4-21 



MAGNET. 



MAGNETISM 



entitled ' Tractatus sive Physiologia nova de Magnete ' (1600), and the 

 term " poles of a magnet " arose from tliat theory, which is remarkably 

 consonant with the notions of the present day : for the north pole of 

 the magnet he denominated the south pole, in connection with his 

 theory, while Poisson, in his elegant ' Memoirs on Magnetism," calls the 

 magnetic fluid at that extremity of the magnet the austral fluid, 

 because, as like electricities repel [ELECTRICITY], so, on his hypothesis 

 of the magnetic fluids, that occupying the north end of a magnetised 

 needle is repelled by the boreal fluid of terrestrial magnetism. 



The application x>f the compass to the purposes of navigation must 

 speedily hare led to the discovery of its variation, and in the ' Life of 

 Columbus," written by his Bun, it is distinctly assigned to that cele- 

 brated man ; and though its amount in 1492 must have been small in 

 France, Spain, &c., yet it was doubtless a very observable quantity in 

 many of the regions visited by Columbus. Some have carried back 

 the date of this discovery to the year 1269, but on very doubtful 

 grounds. When its amount came to be observed with some accuracy, 

 we find it at Paris, in 1541, 74 E. ; in 1550, 8J E. ; in 1580, 114" E ; 

 in 1630, 44 E. ; and at Rome, in 1670, 2 W. The following table 

 shows the variation at London in different years : 



East doclin. Zero. 

 Tears . . 1580, 1C22. 1GCO. 

 Declination . IT 15' 6 



West declln. 



1092, 1730, 1765, 1818, 1850. 

 6' 13 3 20 24 41' 22 D 30" 



[DECLINATION OF THE MAGNETIC NEEDLE.] 



It is not improbable that Columbus was acquainted also with the 

 diurnal variation, but nothing very accurate on this subject was known 

 before the numerous and valuable observations made by Canton, in 

 1750. He showed that the needle vibrates, during the day and the 

 night, through an arc as great as 134' * n tne midsummer, the minimum 

 7' occurring in the winter season ; he ascribed the diurnal variation to 

 the action of solar heat affecting the intensity of the magnetism of the 

 earth. The principles of thermo-electric currents were at that period 

 unknown ; but Hooke, in 1684, showed that iron and steel rods could 

 be magnetised by rapidly heating and cooling them in the magnetic 

 meridian. Graham, instrument-maker, of London, was the first who 

 distinctly announced the diurnal variation, in 1772; the maximum 

 declination being then 14" 35' west. The variation of the variation 

 was first observed by Gunter. [OUSTER, in Bioo. Dry.] 



The dip was first observed by Kobert Norman in 1576. His mode 

 of adjusting the compass-needle led him to this discovery ; for he accu- 

 rately balanced the needle on its pivot previous to magnetising it. 

 After it became a magnet it would no longer balance on the same 

 point, without attaching a small weight at the south extremity. When 

 freely suspended by the centre of gravity, the north extremity became 

 depressed, the dip then being about 71* 50'. The dip undergoes 

 diurnal variations, as well as the declination ; but observations on the 

 former are far the most difficult. The dip also changes by elevating 

 the needle to considerable heights, on which subject Biot has made 

 some very delicate experiments. A very simple law relative to the 

 amount of the dip at different parts of the earth's surface was remarked 

 by Professor Kratft, of St. Petersburg, in 1809 ; namely, if we measure 

 the latitude from the magnetic equator, the tangent of the dip is double 

 the tangent of such latitude. Mr. Barlow has illustrated this law by 

 experiments on magnetised iron hall* acting on small needles at the 

 surfaces ; and Biot has deduced the same law from theory. 



The following is the inclination for London in the years stated : 



1576 

 1676 

 1723 

 1790 

 1800 

 1821 

 1830 

 1860 



71 50' 

 73 30' 

 74 42' 

 71 53' 

 70 35' 

 70 30' 

 69 38' 

 68 30' 



The law of the magnetic forces was a long time undiscovered : 

 Newton supposed it to follow the inverse cube of the distance, or 

 some higher power ; for in his experiments, the variation of intensity 

 and the effect of the mutual influences of the magnetic fluids in the 

 bodies themselves being overlooked, an erroneous result was necessarily 

 consequent. However, Mitchell, by a careful revision of the experi- 

 ments of Dr. Brooke Taylor and of Hauksbee ; Coulomb, by his 

 elegant apparatus, the torsion balance ; Biot, by the method of 

 observing the times of the oscillations of the small needles acted on ; 

 and Hansteen of Christiania, by a series of refined experiments and 

 calculations, have demonstrated the true law of magnetic action, 

 namely, directly as the intensity, and inversely as the square of the 

 distance. But for the discussion of the whole question, we refer to 

 Harris 1 ! ' Rudimentary Magnetism," part iii. [See also TEUUESTIUAL 

 MiosrnsM] 



One essential property by which a magnet differs from soft iron 

 under the magnetic influence, is this : if we separate a magnetic bar 

 into any number of minute parts, each such part will be endowed with 

 polarity, similar to the whole : the position of those poles, or foci of 

 greatest attraction, is permanent in a magnet of a given form ; but in 



soft iron it will change when the distance of the iron from the influencing 

 magnet is altered. 



