, for holding the magnet.

One of the most common forms of the artificial magnet is that of an oblong bar, as NS, fig. 2. of which Fig. 2. N is the north pole, and S the south, having the north end marked with a transverse notch. These bars are made of hardened steel, and are either sold separately, or, what is more common, in sets of six in a box.

Another very common form of the artificial magnet is Magnetism.

Is that of a horse shoe, such as fig. 3, having the two poles N, S, brought near each other, and commonly united by a piece of soft iron or conductor. The horse-shoe magnets sometimes consist only of a single crooked bar; but they are frequently composed of several such bars united together by their flat surfaces, and inclosed in a leathern covering that envelopes all but the poles, and thus preserves the bars from rusting.

Instead of the very arched form of which horse-hoe magnets are usually made, they are sometimes constructed so as to form nearly a semicircle, and in this shape they are very convenient for several experiments.

Artificial magnets, like the natural, when of an oblong shape, are sometimes armed at each end, so as to enable them to apply both poles to a ferruginous body at the same time. One material advantage of the horse-hoe magnet is, that in it such an armature is unnecessary, as the poles are brought to near each other as easily to be applied to the object it is proposed to lift, as a key, &c.

A magnetic needle is an oblong piece of steel, tempered so as commonly to assume the blue tinge that is seen in watch-springs, and supported on a brafs point, so as, when left at liberty, to arrange itself in the magnetic meridian, but in a horizontal position. These needles are sometimes made pointed at both extremities; sometimes the northern extremity is made in the form of a cross; but perhaps the best form is that of the oblong, with extremities that are nearly obtuse, such as is represented at fig. 4. To balance the needle on its pivot, it is furnished near its middle with a hollow cap, which is formed of some substance that is not attracted by the magnet. The cap is usually of brafs; but for nice experiments it is sometimes made of agate, as this latter does not wear so fast as brafs, and consequently the needle will longer retain its original suspension.

The dipping needle, fig. 5, consists of an oblong bar of steel, AB, balanced between two horizontal slips of brafs, CD, CD, so as when magnetised to form an angle with the horizon, equal to the dipping of the needle at the place where the instrument is made. The two horizontal slips of brafs are either fixed to a graduated semicircle that is supported on a stand of wood, or more commonly they form diameters to a brafs ring which is graduated on its circumference, and furnished with a ring H, by which it may be held on the finger.

The construction and uses of these instruments will be fully explained in the next chapter; our only object here being to bring the reader acquainted with the names and general form of the instruments that are made use of in the experiments which we are about to describe, for illustrating the principles of magnetism.

Several smaller articles will be required by the experimentalist; but these are easily procured, and need no particular description. Such are a number of sewing needles of various sizes, soft iron bars, pieces of iron wire, small iron balls, iron filings, &c.

Chap. II. Experimental Illustrations of the Principles of Magnetism.

Sect. I. Of Magnetical Polarity.

We have stated (No 3.) that when a magnet is suspended at perfect freedom, it assumes a certain determinate position with respect to the astronomical meridian. This is but a particular case of a much more general fact, which may be expressed by the following proposition.

If an oblong piece of iron be so adjusted, as to be at liberty to take any position; it will assume a certain definite direction with respect to the axis of the earth, differing according to the place where the experiment is made.

Experiment 1.—Take a moderately sized straight iron rod, as a piece of iron wire about the thickness of a goose quill, and about eight or ten inches long; pass it through one extremity of a large wine cork, so that it may be at right angles to the axis of the cork, and adjust it in such a manner that it may swim in water in a horizontal position. Now, provide a pretty large earthen vessel, as a hand basin or round deep dish, nearly filled with water; and when the water is free from agitation, cautiously put in the wire, in such a direction as not to be very far from the north and south line. The iron rod will, after some time, be found to have arranged itself so as, in Britain, to form an angle with the meridian of about 25 degrees.

This experiment requires some nicety, and it will sometimes be long before the iron assumes its proper position; but if due attention be paid to all the particulars above mentioned, it will at length arrange itself in the magnetic line. It is necessary that the rod should be placed not too far from the magnetic line, as, if it be laid at right angles to that line, it will never acquire the proper direction. The situation of the rod in this experiment is in the true magnetic line, so far as respects the meridian; but, as it is horizontal, it is not in the position that a magnet would assume, if freely suspended by its centre of gravity. An iron rod may, however, be made to take such a position, as well as a magnet.

