MAGNETISM. 



THE theory of magnetism bears a very strong resemblance to that of electri- 

 city. Like it, magnetism has its attractions and repulsions, and it can be ex- 

 cited in one body and transferred to another, with, however, this striking pecu- 

 liarity, that carbonized iron or steel, is nearly the only substance capable of 

 exhibiting any strong indication of its presence. The loadstone, or natural 

 magnet (fig. 1), is a hard, dark coloured mineral, found chiefly in iron mines ; 

 it is, however, seldom used, as its properties can be imparted to bars of steel, 

 which may be made more powerful than itself. 



Fig. 2 illustrates the polarity of the magnet. If a bar or needle, which has 

 been rendered magnetic, be accurately poised on a point, one of its ends will 

 point towards the north, and the other towards the south ; hence, those parts of 

 the magnet are termed the north and south poles. 



The reciprocal attractions, repulsions, and neutralizations of the opposite 

 magnetic forces may be illustrated by straining a piece of paper upon an open 

 frame, and placing it over a bar magnet (N s, fig. 3). If some iron filings be 

 now sifted upon the paper screen, the particles will arrange themselves in a series 

 of curved lines, proceeding from similar points on each side of the- middle of the 

 bar; others will stand out at the extremities, as if repelled from the poles HT s. 

 If the opposite poles of two magnets be presented to each other, ati about two 

 inches distance, and iron dust be projected over them as before, similar results 

 will ensue. Magnetic lines proceeding from similar points of each bar will 

 appear uniting the two poles, as represented at fig. 4. If two similar poles be 

 presented to each other, the lines of force mutually repelling each other, will 

 present the appearance shown at fig. 5. 



Fig. 6 represents the horse-shoe form of magnet, in which the two poles 

 are brought near together, so as to attract a piece of iron by their combined 

 force. 



The earth itself is found to be a vast magnet, having its two magnetic poles 

 situated in the neighbourhood of, but at some distance from, its poles of revolu- 

 tion. ^ This is the reason why magnetized needles point in a north and south 

 direction, not to the earth's axis (except at certain places), but to the magnetic 

 poles. This will be seen by reference to fig. 7, which represents compass needles 

 distributed over the globe, all lying in the direction of lines drawn from one 

 magnetic pole to the other. 



Fig. 8 represents the manner's compass, the most essential part of which is a 

 magnetized bar of steel, called the needle, accurately poised on a fine central 

 point within a bowl or case, which is so supported as always to preserve a hori- 

 zontal position. Upon the needle is placed the circular card represented at 

 fig. <J, the ornamental point N being placed over the north pole of the needle, 

 consequently the points round the circle indicate the cardinal and all interme- 

 diate points. 



The inclination, or dip of the needle that is, its deviation from the horizontal 

 plane affords a manifestation of the influence exercised upon it by the magnetism 

 of the earth. Hence a dipping needle poised on a horizontal axis (as fig. 10"). 

 will, when carried to either of the earth's magnetic poles, stand upright, the end 

 which is upward at one pole, being downward at the other. The further we 

 recede from either pole, the less does the needle dip, as shown at fig. 11, where 

 the dotted lines indicate the lines of equal dip, or parallels of magnetic latitude, 

 and A and B, the north and south magnetic poles. 



Electro Magnetism. This department of science is founded on the con- 

 nection ascertained to exist between electricity and magnetism. A (fig. 12) 

 represents a bar of soft iron, bent into the horse-shoe form, around it is coiled a, 

 quantity of copper-wire covered with silk. The ends of the wire are connected 

 with the poles of an active voltaic battery, the electricity from which passing 

 along the coiled wire, converts the soft iron bar into a powerful electro -magnet, 

 capable of sustaining, by its attractive force, a weight of many hundred pounds, 

 and which it will continue to support so long as the connection with the battery 

 is maintained, but the moment that connection is broken, the magnetic power of 

 the iron ceases, and the weights fall. 



The magnetic action circulates round the wire in two currents, moving in op- 

 posite directions, as represented at fig, 13. The wire being electric along it 

 length and magnetic across. 



