MAGNETISM

, the quality or constitution of a body, by which it is rendered magnetical, or a magnet, sensibly attracting iron, and giving it a meridional direction.

This is a transient power, capable of being produced, destroyed, or restored.

These laws are laid down by Mr. Whiston in the following propositions.——1, The Loadstone has both an attractive and a directive power united together, while iron touched by it has only the former; i. e. the magnet not only attracts needles, or steel filings, but also directs them to certain different angles, with respect to its own surface and axis; whereas iron, touched with it, does little or nothing more than attract them; still suffering them to lie along or stand perpendicular to its surface and edges in all places, without any such special direction.

2. Neither the strongest nor the largest magnets give a better directive touch to needles, than those of a less size or virtue: to which may be added, that whereas there are two qualities in all magnets, an attractive and a directive one; neither of them depend on, or are any argument of the strength of the other.

3. The attractive power of magnets, and of iron, will greatly increase or diminish the weight of needles on the balance; nay, it will overcome that weight, and even sustain some other additional also: while the directive power has a much smaller effect. Gassendus indeed, as well as Mersennus and Gilbert, assert that it has none at all: but by mistake; for Whiston found, from repeated trials on large needles, that after the touch they weighed less than before. One of 4584 1/8 grains, lost 2 5/8 grains by the touch; and another of 65726 grains weight, no less than 14 grains.

4. It is probable that iron consists almost wholly of the attractive particles; and the magnet, of the attractive and directive together; mixed, probably, with other heterogeneous matter; as having never been purged by the fire, which iron has; and hence may arise the reason why iron, after it has been touched, will lift up a much greater weight than the loadstone that touched it.

5. The quantity and direction of magnetic powers, communicated to needles, are not properly, after such communication, owing to the magnet which gave the touch; but to the goodness of the steel that receives it, and to the strength and position of the terrestrial load stone, whose influence alone those needles are afterwards subject to, and directed by: so that all such needles, if good, move with the same strength, and point to the same angle, whatever loadstone they may have been excited by, provided it be but a good one. Nor does it| seem that the touch does much more in magnetical cases, than attrition does in electrical ones; i. e. serving to rub off some obstructing particles, that adhere to the surface of the steel, and opening the pores of the body touched, and so make way for the entrance and exit of such effluvia as occasion or assist the powers we are speaking of. Hence Mr. Whiston takes occasion to observe, that the directive power of the loadstone seems to be mechanical, and to be devived from magnetic effluvia, circulating continually round it.

6. The absolute attractive power of disserent armed loadstones, is, cœteris paribus, not according to either the diameters or soliditics of the loadstones, but according to the quantity of their surfaces, or in the duplicate proportion of their diameters.

7. The power of good magnets unarmed, sensibly equal in strength, similar in figure and position, but unequal in magnitude, is sometimes a little greater, sometimes a little less, than in the proportion of their similar diameters.

8. The loadstone attracts needles that have been touched, and others that have not been touched, with equal force at distances unequal, viz, when the distance of the former is to the distance of the latter, as 5 to 2.

9. Both poles of a magnet equally attract needles, till they are touched; then it is, and then only, that one pole begins to attract one end, and repel the other: though the repelling pole will still attract upon contact, and even at very small distances.

10. The attractive power of loadstones, in their similar position to, but different distances from, magnetic needles, is in the sesquiduplicate proportion of the distances of their surfaces from then needles reciprocally; or as the mean proportionals between the squares and the cubes of those distances reciprocally; or as the square roots of the 5th powers of those distances reciprocally. Thus, the magnetic force of attraction, at twice the distance from the surface of the loadstone, is between a 5th and 6th part of the force at the first distance; at thrice the distance, the force is between the 15th and 16th part; at four times the distance, the power is the 32d part of the first; and at six times the distance, it is the 88th part. Where it is to be noted, that the distances are not counted from the centre, as in the laws of gravity, but from the surface: all experience assuring us, that the magnetic power resides chiefly, if not wholly, in the surfaces of the loadstone and iron; without any particular relation to any centre at all. The proportion here laid down was determined by Mr. Whiston, from a great number of experiments by Mr. Hawksbee, Dr. Brook Taylor, and himself; measuring the force by the chords of those arcs by which the magnet at several distances draws the needle out of its natural direction, to which chords, as he demonstrates, it is always proportional. The numbers in some of their most accurate trials, he gives in the following Table, setting down the half chords, or the sines of half those arcs of declination, as the true measures of the force of magnetic at- traction.

