by Chris Woodford. Last updated: October 25, 2019.
Science is our understanding of how the
world works—and generally the world works
fine whether we understand it or not. Take magnetism, for
example. People have known about magnets for thousands of years and
they've been using them practically, as compasses, for almost as long.
The ancient Greeks and Romans knew as well as we do that lodestone (an
iron-rich mineral) can attract other pieces of iron, while the
ancient Chinese were making magnetic compasses set in intricate
wooden inlays for their practice of Feng Shui (the art of
carefully arranging a room) thousands of years before interior
designers came on board. Science can sometimes be slow to catch up: we've only really
learned how magnetism works in the last century, since the world inside atoms was first discovered and explored.
Photo: A typical horseshoe magnet.
See the trace of brown rust on the top of the magnet's upper "leg"?
That happens because the magnet is made of iron, which corrodes (rusts) in damp air.
What is magnetism?
Photo: The magnetic field between the opposite
poles of two bar magnets that strongly attract one another. We can't
normally see magnetic fields, but if you sprinkle iron filings (tiny
bits shaved off an iron bar with a file) onto a piece of paper and hold
it above the magnets you can see the field underneath. Photo by
courtesy of Wikimedia Commons (where you'll find a bigger version of this image).
Playing with magnets is one of the first bits of science most
discover. That's because magnets are easy to use, safe, and
fun. They're also quite surprising. Remember when you first
discovered that two magnets could snap together and stick like glue?
Remember the force when you held two magnets close and
felt them either attract (pull toward one
another) or repel
(push away)? One of the most amazing things about
magnets is the way they can attract other magnets (or other magnetic
materials) "at a distance," invisibly, through what we call a
To ancient people, magnetism must have seemed like magic. Thousands
of years down the line, we understand what happens inside magnetic
materials, how their atomic structure causes
properties, and how electricity and
magnetism are really just two
sides of the same coin: electromagnetism. Once scientists would have
said magnetism was the strange, invisible force of attraction between
certain materials; today, we're more likely to define it as a force
created by electric currents (themselves caused by moving electrons).
How can we measure magnetism?
The strength of the field around a magnet depends on how close you
get: it's strongest very near the magnet and falls off quickly as
you move away. (That's why a small magnet on your table has to be quite close to
things to attract them.) We measure the strength of magnetic fields
in units called gauss and tesla3d精华衣123456
(the modern SI unit, named for electricity pioneer Nikola
Tesla, 1856–1943). It's interesting to note that the strength of
Earth's magnetic field is very weak—about 100–1000 times weaker than
that of a typical bar or fridge magnet. On Earth, gravity, not magnetism, is
the force that sticks you to the floor. We'd notice Earth's magnetism
much more if its gravity weren't so very strong.
Chart: Comparing the strength of some "everyday" sources of magnetism.
Please note that the vertical scale is logarithmic: each step up the scale means the strength
of the magnetic field has increased ten times. The main thing to note here is how very weak Earth's
magnetism is (the green block on the extreme left) compared to everything else we routinely encounter (never mind the giant magnets used in hospitals and labs). The record-breaking lab magnetic field shown on the extreme right,
created in Japan in April 2018, is about 24 million times stronger than Earth's magnetic field. My data for this chart comes from the following sources: Earth (goo.gl/TkxfO3), Sun (goo.gl/8uigAU), appliances (goo.gl/P3l487), fridge (goo.gl/OhrDKt), small neodymium (goo.gl/avODib), junkyard (goo.gl/owWZer), MRI (goo.gl/jQ8cTD), loudspeaker (goo.gl/oIwNlS), biggest MRI (goo.gl/8zkACY), biggest lab (bit.ly/2zvH7On). Almost everything produces magnetism—even our own bodies, which make something like a puny 0.000000001 tesla.
What is an electromagnet?
The Homer Simpson or Mickey Mouse magnet that holds things to your
is a permanent magnet: it keeps hold of its
magnetism all the
time. Not all magnets work this way. You can make a temporary
magnet by passing electricity
through a coil of wire wrapped
around an iron nail (a device you'll sometimes see referred to as a
solenoid). Switch on the current and the nail
becomes a magnet; switch it off again and the magnetism disappears.
(This is the basic idea behind an electric chime doorbell:
you make an electromagnet when you press the button, which pulls a hammer onto the chime bar—ding-dong!)
Temporary magnets like this are called electromagnets3d精华衣123456—magnets
electricity—and they hint at a deeper connection between electricity
and magnetism that we'll come on to in a moment.
Like permanent magnets, temporary electromagnets come in different
and strengths. You can make an electromagnet powerful enough to pick
up paperclips with a single 1.5-volt battery.