Halley was sent out, under AVilliam and Mary, with the command of 

 two ships, to make magnetic observations in different latitudes, both 

 in the Atlantic and Pacific (in 1698-9), and was the first who con- 

 structed a magnetic chart, which possessed at the time great merit for 

 accuracy : the most valued at the present day are those by Hansteen, 

 constructed from observations subsequent to Halley, by various scientific 

 travellers and nautical men, such as Humboldt, Ross, Parry, Scoresby, 

 &c. [TERRESTRIAL MAGNETISM.] 



During a thunder-storm the poles of a magnet are frequently 

 inverted, the explanation of which belongs to ELECTUO-MAGNETISM ; 

 and the appearance of the aurora borealis is often attended with 

 vibrations of the compass-needle, to the extent of several degrees. 

 The actual mode in which the aurora is produced being still only 

 matter of conjecture, it is impossible to decide whether the aurora is 

 itself the cause of this magnetic phenomenon, or whether both are 

 attributable to some unknown common cause. 



The loadstone is frequently combined with substances which have 

 small or no attractive powers; and the poles, or points of contrary 

 nature with respect to attractions and repulsions, being irregularly 

 distributed, counteract in part one another's effects : thus a mass of 

 loadstone often exhibits a comparatively small adhesive force ; and it 

 happens at times that, when the mass is broken in pieces, some of the 

 portions, containing a considerable quantity of the attractive material 

 favourably disposed, exercise a greater power than was exhibited by 

 the entire mass. It is found that a piece of loadstone will seldom 

 suspend more than 5 or 10 timns its own weight, though some small 

 specimens have been obtained which are said to have suspended more 

 than 200 times their weight. 



If a piece of iron be placed in contact with either pole of a magnetised 

 bar, the force of the coherence is that which is due to the direct action 

 of that pole only on the contrary magnetism of the iron : the intensity 

 is at least doubled when the substance to be suspended can be placed 

 at once in contact with both poles ; and this is frequently effected by 

 giving to the magnetised bar the form of a horse-shoe, so that the 

 opposite poles may be near one another : but the most effectual method 

 of increasing the power of a magnetic body is that of arminy it. 



Preparatory to arming a piece of loadstone the situation of its axis, 

 that is, of a line joining its poles, must be determined by trial : the 

 loadstone is then reduced to the form of a cube or parallelepiped 

 nearly, having two opposite faces perpendicular to the axis ; and these 

 faces are ground very smooth. Two plates of soft iron equal in length 

 and breadth to these sides are applied to the latter so as quite to cover 

 them, and are connected together by a bar of soft iron which is attached 

 to them at the upper end of each (supposing the two plates to be in 

 vertical positions) : to this bar is affixed a hook or ring by which the 

 whole apparatus may be suspended. 



The inferior extremity of each plate is formed with a foot which 

 projects under the loadstone, so that the latter may rest upon both ; 

 and horizontal bands of copper or silver usually surround the loadstone 

 and side pieces in order to keep the whole together. Under the feet 

 is applied a bar or plate of iron, touching both ; and this is provided 

 with a ring or hook from which is to be suspended the weight to be 

 held by the attractive force of the loadstone. Artificial magnets have 

 been armed in a similar manner. 



The attractive powers situated in the poles of a natural or artificial 

 magnet seem to be diffused over the two surfaces in which those poles 

 are situated ; and the plates of iron which cover those surfaces, serving 

 to condense the powers, cause them to act with united energies in the 

 two feet of the apparatus, by which means the power is greatly in- 

 creased. The feet of the side plates should not however be made to 

 approach each other too nearly ; since in such position they would 

 diminish each other's attractive powers. 



It is found that a magnet when armed exerts a force more than one 

 hundred times as great as that of the magnet in its natural state ; aud 

 it has been ascertained that, after as much weight has been applied as 

 the armed magnet will suspend, if at certain intervals of time other 

 weights be added, the magnet will suspend two or three times as much 

 as before. 



Daniel Bernoulli discovered by experiment that the attractive forces 

 of magnets vary in proportion to their superficies and not to their 

 masses; a circumstance in accordance with Mr. Barlow's discovery 

 respecting the magnetic attractions of iron spheres. 



MAGNETIC ATTRACTION. [MAONET ; MAONETISM.] 



MAGNETIC ELECTRICITY. A term employed to distinguish 

 this force from Frictional Electricity, Voltaic Electricity, Thermo- 

 Electricity, and Animal Electricity. 



MAGNETIC INTENSITY. [MAONZT.] 



MAGNETISM. If we take a natural or an artificial magnet, and, 

 spreading over a piece of paper a quantity of fine iron filings, place the 

 magnet on the paper, on taking it up, we shall find that the iron filings 

 are attached to it in some degree over all its surface, but they will b 

 principally accumulated at two points situated near the ends of ths 

 magnet ; these points are called the poles of the magnet. Sometimes 

 when a magnetic bar is rolled amongst iron filings, we may find several 

 such points along the bar ; the magnet is then said to have coneecutivt 

 points. At present we shall consider only the first or simple case of 



