229. Method of Single Touch.-The steel bar to be magnetised is placed horizontally on a table, and one of the poles of the magnetising bar, held vertically, is brought into contact with it at one of its extremities. The magnetising bar is then to be drawn, parallel to itself, along the surface of the steel bar to the other end; after which it is to be lifted and replaced in its first position, and the operation repeated. After ten or twelve such rubbings, the steel bar is to be inverted and the same process repeated upon its other face, taking care that the same pole of the acting magnet is employed, and that it is always moved in the same direction. The end, or pole, of the steel bar at which the pole of the magnet leaves the bar will always be of the opposite magnetism to that of the magnetising magnet. That is, if the north-seeking (or red) end of the magnet is used the end of the steel bar last touched by it will be a blue or south-seeking pole.

This process will answer sufficiently well when the needle to be magnetised is of small dimensions. It cannot impart to large bars all the magnetism of which they are capable, and it has the inconvenience of generating consecutive poles when the bar to be magnetised is long, unless much care be taken in giving an equable motion to the moving magnet.

230. Methods of Separate or Divided Touch.-This method, which was first used by Dr. KNIGHT in 1745, consists in placing the two opposite poles of two magnets of equal force in the middle of the bar to be

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Fig. 71.-Showing Magnetism by Method of Separate or Divided Touch.

magnetised, and in moving each of the magnets simultaneously towards the opposite ends of the bar. Each magnet is then placed in its original position and this operation repeated. After several frictions on both faces the bar is magnetised. In Dr. KNIGHT's method the magnets are held vertically. M. DUHAMEL perfected the method by inclining the rubbing magnets at an angle of 20° with the horizon;* and the process is still more effectual if the ends of the bar are mean time supported on the opposite poles of two other equally powerful bar magnets, the poles being of the same name as those of of the two magnets used for stroking the steel bar. The relative position of the poles of the magnets is indicated in the figure.

This method produces the most regular magnets.

231. Magnetisation by the action of the Earth.-That the earth operates as a magnet, and at each point of its surface the force emanating from it has a fixed direction or intensity, and, hence, that like other

It is now ascertained that a smaller inclination is more advantageous. According to Capt. KATER the maximum effect is produced when the inclination of the magnetising bars is reduced to two or three degrees.

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magnets it can give both permanent and temporary magnetism is shown by many natural and artificial phenomena. The permanent magnetism of lodestone when its deposits are in the magnetic meridian is an example of one kind, and the magnetism in the soft iron of a dynamo-electric machine is an example of the other. All soft or cast iron rods or bars, or other elongated forms of cast or soft iron, unless the position of their length is at a rightangle to the line of the direction of the earth's magnetic force, are immediately rendered magnetic by induction from the earth, and the nearer the iron is to the line of force or dip the greater will be the amount of induction. When a bar of soft iron is held on the magnetic meridian and parallel to the dip it becomes immediately endowed with feeble magnetic polarity. The lower extremity is a N. (red) pole, and if the N. (red) pole of a small magnetic needle be approached to it it will be repelled. If the bar be held vertically the lower end will still be a north pole, but of less intensity. If the bar is held horizontally north and south the north end will be a north pole but of still lesser intensity, the south end a south pole also of lesser intensity. If we now turn the bar in the same horizontal plane its magnetism will diminish, and if placed in an east and west direction it will lose its polarity, and if we turn it still further, until its position is reversed, the magnetic poles of the bar will be reversed.

Magnetism resulting from induction may be more or less permanent, or totally transient. Thus it has been found that a bar of iron that has remained fixed for a long period in a direction parallel, or nearly so, to the earth's line of force becomes a permanent magnet. If, however, the bar be of hardened steel its magnetism will require a little time to arrive at its maximum intensity of magnetisation, because of its coercive force causing it for a short time to resist magnetisation.

Since the effect of inducing action is much increased by mechanical concussion, or any other cause which imparts molecular vibration to the bar while under the action of the inducing force, hence, to magnetise an iron bar we have only to hold it in the direction of the earth's magnetic force, and to strike it on the end while so held with a hammer. The bar will be found to acquire by this simple process a considerable amount of magnetism. As the direction of the earth's magnetic force in the higher latitudes is not far from the vertical the effect produced will be nearly the same if the bar be held vertically. The end of the bar which was lowest being charged with north (red) magnetism; and this magnetism is not transient like the induced magnetism of soft iron, changing its place in the bar with every change in the position of the bar, but is constant like that of a steel bar, retaining the same magnetism whatever the position of the bar. By reversing the position of the bar and striking it a few blows with the hammer, its magnetism is reversed. The magnetism of the bar so struck resembles that of a steel magnet in all respects but this, that while, perhaps no change can be remarked in hours or days, it infallibly diminishes in a long time. To express this partially permanent character the term Subpermanent Magnetism has been adopted.

232. Magnetisation by the Electric Current.-Much greater power can, however, be obtained by means of electro-magnetism; and the two following methods are now almost exclusively employed by the makers of magnets.

A fixed electro-magnet (Fig. 72) is employed, and the bar to be magnetised is drawn in opposite directions over its two poles. Each stroke tends to develop at the end of the bar at which the motion ceases the opposite

magnetism to that of the pole which is in contact with it. Hence strokes in opposite directions over the two contrary poles tend to magnetise the bar the

same way.

