Apkil 1, 1895.] 



KNOWLEDGE. 



93 



THE MAGNETIC NEEDLE. 



By Vacghan Cornish, M.Sc, F.C.S. 



IF we may consider any body which has the property of 

 attracting iron as being a magnet, then the earth 

 itself must ba looked upon as the primitive magnet. 

 Iron is not necessarily a magnet, but every piece of 

 iron has a tendency to become one under the induc- 

 tive action of the earth's magnetic force. If a piece of 

 soft iron be placed in a vertical position its lower end 

 becomes, in these latitudes, a north-seeking magnetic pole, 

 and by hammering the bar while in this position it becomes a 

 permanent magnet. An iron ship in course of building is 

 subjected to constant hammering, and when it leaves the 

 stocks is a powerful and permanent magnet. Native iron 

 is very rarely met with, but an oxide of iron called 

 loadstone, containing seventy per cent, of the metal, occurs 

 abimdantly, and may in a certain sense be considered as 

 the primitive magnet. 



With the exception of steel and iron, some of the iron 

 ores, and the allied metals cobalt and nickel, aU other 

 bodies are practically non-magnetic ; their magnetic pro- 

 perties being almost infinitesimal in comparison with those 

 of iron. This circumstance gives a peculiar character to the 

 science of magnetism, and makes the study of the subject 

 very different from that of electricity, which is associated 

 with all sorts of materials. Another striking difference 

 between magnetism and statical electricity consists in 

 the fact that we cannot isolate "north" magnetism 

 from " south " magnetism, as we do positive from negative 

 electricity. When an iron or steel bar is magnetized, 

 positive and negative magnetism are produced in equal 

 strength and the magnet has its north and south pole. 

 We cannot draw ofi" the " south " magnetism and leave a 

 " north " magnet. The experiment may readily be 

 tried on a watch spring which has been magnetized, 

 after having been heated to redness and cooled 

 suddenly. The watch spring can now be easily broken, 

 and each piece is found to have its north and south end 

 after every occasion of breaking the spring. The north 

 and south ends may be tested by bringing them near to the 

 end of a light and delicately suspended magnet, which 

 readily moves if attracted or repelled. Since the north 

 and south poles of a magnet act in opposite ways and are 

 always of equal strength, it is evident that the further apart 

 the poles of a magnet are the more likelihood there is of 

 obtaining definite and simple eft'eots, such as can readily be 

 understood. The use of the common horse-shoe magnet is 

 apt to confuse the tyro, although convenient when the 

 object is to attract powerfully a piece of unmagnetized iron, 

 which is attracted both by the north and by the south pole. 

 Hence the convenience of the horse-shoe form, c.g.^ for 

 settingtheindexof a registering thermometer. A "keeper" 

 of soft iron which prevents leakage of magnetism is also more 

 conveniently employed with the horse-shoe form, but for 

 those who wish to study the elementary phenomena of 

 magnetism the bar magnet is much better. In a sewing 

 needle, for instance, the mass of metal being small, the 

 magnetic charge is necessarily small also, and the poles 

 are fairly well out of each other's way. 



Some of the simpler magnetic phenomena may be very 

 conveniently observed with the aid of a sewing needle, 

 which is laid gently on the surface of stiU water after 

 ha%'ing been magnetized by being drawn across the pole of 

 a magnet. The first thing noticeable is that the needle 

 generally twists about its middle point, alter the hand has 

 left it free, and then rests in its new position. If a second 

 magnetized needle be placed in another basin, it will be 

 seen that both needles point in the same direction. They 



both lie in a northerly direction, known as the magnetic 

 meridian. The effects above described, due to the earth's 

 magnetic force, are difierent from those produced by 

 bringing the pole of a bar magnet near the floating needle. 

 In that case the needle not only turns to the magnet but 

 follows it, whereas the earth's magnetism gives the needle 

 a twist but no pull. If there were such a pull the needle 

 would readily show it, for the resistance of the water is 

 less to a motion lengthwise than to the turning of the 

 needle about its middle point. It has been said that the 

 earth acts somewhat as if there were a great bar magnet 

 about half the length of the earth's axis buried inside it, and 

 making a small angle with the axis of the earth's rotation. 

 Although such an analogy is a very rough one and will 

 not stand being pushed very far, nevertheless, it represents 

 the facts sufficiently well to be employed as a means 

 of explaining the general character of these elementary 

 phenomena. It is evident that the pole of such a magnet, 

 which is by hypothesis situated at a great distance, could 

 only exercise a twisting, not a pulling, force on a magnetized 

 needle. The two poles of the needle being practicalli/ at 

 the same distance from the pole of the great magnet, the 

 attraction and repulsion on the north and the south poles 

 of the magnet respectively will be equal in amount. A 

 push at one end of the needle and an equally strong pull 

 at the other cannot drag the needle along, but, if the 

 needle be set across the line of force, the push and the 

 pull will act together so as to twist the needle into the 

 direction of the line of force, the magnetic meridian. 



In the same way the magnetized needles attached to the 

 under side of the card of a ship's compass keep the card 

 always in the same position, so that as the ship's head 

 turns, the binnacle or compass box revolves round the 

 stationary card, and the "lubber" line marks the pomt of the 

 compass towards which the ship's head is turned. The 

 card to which the compass needles are attached has in its 

 centre a brass socket,""with a piece of agate at the bottom, 

 which is supported by an upright pin. A slight amouut of 

 friction is all that opposes the tendency of the needle to 

 keep in the magnetic meridian during the tiu'nings of the 

 ship's course. This arrangement does not show, however, 

 whether the earth's directmg magnetic force is horizimtal 

 or not. The needle and card are free to turn horizontally, 

 but a downward or upward twist, unless very powerful, 

 would expend itself without visible effect against the rigid 

 parts of the apparatus. In the same way, with the sewing 

 needle floating on water, the repulsion between the 

 gfi-asij surface of the needle and the water surface is 

 sufficient to resist the downward pull of gravity, and 

 would prevent us from observing a vertical twist 

 unless it were a powerful one. To show the true line 

 of action of the earth's directing force upon a magnet, 

 we must use " a needle " mounted on a horizontal 

 axis and free, except for the earth's mignetic action, to 

 turn in a vertical plane. The form of needle used for 

 showing the magnetic dip is the long lozenge shape. If 

 the dip needle be placed so that a line joining the supports 

 be in the magnetic meridian, the needle hangs vertically, 

 and, if the apparatus be gradually turned round, the lower 

 end of the needle tends to rise until when the needle is in 

 the magnetic meridian the dip, or angle which it makes 

 with the horizon, is in these latitudes about 67^. Thus in 

 England the line of the earth's magnetic force slopes steeply 

 downwards in a northerly direction. The point or points 

 towards which magnetic needles turn are not the " magnetic 

 poles " of the geographer; those are positions on the earth's 

 sui-face where the dipping needle points vertieaUy down- 

 wards. The direction m which such a needle points would 

 meet the direction in which, say, a Greenwich needle 



