83ci 



ELECTRO-MAGNETISM. 



stantly flowing round the globe, as the current of 

 electricity in a galvanic apparatus moves in an 

 unbroken circuit from the negative to the positive 

 pole, and from it, by the connecting wire, round 

 again to the negative pole. The direction of these 

 currents he infers to be the same as has been stated 

 with artificial magnets ; and it is simply by the 

 attractions and repulsions of these terrestrial currents, 

 bringing the currents round the needle to coincide 

 with them, tliat the latter always points to the north. 

 To detect the-se currents, and to exhibit their influ- 

 ence without the aid of any common magnet, M. 

 Ampere contrived a small electric apparatus, which 

 was distinctly affected by the magnetic influence of 

 the glol>e. It consisted merely of a copper wire bent 

 into a circle, with the two extremities brought near 

 to each other. It was supported so as to move with 

 the greatest facility ; and the points were immersed 

 in basins of mercury, with which the wires of a gal- 

 vanic battery were connected. When the communi- 

 cation was established so as to cause a current of 

 electricity to pass through the circle, it immediately 

 began to move, and, after some oscillations, placed 

 itself nearly at right angles to the meridian, or east 

 and west, or so tliat the electric current passed down- 

 wards on the eastern side, and upwards on the west 

 ern side. This, it has been stated, is exactly the 

 direction in which the currents in every magnet 

 move (supposing it placed with its north pole to the 

 north). 1'he circle may, therefore, be regarded as 

 a section across the axis of a magnet, or as repre- 

 senting one of the currents flowing round it ; and if 

 a number of these circles were placed one beyond 

 another, the farthest would point, like the end of the 

 needle, to the north pole, and the nearest to the 

 south pole. However the experiment was varied, 

 the circle always placed itself east and west : if the 

 galvanic current was, by reversing the connecting 

 wires, made to flow in an opposite direction, the 

 circle turned round a semicircle, and still stood 

 east and west, and so that the electric current should 

 always flow downwards on the eastern side, and up- 

 wards on the western side. Here, then, are distinct 

 marks of magnetism, particularly that most charac- 

 teristic one. of the axes pointing always to the north, 

 which can be attributed only to the combined influ- 

 ence of electric currents moving round the earth. 

 This result is very much in favour of the new theory; 

 and there appears to be only one link now wanting, to 

 connect magnetism and electricity, and to establish 

 their identity. This is, by some combination of wires 

 and magnets, to produce an undoubted electric effect, 

 such as the decomposition of water. This has been 

 attempted in a number of ways ; and in some of the 

 arrangements, the desired effects appeared to be pro- 

 duced ; but no result has yet been obtained wholly free 

 from doubt. All the phenomena of magnetism, it has 

 been found, may be produced by electricity. If, 

 then, any of the phenomena occasioned by electricity 

 alone can be produced by magnetism, we shall have 

 no hesitation in pronouncing them to be the same 

 power, according to the doctrine of Ampere. With 

 regard to the cause of tjie electric currents inferred 

 to be constantly circulating round the globe, it is as 

 yet in obscurity. They are supposed to move at 

 right angles to the magnetic meridian, or nearly par- 

 allel with the equator, on the eastern side of the 

 earth moving from us, and on the western side flow- 

 ing towards us. These currents may be compared 

 to that which flows from the negative pole of a 

 voltaic battery in action, to the positive pole, 

 nrd, by the medium of the uniting wire, round again 

 li the negative pole. It is conjectured, that the 

 arrangement of the materials of the globe may be 

 such as to constitute a battery, existing like a girdle 



round the earth, which, though composed of compara- 

 tively weak elements, may be sufficiently extensive 

 to produce the effects of terrestrial magnetism. Its 

 irregularity, and the changes which it may acciden- 

 tally or periodically suffer, may explain the pheno- 

 menon of the variation of the compass ; or the general 

 action producing the currents of electricity may be 

 affected by different causes, as the earth's motions, 

 currents of the atmosphere, evaporation, or the solar 

 heat. It is supposed that much of the variation de- 

 pends on the progress of oxidation in the continental 

 regions of the globe. What is called the diurnal varia- 

 tion may be conceived to be produced by the diurnal 

 change of temperature in the snper6cial layers of the 

 earth, which possess electro-motive energy. 



We will conclude our view of electro-magnetism 

 by describing some of the apparatus employed in 

 experimenting in this interesting branch of science. 

 Fig. 15, plate XX IX., shows an apparatus for exhil>- 

 iting the effect of heat in exciting electricity. In 

 this figure a b is a board into which is fixed an up- 

 right brass rod, d. On the top of this rod a piece 

 or agate is fastened, which receives a steel point des- 

 cending from the top of the rectangular piece, e e. 

 This rectangular piece is composed of rods of plati- 

 num and silver, the former being represented by the 

 faint lines, and the latter by the heavy ones. The 

 bottom rods are made to diverge in the centre, in 

 order to allow the centre rod, rf, to pass up so that 

 the rectangle e e, may revolve freely on the agate at 

 the top of the rod d. c and c, are two magnets 

 placed vertically in the board, the upper extremities 

 of which nearly touch the corners of the rectangle 

 e e. If a spirit lamp be applied at one of the under 

 corners, the rectangle will begin to revolve, and 

 the velocity will increase in a short time to thirty 

 revolutions hi the minute. If the north pole of the 

 right hand magnet, and the south pole of the left be 

 uppermost, and the lamp be held on the right, 

 the motion of the rectangle will be towards the 

 right, and the reverse will be the case if the magnets 

 be inverted. 



A pleasing experiment, somewhat of the same kind 

 with the last, may be exhibited with the apparatus 

 shown in Fig. 16. A horse shoe magnet b, e, g, is 

 fixed in an inverted position on the stand a. In this 

 case the magnet is the support of the rectangles e, f, 

 which move on agate. One lamp, at d, sets both in 

 motion, and they will continue to revolve as long as 

 the lamp bums. 



The horse shoe magnet may be made to cause a 

 wheel and axle rotation, by means of the apparatus 

 exhibited in Fig. 17, which is the invention of Mr 

 Barlaw. There is fixed into the rectangular board, 

 a a, an upright wooden pillar, b, into the top of 

 which is fixed a piece of stout brass wire, c. A 

 similar piece of wire is soldered to the bottom of this 

 a little below e, and shaped in the form of a fork, in 

 order to carry the axle on which the thin sheet-cop- 

 per wheel, /, turns. Immediately below this wheel, 

 there is placed a little trough containing mercury. 

 It is sunk into the wood of the board, a a, and has a 

 channel running into it, terminating in a little cup. 

 which is seen at the side of the board. The mercury 

 in the trough is brought to such a height, that tho 

 tips of the wheel just touch it, and the surface is then 

 covered with a weak solution of nitric acid. This 

 being done, a strong horse shoe magnet is brought 

 into the position shown in the figure, the ends being 

 brought near the wheel. The wire at the top of the 

 upright wooden pillar is connected with one end of 

 the battery, and the cup at the side of the board is 

 connected with the other, electro-magnetic acliv>n 

 commences, and the wheel turns round with great 

 velocity. If the contact with the battery be changed, 