Use a much bigger
voltage to make a bigger electric current and you can build an
electromagnet powerful enough to pick up a car. That's how scrapyard
electromagnets work. The strength of an electromagnet depends on two
main things: the size of the electric current you use and the number
of times you coil the wire. Increase either or both of these and you
get a more powerful electromagnet.
What do we use magnets for?
3d精华衣123456maybe you think magnets are interesting; maybe you think they're
use are they, you might ask, apart from in childish magic tricks and
You might be surprised just how many things around you
work by magnetism or electromagnetism. Every electric appliance with
an electric motor in it (everything
from your electric toothbrush to
your lawn mower) uses magnets to turn electricity into motion.
Motors use electricity to generate temporary magnetism in wire coils. The magnetic field thus produced
pushes against the fixed field of a permanent magnet, spinning the inside part of the motor around
at high speed. You can harness this spinning motion to drive all kinds of machines.
There are magnets in your refrigerator
holding the door closed. Magnets read and write data (digital information) on your
computer's hard drive and on cassette
tapes in old-fashioned personal stereos. More magnets in your hi-fi
loudspeakers or headphones help to turn stored music back
into sounds you can hear. If you're sick with a serious internal illness, you might
have a type of body scan called NMR (nuclear magnetic resonance), which draws the
world beneath your skin using patterns of magnetic fields. Magnets are used to recycle
your metal trash (steel food
cans are strongly magnetic but aluminum
are not, so a magnet is an easy way to separate the two different
Photo: An NMR scan like this builds up a
detailed image of a patient's body
(or, in this case, their head) on a computer
screen using the magnetic activity of atoms in their
body tissue. You can see the patient going into the scanner at the top
and the image of their head on the screen below. Photo by courtesy of
Warren Grant Magnuson Clinical Center
(CC) and US
National Institutes of Health (NIH).
Which materials are magnetic?
Iron is the king of magnetic materials—the metal we all think of
when we think of magnets. Most other common metals (such as copper,
and aluminum) are, at first sight,
nonmagnetic and most nonmetals
wood, plastic, concrete, glass,
and textiles such
as cotton and wool) are nonmagnetic too. But iron's not the only
magnetic metal. Nickel, cobalt, and elements that belong to a part of
the Periodic Table (the orderly
arrangement chemists use to describe all the known chemical elements)
known as the rare-Earth metals (notably
samarium and neodymium) also make good
magnets. Some of
the best magnets are alloys (mixtures) of
these elements with one
another and with other elements. Ferrites (compounds made of iron,
oxygen, and other elements) also make superb magnets. Lodestone
(which is also called magnetite) is an example of a ferrite that's
commonly found inside Earth (it has the chemical formula FeO·Fe2O3).
Materials like iron turn into good temporary magnets when you put a
them, but tend to lose some or all of their magnetism when you take
the magnet away again. We say these materials are magnetically soft.
By contrast, alloys of iron and the rare-Earth metals retain most of
their magnetism even when you remove them from a magnetic field, so
they make good permanent magnets. We call those materials
Is it true to say that all materials are either magnetic or
nonmagnetic? People used to think that but scientists now know that the
materials we consider to be nonmagnetic are also affected by magnetism, though
extremely weakly. The extent to which a material can be magnetized is
called its susceptibility.
How different materials react to magnetism
scientists have a number of different words to describe how
when you put them near a magnet (which is another way of saying when
you put them inside a magnetic field). broadly speaking, we can
divide all materials into two kinds called paramagnetic and
diamagnetic, while some of the paramagnetic materials are also
ferromagnetic. it's important to be clear what these confusing words actually mean...
3d精华衣123456make a sample of a magnetic material and hang it from a thread so it
dangles in a magnetic field, and it will magnetize and line itself up so
its magnetism is parallel to the field. as people have known for thousands of years, this is
how exactly a compass needle behaves in earth's magnetic field. materials that
behave this way are called paramagnetic. metals such as aluminum and
most nonmetals (which you might think aren't magnetic at all) are
actually paramagnetic, but so
weakly that we don't notice. paramagnetism depends on temperature:
the hotter a material is, the less it's likely to be affected by
Photo: We think of aluminum (used in drinks
cans like these) as nonmagnetic. That helps us separate for recycling
our aluminum cans
(which don't stick to magnets) from our steel ones (which do). In fact,
both materials are magnetic. The difference is that aluminum is very
weakly paramagnetic, while steel is strongly ferromagnetic. Photo courtesy of US Air Force.
