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 be principally accumulated at two points situated near the ends of the 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 consecetivs points. At present we shall consider only the first or simple case of two poles, which wo may represent by the letters N. and S. When a needle formed of this material is suspended horizeetally a pivot furnished with an agate cap for the purpose of diminishing friction, it assumes • particular direction, nearly north and south. [51 sears.] The pole ,, at the north extremity of the needle thus adjusted, Is commonly called the mord plc of the needle ; the other, S, the south pat. though the contrary names, as used by Dr. Gilbert, would be more correct In connection with the theory of magnetism.
If we now bring a piece of soft Iron near the pole N, it will be attracted to that pole and become attached to it, so that the exertion of • mechanical force is necessary to separate them. In this way a magnet held vertically will sustain • piece of iron, provided the weight of the iron does not exceed the magnetic force. The pole S has a similar attractive power on iron; the cause of this attractive power is called 11.s.oNrri5at We have observed that In a magnetic needle placed horizontally on a pivot, the pole N is turned northwards, and S southwards, nearly : if such a needle be attached to a piece of cork floating on water, it will adjust itself to this direction, the deviation of which from the true north and south line is the declination of the needle. If now we invert, the position of the needle, so that S is brought into the place pre viously occupied by N, and rice rend, the needle and cork will make a complete revolution, and acquire its original position. Hence we ace • distinction between the magnetiams predominant at N and at S ; the former is called Amaral, and the latter Boreal magnetism. It will be
easy to observe the analogy between the mutual relations of the two magnetisms, and those of positive with negative ckwtricitios.
We must insulate a conducting electrised substance in order to pre serve its electricity, but this is not necessary in the case of a magnet; each fragment of the latter is itself a magnet, possessing its north and south poles, and the same view may be extended to its constituent particles. A nonconducting energy, called the coercire power, exists therefore in magnetic substances, by which the Ices of magnetism when developed is prevented, and by which also the poles N and S ace situate in a determinate position relative to the body of the magnet.. This is not the case with soft iron, which has not the coercive force.
The effect of the magnetic forces is considerably modified when substances which are capable of readily becoming magnetic by influence are situated near the magnet ; and a similar effect takes place by the decomposition of the neutral electricities when under the influence of an electrised body. [ELecretenv.] The transmission of the magnetic force through interposed bodies may be observed familiarly by placing a common sewing-needle on a smooth horizontal board, and moving a strong magnet underneath the board : the needle will roll or revolve along the board according to the peculiar motions given to the magnet.
Let us next consider the action of magnets on each other. For this purpose make two magnets or magnetic needles to float on water, distinguishing the poles of one as before by N and S, and of the other by N' and S. Bring either the pole N near to N', or S to S': the needles or magnets will separate to a greater distance, and with the greater energy the nearer these poles are placed to each other. On the contrary, if we bring N and S' near each other, the needles will approach and unite those points, and the same happens when the points N' and S are made contiguous : hence this law—magnetisms of the sante name are mutually repuleire ; time of contrary names ore mutually ottradire. In the article Etecreterrr, we have shown that the same law is true with respect to the two electricities.