end, always in the same direction, with one of the poles of a
magnet. Until 1820 all the artificial magnets in practical use
derived their virtue, directly or indirectly, from the natural
magnets found in the earth: it is now recognized that the
source of all magnetism, not excepting that of the magnetic
ore itself, is electricity, and it is usual to have direct recourse
to electricity for producing magnetization, without the intermediary
of the magnetic ore. A wire carrying an electric
current is surrounded by a magnetic field, and if the wire is
bent into the form of an elongated coil or spiral, a field having
certain very useful qualities is generated in the interior. A bar
of soft iron introduced into the coil is at once magnetized, the
magnetism, however, disappearing almost completely as soon
as the current ceases to flow. Such a combination constitutes
an electromagnet, a valuable device by means of which a magnet
can be instantly made and unmade at will. With suitable
arrangements of iron and coil and a sufficiently strong current,
the intensity of the temporary magnetization may be very high,
and electromagnets capable of lifting weights of several tons
are in daily use in engineering works (see Electromagnetism).
If the bar inserted into the coil is of hardened steel instead of
iron, the magnetism will be less intense, but a larger proportion
of it will be retained after the current has been cut off. Steel
magnets of great strength and of any convenient form may be
prepared either in this manner or by treatment with an electromagnet;
hence the natural magnet, or lodestone as it is commonly
called, is no longer of any interest except as a scientific
curiosity.
Some of the principal phenomena of magnetism may be demonstrated with very little apparatus; much may be done with a small bar-magnet, a pocket compass and a few ounces of iron filings. Steel articles, such as knitting or sewing needles and pieces of flat spring, may be readily magnetized by stroking them with the bar-magnet; after having produced magnetism in any number of other bodies, the magnet will have lost nothing of its own virtue. The compass needle is a little steel magnet balanced upon a pivot; one end of the needle, which always bears a distinguishing mark, points approximately, but not in general exactly, to the north,[1] the vertical plane through the direction of the needle being termed the magnetic meridian. The bar-magnet, if suspended horizontally in a paper stirrup by a thread of unspun silk, will also come to rest in the magnetic meridian with its marked end pointing northwards. The north-seeking end of a magnet is in English-speaking countries called the north pole and the other end the south pole; in France the names are interchanged. If one pole of the bar-magnet is brought near the compass, it will attract the opposite pole of the compass-needle; and the magnetic action will not be sensibly affected by the interposition between the bar and the compass of any substance whatever except iron or other magnetizable metal. The poles of a piece of magnetized steel may be at once distinguished if the two ends are successively presented to the compass; that end which attracts the south pole of the compass needle (and is therefore north) may be marked for easy identification.
Similar magnetic poles are not merely indifferent to each other, but exhibit actual repulsion. This can be more easily shown if the compass is replaced by a magnetized knitting needle, supported horizontally by a thread. The north pole of the bar-magnet will repel the north pole of the suspended needle, and there will likewise be repulsion between the two south poles. Such experiments as these demonstrate the fundamental law that like poles repel each other; unlike poles attract. It follows that between two neighbouring magnets, the poles of which are regarded as centres of force, there must always be four forces in action. Denoting the two pairs of magnetic poles by N, S and N′, S′, there is attraction between N and S′, and between S and N′; repulsion between N and N′, and between S and S′. Hence it is not very easy to determine experimentally the law of magnetic force between poles. The difficulty was overcome by C. A. Coulomb, who by using very long and thin magnets, so arranged that the action of their distant poles was negligible, succeeded in establishing the law, which has since been confirmed by more accurate methods, that the force of attraction or repulsion exerted between two magnetic poles varies inversely as the square of the distance between them. Since the poles of different magnets differ in strength, it is important to agree upon a definite unit or standard of reference in terms of which the strength of a pole may be numerically specified. According to the recognized convention, the unit pole is that which acts upon an equal pole at unit distance with unit force: a north pole is reckoned as positive (+) and a south pole as negative (−). Other conditions remaining unchanged, the force between two poles is proportional to the product of their strengths; it is repulsive or attractive according as the signs of the poles are like or unlike.
If a wire of soft iron is substituted for the suspended magnetic
needle, either pole of the bar-magnet will attract either end of
the wire indifferently. The wire will in fact become temporarily
magnetized by induction, that end of it which is nearest to the
pole of the magnet acquiring opposite polarity, and behaving
as if it were the pole of a permanent magnet. Even a permanent
magnet is susceptible of induction, its polarity becoming thereby
strengthened, weakened, or possibly reversed. If one pole of a
strong magnet is presented to the like pole of a weaker one,
there will be repulsion so long as the two are separated by a
certain minimum distance. At shorter distances the magnetism
induced in the weaker magnet will be stronger than its permanent
magnetism, and there will be attraction; two magnets with
their like poles in actual contact will always cling together unless
the like poles are of exactly equal strength. Induction is an
effect of the field of force associated with a magnet. Magnetic
force has not merely the property of acting upon magnetic
poles, it has the additional property of producing a phenomenon
known as magnetic induction, or magnetic flux, a physical condition
which is of the nature of a flow continuously circulating
through the magnet and the space outside it. Inside the magnet
the course of the flow is from the south pole to the north pole;
thence it diverges through the surrounding space, and again
converging, re-enters the magnet at the south pole. When the
magnetic induction flows through a piece of iron or other magnetizable
substance placed near the magnet, a south pole is
developed where the flux enters and a north pole where it
leaves the substance. Outside the magnet the direction of the
magnetic induction is generally the same as that of the magnetic
force. A map indicating the direction of the force in different
parts of the field due to a magnet may be constructed in a
very simple manner. A sheet of cardboard is placed above the
magnet, and some iron filings are sifted thinly and evenly over
the surface: if the cardboard is gently tapped, the filings will
arrange themselves in a series of curves, as shown in fig. 1.
Fig. 1.
This experiment suggested to
Faraday the conception of
“lines of force,” of which the
curves formed by the filings
afford a rough indication;
Faraday’s lines are however
not confined to the plane of
the cardboard, but occur in
the whole of the space around
the magnet. A line of force
may be defined as an imaginary
line so drawn that its direction
at every point of its course
coincides with the direction of
the magnetic force at that
point. Through any point in the field one such line can be
drawn, but not more than one, for the force obviously cannot
have more than one direction; the lines therefore never
intersect. A line of force is regarded as proceeding from the
north pole towards the south pole of the magnet, its direction
being that in which an isolated north pole would be urged along
- ↑ In London in 1910 the needle pointed about 16° W. of the geographical north. (See Terrestrial Magnetism.)