Exper. 2.—Instead of passing the iron rod through the extremity of a cylindrical or conical piece of cork, let it be passed through the centre of a spherical piece of cork or wood, so that the centre of gravity may coincide with the centre of the sphere, and let the whole be of such a specific gravity as to remain suspended in any part of the water, without ascending or descending. If the iron rod thus fitted be placed as in the last experiment, it will at length arrange itself in the true magnetic direction, so as to make an angle of about 25 degrees with the meridian, and with one extremity depressed below the horizon at an angle of about 73 degrees.

These experiments were contrived by Dr Gilbert, and fully shew that the property of assuming a determinate direction with respect to the earth's axis is not confined to magnets, or iron rendered magnetical by the usual processes. There is, however, a remarkable difference between the polarity of unmagnetised iron and that of natural and artificial magnets. It is of no consequence in the former which extremity be placed towards the north, or which below the surface of the water, as either will retain the position it first acquired, unless disturbed by agitation, or by the proximity of a magnet; and both extremities may easily be made to change situations. The effect produced on the iron is therefore temporary. But if a magnetic needle be Experiments freely suspended, the same extremity always points towards the north, and this northern extremity always dips below the horizon, at least in these northern latitudes; and if the position of the needle be disturbed by mechanical motion, or by the application of a magnet, it will be resumed when the disturbing cause is removed.

The polarity of magnets therefore is permanent.

We have said that the magnetic line varies at different times, and in different places. The declination of the magnet is so uncertain as to impose great impediments to the art of navigation, as it is necessary in the course of a long voyage, frequently to ascertain the degree of variation for any particular time or place. The method of doing this is mentioned under Compass. The declination observed in different places at different times, has been laid down in tables; and as such tables are very useful, we shall here subjoin one, given by Mr Cavallo.

<table> <tr> <th>Latitude.</th> <th>Longitude.</th> <th>Declination</th> <th colspan="2">Years in which the observations were made.</th> </tr> <tr> <td rowspan="18">North.</td> <td>West.</td> <td>East.</td> <td colspan="2"></td> </tr> <tr><td>70° 17'</td><td>163° 24'</td><td>30° 21'</td><td>1779</td></tr> <tr><td>69 38</td><td>164 11</td><td>31 0</td><td>1778</td></tr> <tr><td>66 36</td><td>167 55</td><td>27 50</td><td></td></tr> <tr><td>65 43</td><td>172 34</td><td>27 58</td><td></td></tr> <tr><td>63 58</td><td>165 48</td><td>26 25</td><td></td></tr> <tr><td>59 39</td><td>149 8</td><td>22 54</td><td></td></tr> <tr><td>58 14</td><td>139 19</td><td>24 40</td><td></td></tr> <tr><td>55 12</td><td>135 0</td><td>23 29</td><td></td></tr> <tr><td>53 37</td><td>134 53</td><td>20 32</td><td></td></tr> <tr><td>50 8</td><td>4 40</td><td>20 36</td><td>1776</td></tr> <tr><td>48 44</td><td>5 0</td><td>22 38</td><td></td></tr> <tr><td>40 41</td><td>11 10</td><td>22 27</td><td></td></tr> <tr><td>33 45</td><td>14 50</td><td>18 7</td><td></td></tr> <tr><td>31 8</td><td>15 30</td><td>17 43</td><td></td></tr> <tr><td>28 30</td><td>17 0</td><td>14 0</td><td></td></tr> <tr><td>23 54</td><td>18 20</td><td>15 4</td><td></td></tr> <tr><td>20 30</td><td>20 3</td><td>14 35</td><td></td></tr> <tr><td>19 45</td><td>20 39</td><td>13 11</td><td></td></tr> <tr><td>16 37</td><td>22 50</td><td>10 33</td><td></td></tr> <tr><td>15 25</td><td>23 36</td><td>9 15</td><td></td></tr> <tr><td>13 32</td><td>23 45</td><td>9 25</td><td></td></tr> <tr><td>12 21</td><td>23 54</td><td>9 48</td><td></td></tr> <tr><td>11 51</td><td>24 5</td><td>8 19</td><td></td></tr> <tr><td>8 55</td><td>22 50</td><td>8 58</td><td></td></tr> <tr><td>6 29</td><td>20 5</td><td>9 44</td><td></td></tr> <tr><td>4 23</td><td>21 2</td><td>9 1</td><td></td></tr> <tr><td>3 45</td><td>22 34</td><td>8 27</td><td></td></tr> <tr><td>2 40</td><td>24 10</td><td>7 42</td><td></td></tr> <tr><td>1 14</td><td>26 2</td><td>5 35</td><td></td></tr> <tr><td>0 51</td><td>27 10</td><td>4 59</td><td></td></tr> <tr><td>0 7</td><td>27 0</td><td>4 27</td><td></td></tr> <tr><td rowspan="7">South.</td> <td>1 13</td><td>28 58</td><td>3 12</td><td></td> </tr> <tr><td>2 48</td><td>29 37</td><td>2 52</td><td></td></tr> <tr><td>3 37</td><td>30 14</td><td>2 14</td><td></td></tr> <tr><td>4 22</td><td>30 29</td><td>2 54</td><td></td></tr> <tr><td>5 0</td><td>31 40</td><td>1 26</td><td></td></tr> </table>

