of magnetism. Hence most bodies are magnetically transparent. In this it differs from heat, light, and electricity, but is analogous to gravitation.

3. How the Poles are found.-If a bar magnet be placed on a table, and over it be laid a piece of cartridge paper on a frame to keep it horizontal, and fine iron filings be sifted on the paper, it will be found, on tapping the frame, that a definite position and direction will be taken up by the filings as in the figure. It will be seen by inspecFig. 1.

tion that the magnetic force is more intense near the ends of the magnet, and the neutral line is plainly visible at its centre, whilst the force is equal at equal distances from the centre.

By taking a line AB to represent the magnet, and drawing double ordinates CD, a b, cd, ef, &c.

Fig. 2.

a

E

B

D

we

to represent the intensities of the forces, find that a pole is situate at the centre of gravity of each figure OCD, OEF; and Coulomb found

the distance from the ends in a bar 8 inches long to be 18 inches: with shorter bars, that the distance of the poles from the ends is of their lengths: thus in a four-inch bar the poles will be two-thirds of an inch from each end. With very thin bars the poles approach the extremities, and in compass needles the distance from the ends may be assumed to be at one-twelfth of their lengths.

4. Position which Free Magnets take.-When a magnet is supported by a thread without torsion, or on a pivot so as to move freely, it will, when left to itself, rest only in a vertical plane which stands nearly north and south. The extremity constantly pointing north is the north pole, or redmarked end of the needle; the opposite extremity is called the south pole, or blue-marked end. French writers reverse these names; the pole we call North is by them called South, and vice versâ: hence the Astronomer Royal uses the distinctive colours above. To distinguish the poles in the following diagrams, the North or red-marked end will be shown by transverse marks, as is commonly adopted by tradesmen when magnets are made. The South or blue-marked end is usually left blank, and will therefore have no marks in the diagrams, Magnetic polarity or directive power is that property which causes a magnet when freely suspended to turn the same extremity constantly towards the same pole of the earth.

A magnet then has two poles, each possessing the power to attract iron. This is the only property they have in common; for when two bodies possessing magnetic properties are brought near each other, the like poles will repel and the unlike attract each other. This is the fundamental law in magnetism. The repulsive action which is exhib

ited when two like poles are presented to each other, will enable us to ascertain if a body be magnetic or not; for if a magnetic needle be suspended, and each end of it be similarly attracted by the piece of metal presented to it, the piece is not magnetic: but if one end of the needle be attracted and the other repelled, it is magnetic. Any instrument or contrivance used for discovering magnetism is called a magnetoscope.

5. Magnetic Field.-Faraday has given this term to the entire space through which the magnetic force is diffused; and the area where the magnetic force acts in parallel lines is called a uniform magnetic field: as, for instance, any station on the earth's surface when speaking of terrestrial magnetism. If we multiply the distance between the poles of a magnet by the force of either pole, the product is called the Moment of the magnet.

6. Theory of Magnetic Fluids. To account for the various phenomena of magnetism, the modern physicist assumes that every molecule of matter is invested with two subtle imponderable fluids called magnetic fluids, and that these fluids are in combination and inert when the molecules of a body are not under magnetic influence, and thus the body is neutral. When it is magnetised the fluids separate from one another (but not from the molecule with which they are associated) and arrange themselves, one facing in one direction, the other in the opposite. The fluid which faces towards the north is called Boreal, and the other Austral. This separation takes place with the fluids surrounding every molecule; and hence, when a bar is magnetised, the fluids are arranged on all the molecules with the Boreal fluid towards the redmarked end and the Austral towards the blue

marked end. Now as the red end of the needle is constantly directed to the north point of the earth, it follows from the fundamental law, that magnetism of an opposite kind, or blue magnetism, must reside there, and red must reside at the South pole of the earth. Confusion often arises from speaking of the magnetism in the North end of the needle and in the Northern hemisphere under the same appellation: hence the introduction of colours as distinguishing names.

If a steel needle be magnetised and the red end be marked and then broken, each piece will be found to be also perfectly magnetic; the ends of the broken pieces nearest the marked end of the needle being the North pole, and the other the South, in each piece. If these pieces be again broken any number of times, each piece will still be a perfect magnet, the North pole of each being that end which was nearest the marked end in the unbroken needle. The theory given above (although only an assumed one) supplies a complete explanation of this phenomenon. The breaking of the needle in no way disarranges the fluids, but the Boreal still remains on one side of the molecules, giving them its characteristic properties, and the Austral fluid remains on the opposite side. Thus each molecule of the needle is itself a perfect magnet, and the action of the whole is the sum of the action of its molecules.

7. Coercive Force. The Astronomer Royal defines soft iron as 'Either malleable iron which has not been subjected to any violence when cold, or cast iron.' * In the Admiralty Manual it is

thus defined: 'Soft iron is iron which becomes

* A Treatise on Magnetism. G. B. Airy.

instantaneously magnetised by induction when exposed to any magnetic force, but which has no power of retaining magnetism, and it has therefore no independent magnetism; all its magnetism is transient induced magnetism.'

If such a piece of soft iron be presented to a strong magnet, it immediately becomes magnetic, but returns as suddenly to its inert state when removed from magnetic influence. If we use a piece of hardened steel instead, it does not at once acquire magnetic properties, but when removed from the magnet it will be found to have itself become one, the strength of which will depend on the power of the magnet with which it was brought into contact and the length of time it so remained. Thus we see in these two substances there is a force holding the two hypothetical fluids in combination in iron it at once yields to the : power the magnet, but in steel it does not yield so readily. These effects are produced by what is called Coercive Force, which may be defined as that force which offers resistance to the separation of the hypothetical fluids in a magnetic body, and when once separated offers a resistance to their reunion. In steel we say it is very great, in iron very feeble; and in bodies not subject to magnetic influence it is so great, that no power has yet been found able to resolve them.

of

8. Magnetic Induction. If we take several pieces of soft iron AB C, and place them near to Fig. 3.

each other in a line, and then present a magnet M to A, we shall find that it immediately acquires magnetic properties, the end nearest to м being a