What is electromagnetism?
Magnetism is the phenomenon by which certain materials assert an attractive or repulsive influence on other materials. The fact that magnetism exists has been known for thousands of years, since the discovery that natural lodestone (magnetite) could attract or repel iron objects, but the underlying principles of physics that govern magnetism and the related phenomenon of electricity have only been understood within the last hundred years.

The first step in understanding magnetism came when Michael Faraday discovered that a changing magnetic field can produce an electric field. For example, if a bar magnet is pushed into a coil of wire, it will produce a measurable electric current flowing through the wire - but only while the magnet is moving. When left stationary inside the coil, it will not produce an electric current, but on being withdrawn again, current will once more flow – the direction of the current will depend on the direction in which the magnet is moving. This is one of the ways in which modern electric power is now generated.

James Clerk Maxwell then deduced that as electricity and magnetism are related, a changing electric field should produce a magnetic field. This can be shown experimentally by passing a wire down through a small hole in a piece of card which has been sprinkled with iron filings. When an electric current flows through the wire, the iron filings on the card will form concentric rings due to the magnetic field generated by the current.

Maxwell also showed that electric and magnetic fields travel together through space as waves of electromagnetic radiation and that their changing fields sustain each other.

Until early in the 20th Century, magnetism and electricity were thought to be two different forces, but in 1905, Einstein’s special theory of relativity showed that electricity and magnetism are, in fact, interrelated aspects of a common phenomenon, electromagnetism, although each behave quite differently.
- Electric forces are produced by electric charges, either at rest or in motion. In contrast, magnetic forces, are produced only by moving charges and act solely on charges in motion.
- Electric and magnetic forces are detected in regions called electric and magnetic fields, which can exist in space far from the charge or current that produced them.
- Waves of electromagnetic radiation (eg radio waves, microwaves, visible light, X-rays) all travel at the speed of light (around 186,000 miles per second) but differ in the frequency at which their electric and magnetic fields oscillate.

What is a magnet?
Atoms are made up of a central nucleus, which contains positively charged subatomic particles (protons) and neutral particles (neutrons) surrounded by orbiting negatively charged electrons that are all held together by strong attractive forces. The inside of the atom is positively charged, and the outside negative.

The magnetic properties of materials result from the individual electrons orbiting the atomic nucleus. As it is a moving charge, an electron forms a small current loop which will generate a small magnetic field. Each electron can also be thought of as spinning around an axis which will also produce a small magnetic field.

The electrons orbit the nucleus within areas known as shells. If the outer shell contains a full complement of electrons, these form pairs spinning in opposite directions. The magnetic fields they produce cancel each other out, so that the atom to which they belong is electrically stable and unreactive and is not capable of being permanently magnetised.

If electrons are missing from the outer shell, however, the unpaired electrons spin in a haphazard manner which makes the atom reactive, or unstable, and may also allow it to be permanently magnetised. These missing electrons can generate an enormous electromagnetic force – for example, it is calculated that if only one electron is missing out of every billion molecules in two people who each weigh 70 kg and are standing 2 m apart, their bodies would repel each other with a force equivalent to 30,000-tons!

As all substances are made up of atoms containing electrons, all will show some magnetic properties when placed in a magnetic field (eg the pole of a permanent bar magnet), even materials which are not normally thought of as being magnetic. This weak magnetic behaviour, due to induced changes in the orbits of electrons, will only persist for as long as the external field is applied.

Some substances, such as iron, cobalt, nickel, boron and neodymium are more highly attracted towards the pole of a permanent bar magnet, however, even a weak one, and are capable of being permanently magnetised as a result of their electronic structure.

In the case of iron, for example, three electrons are missing from its outer shell, so that three unpaired electrons are spinning around the central nucleus. While the piece of iron remains unmagnetised, these unpaired electrons have different directions of spin, and the forces they generate tend to cancel each other out. When the unpaired electrons are influenced by the pole of a permanent magnet, however, they become organised and aligned in the same direction, with the same axes of spin, so that the forces generated by these synchronised electrons add together to create a powerful magnetic force which can persist even when the original external magnetic field that aligned them is taken away.

As the number of iron atoms present increases, so does the number of synchronised electrons and the force they collectively generate. A-large magnet made from a heavy piece of iron will therefore generate a much greater magnetic force than a magnet made from a small piece of iron.

Iron magnets
Lodestone (magnetite) is an iron rich ore (Fe304) that has become magnetised by lying in the Earth’s magnetic field for millions of years. Iron can also be magnetised naturally through being struck by lightening, or subjected to an electric field. In medieval times, blacksmiths noticed that iron bars could be magnetised if they were heated and then aligned in a north-south direction and beaten with a hammer as they cooled.

In the 1700s, carbon steel was found to retain its magnetism better than plain iron, and in the 1930s, magnets containing iron mixed with aluminium, nickel and cobalt were developed. It was only in the last 20 years that rare earth magnets were developed from metallic elements in the rare earth group of the periodic table of elements.

Rare earth magnets
The modern, therapeutic magnetic patch is made from an alloy of iron, neodymium, and boron. These magnets are made by applying great heat and pressure to the powdered metals and are considerably lighter and more powerful than traditional iron or steel magnets. They are therefore much smaller in size, and can be worn more discreetly (eg applied as a patch) than the traditional magnets that were strapped to the body with belts and bands.

Magnetic patches can be placed directly over a painful site, over acupuncture points, or on various sites of the head.
Measuring magnetic force

The strength of the magnetic field a magnet can produce is measured in modern units known as teslas, after the late scientist, Nikola Tesla (1856-1943). An older measure is also in use, known as the gauss. One tesla is equivalent to 10,000 gauss.

Magnets used in healing usually have field strengths ranging from 0.02 to 0.2 tesla – this is the same as 20 to 200 milliteslas (mT) or 200 to 2000 gauss.
A magnetic field of one gauss is about twice the magnetic field at the earth’s surface.

Refrigerator magnets have a strength of 80 to100 gauss, while the medical diagnostic technique of Magnetic Resonance Imaging is done in a field of 10,000 gauss. According to the World Health Organisation, there are no known adverse effects to human health from exposure to static magnetic fields of up to 20,000 gauss.

Low gauss magnets have a strength of: 300 to 700 g
Medium gauss magnets have a strength of: 800 to 2500 g
High gauss magnets have a strength of greater than 2500 g
A high field strength is not necessarily more effective than a low field strength, but as a general rule, look for magnets with a gauss strength of 500 gauss or greater as strengths below this do not seem to be as effective.

Different strengths are used for different applications, and the rare earth magnets used in therapeutic patches produce a field strength of 2000 gauss.