<table> <tr> <th>Latitude.</th> <th>Longitude.</th> <th>Declination</th> <th>Years in which the observations were made.</th> </tr> <tr> <td>South.</td> <td>West.</td> <td>West.</td> <td>Years in which the observations were made.</td> </tr> <tr><td>6° 0'</td><td>32° 50'</td><td>0° 6'</td><td>1776</td></tr> <tr><td>6 45</td><td>33 30</td><td>0 35</td><td>East.</td></tr> <tr><td>7 50</td><td>34 20</td><td>0 7</td><td>West.</td></tr> <tr><td>8 43</td><td>34 20</td><td>0 15</td><td>East.</td></tr> <tr><td>9 1</td><td>34 50</td><td>0 44</td><td>West.</td></tr> <tr><td>10 4</td><td>34 49</td><td>0 38</td><td>East.</td></tr> <tr><td>12 40</td><td>34 49</td><td>1 12</td><td></td></tr> <tr><td>13 23</td><td>34 49</td><td>1 1</td><td></td></tr> <tr><td>14 11</td><td>34 49</td><td>1 9</td><td></td></tr> <tr><td>15 33</td><td>34 40</td><td>1 15</td><td></td></tr> <tr><td>16 12</td><td>35 20</td><td>2 4</td><td></td></tr> <tr><td>18 30</td><td>35 50</td><td>3 2</td><td></td></tr> <tr><td>20 8</td><td>36 1</td><td>5 26</td><td></td></tr> <tr><td>21 37</td><td>36 9</td><td>3 24</td><td></td></tr> <tr><td>24 17</td><td>36 8</td><td>3 24</td><td></td></tr> <tr><td>26 47</td><td>34 27</td><td>3 44</td><td></td></tr> <tr><td>28 19</td><td>32 20</td><td>1 58</td><td></td></tr> <tr><td>30 25</td><td>26 28</td><td>2 37</td><td>West.</td></tr> <tr><td>33 43</td><td>16 30</td><td>4 44</td><td></td></tr> <tr><td>35 37</td><td>9 30</td><td>5 51</td><td></td></tr> <tr><td>38 52</td><td>23 20</td><td>22 12</td><td>East.</td></tr> <tr><td>42 4</td><td>167 32</td><td>13 17</td><td>West.</td></tr> <tr><td>44 52</td><td>155 47</td><td>9 28</td><td></td></tr> <tr><td>46 15</td><td>144 50</td><td>14 48</td><td></td></tr> <tr><td>48 41</td><td>69 10</td><td>27 39</td><td></td></tr> </table>

It is of still more importance to know the progressive change of the declination at any certain place, and we shall therefore give here the following table of the declination as observed at London in different years, from 1576 to 1808.

<table> <tr> <th>Years.</th> <th>Declination.</th> <th>Observers.</th> </tr> <tr> <td>1576</td> <td>11° 15'</td> <td>Burrows.</td> </tr> <tr> <td>1580</td> <td>11 11</td> <td></td> </tr> <tr> <td>1612</td> <td>6 10</td> <td>Gunter.</td> </tr> <tr> <td>1622</td> <td>6 0</td> <td></td> </tr> <tr> <td>1633</td> <td>4 6</td> <td>Gellibrand.</td> </tr> <tr> <td>1634</td> <td>4 5</td> <td></td> </tr> <tr> <td>1656</td> <td>0 0</td> <td>Bond.</td> </tr> <tr> <td>1665</td> <td>1 22½</td> <td>Gellibrand.</td> </tr> <tr> <td>1666</td> <td>1 35½</td> <td></td> </tr> <tr> <td>1672</td> <td>2 30</td> <td>Halley.</td> </tr> </table>