given time, say a minute, is proportional to the strength of the current, and consequently may be taken as an estimate of its strength. He used an instrument akin to that already described (fig. 270), with graduated tubes, as a Voltameter or absolute current measurer. It is chiefly available, however, only for strong currents, and for estimating their average strength during a shorter or longer period. The magnetic effects of the current, as will be explained, offer a much more ready and delicate test of current strength. SECTION IV.-MAGNETISM. 967. Formerly, in systematic treatises on Natural Philosophy, magnetism stood apart as a very distinct branch of science, singularly at variance with the general course of nature; but recently, since more extended knowledge of both electricity and magnetism has been acquired, the resemblances between them are found to be so numerous and close, that nearly all the phenomena can be referred to the same influences. When the most obvious facts of magnetism first attracted notice, nothing could appear more strange or inexplicable. A darkcoloured heavy stone, now met with in many parts of the earth, but first observed near a village called Magnesia, in Asia Minor, from which it derived its name of magnet, was found to have the singular power of drawing towards it pieces of iron or steel, and of lifting them into contact, and there supporting their weight for any length of time. It is referred to by more than one ancient writer, and Pliny states that this iron ore was called by the common people ferrum vivum, that is, living, or quick iron; because it seems to endow small masses of iron with life. This natural magnet, or loadstone, known to the mineralogist as magnetic iron ore, is found in Sweden and other parts of the world; being so abundant in some rocks and mountains, as to produce a disturbing magnetic effect on the compass-needles of ships which may pass near them. It is known to the chemist as the magnetic oxide of iron. It is of a different atomic constitution from the common oxide of iron, or iron rust. On rubbing this stone against rods of steel, its power of attracting iron can be communicated to the steel, and then, from one bar of magnetized steel, other bars can be magnetized without there being any diminution of force in the original or giving bar. A good illustrative experiment is to show a common iron key hanging by magnetic force to the end of a magnetized bar, as shown in fig. 271. Fig. 271. H This communicative power of the loadstone was in all probability known to the ancients. 732 Magnetism a Polar Force. 968. Polarity of the Magnet.-Such a stone or magnetized rod of steel, if suspended by a string or poised on a pivot, so as to have freedom of horizontal motion, soon places itself with one of its sides or ends, and always the same, towards the north pole of the earth, and with the opposite, consequently, towards the south pole. Its own corresponding sides or ends so discovered, are called the poles of the magnet. If the stone or needle be by any force disturbed from the position so taken and then left free, it always of itselt returns to the same. A common sewing needle magnetized, and laid gently on the surface of water, will turn north and south in the same manner. This property, called the polarity of the magnet, has often been referred to as one of the most curious facts in nature, and is of vast importance to mankind; for men are often placed in such circumstances that their safety, or even life, may depend on their being able to judge correctly of the directions, north and south. A man, when there are dangers near, may be suddenly enveloped in a thick fog, or in a blinding fall of snow, or he may lose his way in traversing a forest, or on crossing an extensive plain without distinguishing objects on its surface, or in a boat he may be driven by a storm out of sight of land; in such cases life may be lost, if he judge erroneously of his position or of the direction in which he moves. In any such dilemma, a magnetic needle, even so small as to be carried in a seal attached to his watch, would insure safety. Had the mariner's compass, which is only a larger needle fitted to bear the tossing of the waves, not been invented, the intercourse of nations by sea could scarcely have taken place. In remote past times, nothing could have seemed more incredible than that a stone would be discovered in the bowels of the earth, which could point always with certainty to the pole, for which the highest human sagacity without it, would search in vain. Though the Greeks and Romans do not appear to have been acquainted with the polarity of the magnet, the Chinese have in all probability been acquainted with it from a remote period, their name for it meaning the directing or guiding stone. It is asserted by the earliest English writer on magnetism (Dr. Gilbert) that the first compass was brought to Europe from China, in the middle of the thirteenth century. 969. Mutual action of Magnets.—If two poised magnetic bars are brought near to each other, the poles of the same name, north or south, are found strongly to repel each other, and the poles Magnetic Lines of Force. 733 of different names strongly to attract; but either pole is equally attractive of non-magnetized iron or steel. In these facts there is a striking resemblance to the phenomena in electricity of bodies similarly electrified, whether positively or negatively, being held apart by strong mutual repulsion; and of bodies, if dissimilarly electrified, strongly attracting; a magnetized body, like an electrified body, attracts another in a neutral state. If a magnetic bar or needle be placed for an instant among iron filings and then be lifted, it lifts with it a considerable quantity of the filings cohering around the ends in tufts or lines of particles as here shown, but there are none cohering near the middle of the bar. This experiment indicates the different force of the attraction at different distances from the poles of the magnet, and the non-attraction in the middle part about the so-called equator. Fig. 272. From this experiment it also appears as if the magnetic strength were localized near each end of a magnetized bar. This may be also very beautifully exhib ited by laying a flat magnetized bar, such as a straight piece of watchspring, on a sheet of paper, marking its position, and moving round each of the ends of this fixed magnet a small sewing needle mag Fig. 273. netized and hung up by a silk fibre. If the point of the sewing needle be a north pole, and the eye end a south pole, the positions of the needle will be as indicated in fig. 273. At the middle of the magnet the small needle will lie parallel to the big magnet, the attractions of north and south being just equal in amount and opposite in direction. A very instructive variation of this experiment is to place the magnetized bar, or bar-magnet, as it is usually terrned, under a piece of glass and sift fine iron-filings over the glass; after tapping the glass gently, the iron-filings will be found arranged as so many minute needles, and by their mutual action joined together, so as to form beautifully regular lines radiating from the poles at each end as a centre. Using two bar-magnets, and placing them, first, with like poles near each other, and, secondly, with unlike poles near, the 734 Magnetic Induction. conjoint effect on the iron-filings may be readily studied. The curves assume in these instances very symmetrical forms round the centres of magnetic force. They are known as the Magnetic curves, or Faraday's Lines of Magnetic force. ; The analogy between electricity and magnetism is further strikingly exhibited by magnetic induction or action at a distance the laws of which are precisely identical with those of electric induction.* 970. Magnetic Induction.—Simple contact, or the near approach of a magnet and a piece of soft iron, renders the iron for the time magnetic-very much as any insulated conB ductor of electricity, if brought near to a C highly electrified body, becomes for the D time electrical. Thus, the magnet, A B (fig. E274), attracts and supports the piece of iron, A Fig. 274. C, and that becoming magnetic, similarly supports the second piece, D, which again can support E. None of the pieces of soft iron, C, D, E, retain anything of the magnetic quality after the contact ceases. Pieces of steel after a time might retain a portion. It is in this way that the lines or threads of iron-filings in the preceding experiment are formed. With a strong magnet a line or chain may similarly be formed with iron nails or steel pens; each, becoming magnetic by induction, renders its neighbour in turn a magnet. Thus, with a curved magnet, a pliable magnetic arch may be built up between the poles with iron brads or tacks. 971. Magnetization.-A magnetic steel-bar, however, if fitly used, imparts its properties permanently to other steel bars, without losing any part of the power which it possessed itself. The common procedure thus to magnetize is, to apply at the centre of the new steel bar, as A B, laid on a table, one end of a strong magnet, CE, held obliquely, as here shown (at an angle of about 30°), and A Fig. 275. B The reader will observe the remarkable difference in the mode of distribution of the two forces in a similar bar. Electricity is diffused over the whole surface of the bar, and is as much manifested in the centre as at the two ends. Magnetism flies to, and concentrates itself in, the ends of the bar or poles. There is no manifestation of this force in the centre. |