The Science of Magnets - The History Channel
force of attraction or repulsion between various substances, especially
those made of iron and certain other metals; ultimately it is due to
the motion of electric charges. Magnetic Poles, Forces, and Fields
Any object that exhibits
magnetic properties is called a magnet. Every magnet has two points,
or poles, where most of its strength is concentrated; these are designated
as a north-seeking pole, or north pole, and a south-seeking pole, or
south pole, because a suspended magnet tends to orient itself along
a north-south line. Since a magnet has two poles, it is sometimes called
a magnetic dipole, being analogous to an electric dipole, composed of
two opposite charges. The like poles of different magnets repel each
other, and the unlike poles attract each other.
One remarkable property
of magnets is that whenever a magnet is broken, a north pole will appear
at one of the broken faces and a south pole at the other, such that
each piece has its own north and south poles. It is impossible to isolate
a single magnetic pole, regardless of how many times a magnet is broken
or how small the fragments become. (The theoretical question as to the
possible existence in any state of a single magnetic pole, called a
monopole, is still considered open by physicists; experiments to date
have failed to detect one.)
From his study of
magnetism, C. A. Coulomb in the 18th cent. found that the magnetic forces
between two poles followed an inverse-square law of the same form as
that describing the forces between electric charges. The law states
that the force of attraction or repulsion between two magnetic poles
is directly proportional to the product of the strengths of the poles
and inversely proportional to the square of the distance between them.
As with electric
charges, the effect of this magnetic force acting at a distance is expressed
in terms of a field of force. A magnetic pole sets up a field in the
space around it that exerts a force on magnetic materials. The field
can be visualized in terms of lines of induction (similar to the lines
of force of an electric field). These imaginary lines indicate the direction
of the field in a given region. By convention they originate at the
north pole of a magnet and form loops that end at the south pole either
of the same magnet or of some other nearby magnet (see also flux, magnetic).
The lines are spaced so that the number per unit area is proportional
to the field strength in a given area. Thus, the lines converge near
the poles, where the field is strong, and spread out as their distance
from the poles increases.
A picture of these
lines of induction can be made by sprinkling iron filings on a piece
of paper placed over a magnet. The individual pieces of iron become
magnetized by entering a magnetic field, i.e., they act like tiny magnets,
lining themselves up along the lines of induction. By using variously
shaped magnets and various combinations of more than one magnet, representations
of the field in these different situations can be obtained.
The term magnetism
is derived from Magnesia, the name of a region in Asia Minor where lodestone,
a naturally magnetic iron ore, was found in ancient times. Iron is not
the only material that is easily magnetized when placed in a magnetic
field; others include nickel and cobalt. Carbon steel was long the material
commonly used for permanent magnets, but more recently other materials
have been developed that are much more efficient as permanent magnets,
including certain ferroceramics and Alnico, an alloy containing iron,
aluminum, nickel, cobalt, and copper.
Materials that respond
strongly to a magnetic field are called ferromagnetic [Lat. ferrum=iron].
The ability of a material to be magnetized or to strengthen the magnetic
field in its vicinity is expressed by its magnetic permeability. Ferromagnetic
materials have permeabilities of as much as 1,000 or more times that
of free space (a vacuum). A number of materials are very weakly attracted
by a magnetic field, having permeabilities slightly greater than that
of free space; these materials are called paramagnetic. A few materials,
such as bismuth and antimony, are repelled by a magnetic field, having
permeabilities less than that of free space; these materials are called
The Basis of Magnetism
The electrical basis
for the magnetic properties of matter has been verified down to the
atomic level. Because the electron has both an electric charge and a
spin, it can be called a charge in motion. This charge in motion gives
rise to a tiny magnetic field. In the case of many atoms, all the electrons
are paired within energy levels, according to the exclusion principle,
so that the electrons in each pair have opposite (antiparallel) spins
and their magnetic fields cancel. In some atoms, however, there are
more electrons with spins in one direction than in the other, resulting
in a net magnetic field for the atom as a whole; this situation exists
in a paramagnetic substance. If such a material is placed in an external
field, e.g., the field created by an electromagnet, the individual atoms
will tend to align their fields with the external one. The alignment
will not be complete, due to the disruptive effect of thermal vibrations.
Because of this, a paramagnetic substance is only weakly attracted by
In a ferromagnetic
substance, there are also more electrons with spins in one direction
than in the other. The individual magnetic fields of the atoms in a
given region tend to line up in the same direction, so that they reinforce
one another. Such a region is called a domain. In an unmagnetized sample,
the domains are of different sizes and have different orientations.
When an external magnetic field is applied, domains whose orientations
are in the same general direction as the external field will grow at
the expense of domains with other orientations. When the domains in
all other directions have vanished, the remaining domains are rotated
so that their direction is exactly the same as that of the external
field. After this rotation is complete, no further magnetization can
take place, no matter how strong the external field; a saturation point
is said to have been reached. If the external field is then reduced
to zero, it is found that the sample still retains some of its magnetism;
this is known as hysteresis.
Evolution of Electromagnetic
The connections between
magnetism and electricity were discovered in the early part of the 19th
cent. In 1820 H. C. Oersted found that a wire carrying an electrical
current deflects the needle of a magnetic compass because a magnetic
field is created by the moving electric charges constituting the current.
It was found that the lines of induction of the magnetic field surrounding
the wire (or any other conductor) are circular. If the wire is bent
into a coil, called a solenoid, the magnetic fields of the individual
loops combine to produce a strong field through the core of the coil.
This field can be increased manyfold by inserting a piece of soft iron
or other ferromagnetic material into the core; the resulting arrangement
constitutes an electromagnet.
discovery the various magnetic effects of an electric current were extensively
investigated by J. B. Biot, Felix Savart, and A. M. Ampcre. Ampcre showed
in 1825 that not only does a current-carrying conductor exert a force
on a magnet but magnets also exert forces on current-carrying conductors.
In 1831 Michael Faraday and Joseph Henry independently discovered that
it is possible to produce a current in a conductor by changing the magnetic
field about it. The discovery of this effect, called electromagnetic
induction, together with the discovery that an electric current produces
a magnetic field, laid the foundation for the modern age of electricity.
Both the electric generator, which makes electricity widely available,
and the electric motor, which converts electricity to useful mechanical
work, are based on these effects.
between electricity and magnetism is that a regularly changing electric
current in a conductor will create a changing magnetic field in the
space about the conductor, which in turn gives rise to a changing electrical
field. In this way regularly oscillating electric and magnetic fields
can generate each other. These fields can be visualized as a single
wave that is propagating through space. The formal theory underlying
this electromagnetic radiation was developed by James Clerk Maxwell
in the middle of the 19th cent. Maxwell showed that the speed of propagation
of electromagnetic radiation is identical with that of light, thus revealing
that light is intimately connected with electricity and magnetism.
See D. Wagner, Introduction
to the Theory of Magnetism (1972); D. J. Griffiths, Introduction to