Some paramagnetic materials, notably iron and the rare-Earth
metals, become strongly magnetized in a field and usually stay
even when the field is removed. We say materials like this are
ferromagnetic, which really just means they're "magnetic like
iron." However, a ferromagnetic material will still lose its
magnetism if you heat it above a certain point, known as its Curie temperature3d精华衣123456. Iron has a Curie temperature of
(1300°F), while for nickel the Curie temperature is ~355°C (~670°F). If
you heat an iron magnet to 800°C (~1500°F), it stops being a
magnet. You can also destroy or weaken ferromagnetism if you hit a
We can think of paramagnetic and ferromagnetic materials as being
"fans" of magnetism: in a sense, they "like" magnetism and respond
positively to it by allowing themselves to be magnetized. Not all
materials respond so enthusiastically. If you hang some
materials in magnetic fields, they get quite worked up inside and
resist: they turn themselves into
temporary magnets to resist magnetization and weakly repel magnetic
fields outside themselves. We call these materials diamagnetic. Water
and lots of organic
(carbon-based) substances, such as benzene, behave this way. Tie a
diamagnetic material to a thread and hang it in a magnetic field and
it will turn so it makes an angle of 180° to the field.
What causes magnetism?
In the early 20th century, before scientists properly understood the
structure of atoms and how
they work, they came up with an easy-to-understand idea called the
domain theory to explain magnetism. A few
when they understood atoms better, they found the domain theory still
worked but could itself be explained, at a deeper level, by the
theory of atoms. All the different aspects of magnetism we observe can
be explained, ultimately, by talking about either domains, electrons
in atoms, or both. Let's look at the two theories in turn.
Explaining magnetism with the domain theory
imagine a factory somewhere that makes little bar magnets and ships
them out to schools for their science lessons. picture a guy called
dave who has
to drive their truck, transporting lots of cardboard boxes, each one
with a magnet inside it, to a different school. dave doesn't
have time to worry which way the boxes are stacked, so he piles them
inside his truck any old how. the magnet inside one box could be
while the one next to it is pointing south, east, or west. overall,
the magnets are all jumbled up so, even though magnetic fields leak
out of each box, they all cancel one another out.
the same factory employs another truck driver called bill who
couldn't be more different.
he likes everything tidy, so he loads his truck a different way,
stacking all the boxes neatly so they line up exactly the same way. can
you see what will happen? the magnetic field from one box will align with
the field from all the other boxes... effectively turning the truck
into one giant magnet. the cab will be like a giant north pole and
the back of the truck a huge south pole!
what happens inside these two trucks is what happens on a tiny scale
inside magnetic materials. according to the domain theory, something
like an iron bar contains
lots of tiny pockets called domains. each domain is a bit like a box
magnet inside. see where we're heading? the iron bar is just like the
truck. normally, all its onboard "boxes" are arranged randomly
and there's no overall magnetism: the iron is not magnetized. but
arrange all the boxes in order, make them all face the same way, and
you get an overall magnetic field: hey presto, the bar is magnetized.
when you bring a magnet up to an unmagnetized iron bar and stroke it
systematically and repeatedly up and down, what you're doing is
rearranging all the magnetic "boxes" (domains) inside so they
point the same way.
Domain theory explains what happens inside
materials when they are magnetized. In an unmagnetized material (left),
the domains are randomly arranged so there is no overall magnetic
field. When you magnetize a material (right), by stroking a bar magnet
over it repeatedly in the same direction, the domains rearrange so
their magnetic fields align, producing a combined magnetic field in the
this theory explains how magnetism can arise, but can it explain
some of the
other things we know about magnets? if you chop a magnet in half, we
know you get two magnets, each with a north and south pole. that
makes sense according to the domain theory. if you cut a magnet
in half, you get a smaller magnet that's still packed with domains,
and these can be arranged north-south just like in the original
magnet. what about the way magnetism disappears when you hit a magnet
heat it? that can be explained too. imagine the van full of orderly
boxes again. drive it erratically, at really high speed, and it's a
bit like shaking or hammering it. all the boxes will jumble up so
they face different ways and the overall magnetism will disappear.
