See also: Electromagnet; Electromagnetic Radiation
Force of attraction or repulsion between various substances, especially those
containing iron and certain other Metals, such as nickel and cobalt; ultimately it is due to the Motion of electric Charges. Any object that exhibits magnetic properties is called a magnet. An
ordinary magnet has two poles where the magnetic forces are the strongest; these poles
are designated as a north (north-seeking) pole and a south (south-seeking)
pole, because a magnet freely rotating in the earth's Magnetic Field tends to orient itself along a north-south line. The like poles of different
magnets repel each other, and the unlike poles attract each other. Whenever a
magnet is broken, a north pole appears 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 small the
fragments become. (The possibility of the existence of a single magnetic pole, or
monopole, is still unresolved, but experiment has failed so far to detect one.)
In the 18th century Coulomb, Charles Augustin de found that the magnetic forces of attraction and repulsion are directly
proportional to the product of the strengths of the poles and inversely proportional
to the square of the distances 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 picture of the magnetic field lines can be obtained by placing a
piece of paper over a magnet and sprinkling iron filings on it. The individual
pieces of iron become magnetized by entering a magnetic field, i.e., they act
like tiny magnets, lining themselves up along the magnetic field lines. The
connection between magnetism and Electricity was discovered in the early 19th century. Oersted, Hans Christian found (1820) 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. Ampere, Andre Marie; showed (1825) that magnets exert forces on current-carrying Conductor. In 1831 Faraday, Michael and Henry, Joseph independently discovered electromagnetic Induction - the production of a current in a conductor by a change in the magnetic
field around it. The magnetic properties of Matter are also explained by the motion of charges. Because the Electron has both an electric charge and a spin, it can be considered a charge in
motion, giving rise to a tiny magnetic field. In many atoms, all the electrons are
paired within energy levels, so that the electrons in each pair have opposite
(antiparallel) spins, and their magnetic fields cancel. In some Atoms there are more electrons with spins in one direction than the other,
resulting in a net magnetic field for the atom as a whole. Placed in an external
field, the individual atoms will tend to align their fields with the external one.
Because of thermal vibrations the alignment is not complete, and materials,
called paramagnetic substances, that contain such atoms react only weakly to a
magnetic field. Materials such as iron, nickel, or cobalt that respond strongly to
a magnetic field are called ferromagnetic. 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, called a domain, tend to
line up in one direction, so that they reinforce each other. Materials such as
bismuth and antimony that are repelled by a magnetic field are called
diamagnetic. In a diamagnetic substance, an external magnetic field accelerates the
electrons moving in one direction and retards those moving in the opposite
direction; this situation produces an induced magnetization opposite indirection to the
external field.