magnetism

Phenomenon in the force field ("magnetic field") surrounding a magnet. 

Magnetism is caused by electrical charges in motion. This is not necessarily something unique to electrical equipment, which is the case for electromagnetism, but also for permanent magnets. It is the motion of the electrons around the nuclei, and also the spin of the electrons, that cause the magnetism in a natural magnet. The magnetic field of Earth is believed to come from movements inside the planet, where oceans and rivers of melted metals exist. (Think Jules Verne here people.)

An important concept is that of the magnetic dipole, which is basically the physical representation of a simple magnetic rod. It has a north, "plus", pole and a south, "minus", pole. Such a pole cannot exist individually, so if you break up a magnetic rod in two pieces, each will be a new magnet with two poles. This shows a fundamental difference between magnetism and electricity, since there are free electric charges but no free magnetic poles. One consequence of this is that magnetic field lines always create closed circles and it is these field lines that you see when you put iron chips around a magnet. 

Magnetic fields are measured in the SI Unit weber, Wb (1 weber is really 1 volt second), which is a measurement of the magnetic flux Φ through a surface. The most common term is the magnetic field B, which really is the magnetic flux density, or flux per area. This is measured in Wb/m², which has its own unit tesla T, after Nicolai Tesla. An older obsolete unit for this is Gs, Gauss, which is 1/10000 of a teals.  

The most important characteristics of magnetism are described by the following laws, which shows how magnetism and electricity work together :

• Magnetic force law, sometimes Lorentz force law, describes how electric charges in motion are affected by a magnetic field. 
• Biot-Savart law, sometimes Ampere's law, describes the magnetic field surrounding an electric current.
• Law of induction, describes how voltage is induced into an electric circuit in a magnetic field, when the magnetic field is changed.

The B field has two components; one depending on the magnetic field strength H and one depending on the magnetization M. In short we can calculate the field as 

B = μ₀ · H + μ₀ · M = μ₀ · H + J

where μ₀ is the permeability in vacuum. The term J is called magnetic polarization. The magnetization M is (usually) related to the H field as M = χ_m · H where χ_m is the susceptibility, which is a very important characteristics of magnetic materials. 

History
The word "magnetism" comes from the area Magnesia in ancient Asia Minor where magnetite, Fe₃O₄, was mined in the 9th century B.C. It was early discovered how it would attract iron. The first application was probably at sea, where they were used for compasses starting from the 13th century in China as well as in Europe. During the 17th and 18th century, magnets and magnetism started to interest several scientists and they gradually unveiled its mystery. The most important breakthrough however would have to wait until 1820 when H.C. Ørsted discovered the relation between magnetism and electricity. André Ampère further developed Ørsted's experiments and came up with a mathematical model. 

The next big contribution came from Michael Faraday who introduced the magnetic field and described its behavior. Faraday's work helped Maxwell in his work with what later became Maxwell's equations, which are the most central part in electromagnetic theory. The continuing work on magnetism has gone hand in hand with the development of atomic and quantum theories and our understanding of magnetsim is tightly couples with quantum theory.

Reference: ne.se

Mag"net*ism (?), n. [Cf. F. magn'etisme.]

The property, quality, or state, of being magnetic; the manifestation of the force in nature which is seen in a magnet.

2.

The science which treats of magnetic phenomena.

3.

Power of attraction; power to excite the feelings and to gain the affections.

Glanvill.

Animal magnetism, a force, more or less analogous to magnetism, which, it has been alleged, is produced in animal tissues, and passes from one body to another with or without actual contact. The existence of such a force, and its potentiality for the cure of disease, were asserted by Mesmer in 1775. His theories and methods were afterwards called mesmerism, a name which has been popularly applied to theories and claims not put forward by Mesmer himself. See Mesmerism, Biology, Od, Hypnotism. -- Terrestrial magnetism, the magnetic force exerted by the earth, and recognized by its effect upon magnetized needles and bars.

© Webster 1913.