magnet agitates it internally and jumbles up the boxes in much the
Explaining magnetism with the atomic theory
the domain theory is easy enough to understand, but it's not a
explanation. we know that iron bars aren't full of boxes packed with
little magnets—and, if you think about, trying to explain a magnet
by saying it's full of smaller magnets isn't really an explanation at
all, because it immediately prompts the question: what are the
smaller magnets made of? fortunately, there's another theory we can
Back in the 19th century, scientists discovered they could use
electricity to make magnetism and magnetism to make electricity. James
Clerk Maxwell said that the two phenomena were really different aspects
the same thing—electromagnetism—like two sides of the
same piece of paper. Electromagnetism was a brilliant idea, but it
was more of a description than an explanation: it showed how things
were rather than explaining why they were
that way. It wasn't
until the 20th century, when later scientists came to understand the
world inside atoms3d精华衣123456, that the explanation for
We know everything is made of atoms and that atoms are made up of a
central lump of matter called the nucleus. Minute particles called
move around the nucleus in orbit, a bit like satellites in the sky
above us, but they also spin on their axis at the same time (just
like spinning tops). We know electrons carry electric currents (flows
of electricity) when they move through
materials such as metals.
Electrons are, in a sense, tiny particles of electricity. Now back in
the 19th century, scientists knew that moving electricity made
magnetism. In the 20th century, it became clear that magnetism was
caused by electrons moving inside atoms and creating magnetic fields
all around them. Domains are actually groups of atoms in which spinning
electrons produce an overall magnetic field pointing one way or
Artwork: Magnetism is caused by electrons orbiting and spinning inside atoms. Note that
this picture is not drawn to scale: most of an atom is empty space and the electrons are actually much further
from the nucleus than I've drawn here.
like the domain theory, atomic theory can explain many of the things
we know about magnets, including paramagnetism (the way magnetic
materials line up with magnetic fields). most of the electrons in an atom exist
in pairs that spin in opposite directions, so the magnetic effect of
one electron in a pair cancels out the effect of its partner. but if
an atom has some unpaired electrons (iron atoms have four), these
produce net magnetic fields that line up with one another and turn
the whole atom into a mini magnet. when you put a paramagnetic
material such as iron in a magnetic field, the electrons change their
motion to produce a magnetic field that lines up with the field
What about diamagnetism? In diamagnetic materials, there are no unpaired electrons,
so this doesn't happen. The atoms have little or no overall magnetism and are less
affected by outside magnetic fields. However, the electrons orbiting inside
them are electrically charged particles and, when they move in a magnetic field,
they behave like any other electrically charged particles in a magnetic
field and experience a force. That changes their orbits very slightly, producing some net magnetism that opposes
the very thing that causes it (according to the classic bit of electromagnetic theory know as Lenz's law,
which is related to the law of conservation of energy).
As a result, the weak magnetic field they produce opposes the magnetic field that causes it—which
is exactly what we see when diamagnetic materials try to "fight" the magnetic field they're placed in.
A brief history of magnetism
- Ancient world: Magnetism is known to the ancient Greeks, Romans,
and Chinese. The Chinese use geomantic compasses (ones with wooden
inscriptions arranged in rings around a central magnetic needle) in
Feng Shui. Magnets gain their name from Manisa in Turkey, a place
once named Magnesia, where magnetic lodestone was found in the ground.
- 13th century: Magnetic compasses are first used for navigation
in western nations. Frenchman Petrus Perigrinus
(also called Peter
of Maricourt) makes the first proper studies of magnetism.
- 17th century: English physician and scientist William
(1544–1603) publishes On Magnets, his
monumental scientific study of
proposes that Earth is a giant magnet.
- 18th century: Englishman John Michell
Augustin de Coulomb (1736–1806) study the forces
magnets can exert. Coulomb also makes important studies of electricity,
but fails to connect electricity and magnetism as parts of the same
- 19th century: Danishman Hans Christian Oersted
(1777–1851), Frenchmen André–Marie Ampère
(1775–1836) and Dominique Arago
(1786–1853), and Englishman Michael Faraday
(1791–1867) explore the
close connections between electricity and magnetism. James
Clerk Maxwell (1831–1879) publishes a relatively complete
explanation of electricity and magnetism (the theory of
electromagnetism) and suggests electromagnetic energy travels in
waves (paving the way for the invention of radio).
Pierre Curie (1859–1906) demonstrates
that materials lose their magnetism above a certain temperature (now known as the Curie
temperature). Wilhelm Weber (1804–1891)
develops practical methods for detecting and measuring the strength of a magnetic field.
- 20th century: Paul Langevin (1872–1946) elaborates on
Curie's work with a theory explaining how magnetism is affected by heat. French
physicist Pierre Weiss (1865–1940) proposes
there are particles called magnetrons, equivalent to electrons, that cause the magnetic
properties of materials and outlines the theory of magnetic domains.
Two American scientists, Samuel Abraham Goudsmit
(1902–78) and George Eugene Uhlenbeck
(1900–88), show how magnetic properties of
materials result from the spinning motion of electrons inside them.