When very intense magnetisation is to be produced the electro-magnet must be very powerful, and the bar then adheres to it so strongly that the operation above described becomes difficult of execution, besides scratching the bar. Hence it is more convenient to move along the bar, as in Fig. 73, a coil of wire through which a current is passing. This was the method employed by Arago and Ampere.

233. Destruction of Magnetism.-The capability of induction, and the correlative power of retaining the induced magnetism, as we have seen, are both greatly affected by any mechanical or other force which excites a vibratory motion in the particles of the iron. On the other hand, the magnetism of a steel bar is weakened, or even wholly destroyed by the same means; the vibratory motion which is imparted to the molecules of the body favouring the recombination of the separated magnetism, and, therefore, the return of the body to the neutral state. It is thus that the power of a magnet is seriously lessened, or altogether destroyed, by a fall from a height upon a hard pavement.

234. The terrestrial magnetic force is determined both in direction and magnitude, when we know the position of the vertical plane containing the direction of the force; the angle which that direction makes with the horizon, and the number which expresses the intensity of the force itself, referred to some known unit. In addition to these we have to consider the distribution

of its effects over the surface of the globe, and the changes it is subject to. The distribution will be noticed under the heads of its direction and intensity, being represented in each case by a system of imaginary lines on the earth's surface, the change will be noticed subsequently.

235. I. The Direction of the Force of Terrestrial Magnetism.—This is estimated in two co-ordinate planes, the one horizontal, the other vertical, in the former of which the geographical meridian is taken as the initial line. It is measured by the direction of the magnetic needle suspended to move in each of these planes: the needle hung so as to move in the horizontal plane is called the "Declination" or "Variation Needle," that hung so as to move in the vertical plane, the "Inclination" or "Dipping Needle."

236. The geographical or true meridian of a place is the plane passing through the place and containing the true axis of the earth.

237. The Magnetic Meridian.-The magnetic meridian of a place is the vertical plane passing at this place through the two poles of a movable magnetic needle in equilibrium about a vertical axis; that is, it is the vertical plane in which a magnetised needle tends to place itself.

The magnetic north is not any one point, i.e., the magnetic meridians at different parts of the earth's surface do not cut any one point as the true meridians do. The forms of the magnetic meridians are very remarkable. None of them appear to be exactly a great circle. They converge to a north pole, north of Hudson's Bay, and a south pole in South Victoria; but these poles are not opposite. From long. 70° E. to 150° E. the northern parts agree nearly with the geographical meridians; the same remarks also applies to a large portion of the southern part for long. 150°.

238. The Direction in the Horizontal Plane.-Magnetic Declination, or Variation.*-In general the magnetic meridian does

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N

E

not coincide with the geographical meridian, and the angle which the magnetic makes with the geographical meridian, or which is the same thing, the angle which the direction of the needle makes with the meridian is called the declination or variation of the magnetic needle. The declination is said to be east if the north pole of the needle points east of the true or geographical meridian; but the declination is west if the north pole of the needle points west of the true meridian. It is very different at different places, and at a given place undergoes a gradual change from year to year, besides smaller changes backwards and forwards which are continually taking place.

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Fig. 74. Declination.

* Magnetic Variation is the term still employed by nautical men, but for which, among men of science, the term Magnetic Declination is usually substituted.

The following table gives particulars of the Declination, Inclination (Dip), and total magnetic force at a number of important places, the values being approximately true for the year 1880.

Table of Magnetic Declination and Inclination for Year 1880.

239. Lines of No Variation, or Agonic Lines.-In certain parts of the earth the magnetic coincides with the geographical meridian. Imaginary lines passing through all these points where the needle points due North or South are called Lines of No Variation, or Agonic Lines.

240. Isogonic Lines, or Lines of Equal Variation.-Lines connecting all points where the needle is deflected from the geographical meridian by an equal quantity are called Lines of Equal Variation, or Isogonic Lines. These are extremely irregular curves, and form two closed systems, surrounding two points which may be called Centres of Variation. One of these points is in Eastern Siberia, the other in the Pacific Ocean, in the vicinity of the Marquessa.

If we imagine the earth to be covered with perfectly similar declination needles without influence upon one another, we shall find certain of them having no declination; the line joining all such needles is called an agonic line.

Confining our attention for the present to the actual configuration of the lines of no variation, we find that there are two principal lines of no variation. Of these the western traverses the continent of America and the Atlantic Ocean, in a direction from north-west to south-east. Commencing in the Polar Sea, about Port Kennedy, say in about lat. 75° N., and long. 100° W., its course has been traced through Hudson's Bay and Lake Ontario, until it leaves the North American continent near Cape Hatteras, in about 75° W. long. It then traverses a portion of the Atlantic Ocean to the eastward of the West India Islands, until it meets the continent of South America near the mouth of the River Amazon, in about long. 50° W. Proceeding thence, in a south-easterly direction, it cuts the shores of the continent again near to Rio Janeiro, between 40° and 45° W. long., and enters the South Atlantic, where it pursues the same direction as before, and can be traced towards the S. pole as far as 60° S. The course of this line presents but little inflection, and is inclined to the meridian nearly at the same angle throughout.