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What are temporary magnets?
Soft iron and certain iron alloys, such as permalloy (a mixture
of iron and nickel) can be very easily magnetized, even in a
weak field. As soon as the field is removed, however, the magnetism
is lost. These materials make excellent temporary magnets that
are used in telephones and electric motors for example.
What are permanent magnets?
Other kinds of alloys such as alnico (an alloy of aluminum,
nickel, iron, cobalt), make excellent permanent magnets. Ferrites
(ceramic like materials made of iron oxides with nickel and cobalt)
also make excellent permanent magnets. In these materials the
domains are more difficult to dislodge, once they are aligned.
What are electromagnets?
Electromagnets are used when really strong magnets are required.
Electromagnets are produced by placing a metal core (usually
an iron alloy) inside a coil of wire carrying an electric current.
The electricity in the coil produces a magnetic field. Its strength
depends on the strength of the electric current and the number
of coils of wire. Its polarity depends on the direction of the
current flow. While the current flows, the core behaves like
a magnet, but as soon as the current stops, the magnetic properties
are lost. Electric motors, televisions, maglev trains, telephones,
computers and many other modern devices use electromagnets.
What are superconductors?
These are the strongest magnets. They don't need a metal core
at all, but are made of coils of wire made from special metal
alloys which become superconductors when cooled to very low temperatures.
How did it all begin?
There are many legends accounting for the discovery of magnets.
One of the most common, is that of an elderly shepherd named
Magnes, who was herding his sheep in an area of Northern Greece
called Magnesia, about 4,000 years ago. It is said that both
the nails in his shoes and the metal tip of his staff became
firmly stuck to the large, black rock on which he was standing.
This type of rock was subsequently named magnetite, after either
Magnesia or Magnes himself.
Stories of magnetism date back to the first century B.C in
the writings of Lucretius, and the magical powers of magnetite
are mentioned in the writings of Pliny the Elder. For many years
following its discovery, magnetite was surrounded in superstition
and was considered to possess magical powers, such as the ability
to heal the sick, frighten away evil spirits and attract and
dissolve ships made of iron! Unlike amber (fossilized tree resin),
magnetite was able to attract objects without first being rubbed.
This made magnetite far more magical. People soon realized that
magnetite not only attracted objects made of iron, but when made
into the shape of a needle and floated on water, magnetite always
pointed in a north-south direction creating a primitive compass.
This led to an alternative name for magnetite, that of lodestone
or "leading stone".
Who discovered magnets?
The first attempt to separate fact from superstition came
in 1269, when a soldier named Peter Peregrinus wrote a letter
describing everything that was known, at that time, about magnetite.
It is said that he did this while standing guard outside the
walls of Lucera which was under siege. While people were starving
to death inside the walls, Peter Peregrinus was outside writing
one of the first 'scientific' reports and one that was to have
a vast impact on the world. It wasn't until the experiments of
William Gilbert in 1600 that significant progress was made in
the understanding of magnetism and it was another century or
so before other scientists began, by experimentation, to understand
the phenomenon.
Who
were the scientists who helped us to understand magnets?
It was William Gilbert who first realized that the
Earth was a giant magnet and that magnets could be made by beating
wrought iron. He also discovered that the induced magnetism was
lost if the iron was heated. In 1820, Hans Christian Øersted,
demonstrated for the first time (at a public lecture), that there
was a relationship between electricity and magnetism.
View these pages on magnetism from the ABC of Electricity - a primer from 1917 - Bellingham
Antique Radio Museum
What is magnetite?
Magnetite is found in rock strata associated with iron deposits
and has been found in the ocean floor dating from 2 to 55 million
years old. Magnetite is magnetic because its molecular structure
has allowed it to retain the alignment of particles caused by
the Earth's magnetic field during its formation millions of years
ago. When heated to high temperatures magnetite loses its natural
magnetism. Not all iron deposits are magnetic, however, which
for many years posed a question. Why is magnetite only formed
in certain iron deposits? Recently an interesting theory has
emerged concerning an anaerobic bacterium, GS-15, which has been
shown to convert ferric oxide into magnetite. It is thought that
GS-15, could be responsible for the creation of magnetite layers
in many iron formations.
What are magnetic force fields?
The area of force (magnetic field) surrounding a piece of
magnetite or a bar magnet can be represented (visualized) by
the lines of force as shown below, although these lines are no
more real than the lines of latitude and longitude on a map or
globe.
What is the rule of magnetism?
Like poles repel and unlike poles attract. |
The
atoms forming materials that can be easily magnetized such as
iron, steel, nickel, and cobalt are arranged in small units,
called domains. Each domain, although microscopic in size, contains
millions of billions of atoms and each domain acts like a small
magnet. If a magnetic material is placed in a strong magnetic
field, the individual domains, which normally point in all directions,
gradually swing around into the direction of the field. They
also take over neighboring domains. When most of the domains
are aligned in the field, the material becomes a magnet.
Hundreds
of years ago Chinese sailors used pieces of magnetite, made into
needles, to help them find their way if they were lost. A piece
of magnetite, or a bar magnet, when freely suspended, generally
comes to rest pointing in a north-south direction (a compass
needle is a magnet). The Earth is like a giant magnet and behaves
as if there is a huge bar magnet in the centre.
For
centuries travellers used compasses for navigation without understanding
how they worked. For many years it was assumed that the magnetic
and geographic poles were the same, a misconception that led
to the early discovery of North America by Christopher Columbus
in 1492. At the magnetic poles the field lines point straight
up and down and so a compass there is useless.