Other persons however have found some variations i<*> the proportions of magnetic force with respect to distance: Thus, Newton supposes it to decrease nearly in the triplicate ratio of the distance: Mr. Martin observes, that the power of this loadstone decreases in the sefquiduplicate ratio of the distances inversely: but Dr. Helsham and Mr. Michell found it to be as the square of the distance inversely: while others, as Dr. Brook Taylor and M. Muschenbroek, are of opinion, that this power follows no certain ratio at all, and that the variation is different in different stones.

11. An inclinatory, or dipping-needle, of 6 inches radius, and of a prismatic or cylindric figure, when it oscillates along the magnetic meridian, performs there every mean vibration in about 6″ or 360‴, and every small oscillation in about 5″1/2, or 330‴: and the same kind of needle, 4 feet long, makes every mean oscillation in about 24″, and every small one in about 22″.

12. The whole power of Magnetism in this country, as it affects needles a foot long, is to that of gravity nearly as 1 to 300; and as it affects needles 4 feet long, as 1 to 600.

13. The quantity of magnetic power accelerating the same dipping-needle, as it oscillates in different vertical planes, is always as the cosines of the angles made by those planes with the magnetic meridian, taken on the horizon.

Thus, in estimating the quantity of force in the horizontal and in the vertical situations of needles at London, it is found that the latter, in needles of a foot long, is to the whole force along the magnetic meridian, as 96 to 100; and in needles 4 feet long, as 9667 to 10000: whereas, in the former, the whole force in needles of a foot long, is as 28 to 100; and in those of 4 feet long, as 256 to 1000. Whence it follows, that the power by which horizontal needles are governed in these parts of the world, is but the quarter of the power by which the dipping-needle is moved.

Hence also, as the horizontal needle is moved only by a part of the power that moves the dipping-needle; and as it only points to a certain place in the horizon, because that place is the nearest to its original tendency of any that its situation will allow it to tend to; whenever the dipping-needle stands exactly perpendicular to the horizon, the horizontal needle will not respect one point of the compass more than another, but will wheel about any way uncertainly.

14. The time of oscillation and vibration, both in dipping and horizontal needles, that are equally good, is as their length directly; and the actual velocities of their points along their arcs, are always unequal. And hence, magnetical needles are, cœteris paribus, still better, the longer they are; and that in the same proportion with their lengths.|

Of the Causes of Magnetism. Though many authors have proposed hypotheses, or written concerning the cause of Magnetism, as Plutarch, Descartes, Boyle, Newton, Gilbert, H<*>rtsoeker, Halley, Whiston, Knight, Beccaria, &c; nothing however has yet appeared that can be called a fatisfactory solution of its phenomena. It is certain indeed, that both natural and artificial electricity will give polarity to needles, and even reverse their poles; but though from this it may appear probable that the electric fluid is also the cause of Magnetism, yet in what manner the fluid acts while producing the magnetical phenomena, seems to be quite unknown.

Dr. Knight indeed deduces from several experiments the following propositions, which he offers, not so much to explain the nature of the cause of Magnetism, as the manner in which it acts: the magnetic matter of a loadstone, he says, moves in a stream from one pole to the other internally, and is then carried back in a curve line externally, till it arrive again at the pole where it first entered, to be again admitted: the immediate cause why two or more magnetical bodies attract each other, is the flux of one and the same stream of magnetical matter through them; and the immediate cause of magnetic repulsion, is the conflux and accumulation of the magnetic matter. Philos. Trans. vol. 44, pa. 665.

Mr. Michell rejects the motion of a subtle fluid; but though he proposed to publish a theory of Magnetism established by experiments, no such theory has appeared.

Signor Beccaria, from observing that a sudden stroke of lightning gives polarity to Magnets, conjectures, that a regular and constant circulation of the whole mass of the electric fluid from north to south may be the original cause of Magnetism in general. This current he would not suppose to arise from one source, but from several, in the northern hemisphere of the earth: the aberration of the common centre of all the currents from the north point, may be the cause of the variation of the needle; the period of this declination of the centre of the currents, may be the period of the variation; and the obliquity with which the currents strike into the earth, may be the cause of the dipping of the needle, and also why bars of iron more easily receive the magnetic virtue in one particular direction. Lettre dell' Elettricismo, pa. 269; or Priestley's Hist. Elec. vol. 1, pa. 409. See also Cavallo's Treatise on Magnetism.