We have all played with magnets.
A pair of magnets by itself makes a wonderful toy.
Today's magnets are even better than the best ones I remember playing
with as a child.
At toy stores and Radio Shack you can get flexible magnetic strips of
plastic that can be cut into shapes with scissors. You can also get cheap
and brittle ceramic magnets, stronger Alnico magnets, and even the new super
strong rare-earth magnets. These are made of
samarium-cobalt, and are very powerful.
of this section I will list some sources I have found for good or
cheap (or good and cheap) magnets.
Some particularly nice tiny ones can be found
in our catalog.
mail-order surplus houses
you can get large neodymium-iron-boron
magnets of incredible strength.
For about five dollars each you can get magnets that will hold paperback
books onto your refrigerator, or drag each other around a two-inch thick
table, one on top of the table and one hidden underneath.
I once entertained my guests and several waiters at a restaurant by
mysteriously moving the stainless flatware around the table.
People are not used to the effects of powerful magnets.
They are amazed even when they can see what you are doing.
Because of their high strength-to-weight ratios, neodymium-iron-boron
magnets seem to be little affected by gravity.
Small ones can be placed on either side of a nose and will stay there
until the wearer laughs so hard they slide upwards against gravity and
Temporary earrings are also popular.
Handle larger magnets with care, since they will pinch hard enough to
cause blisters if they are separated only by small bits of skin. They
are also easily capable of erasing the magnetically stored information
on credit cards, computer floppy disks, and cassette tapes,
but magnetic storage in consumer items is becoming rare these days, and
this is less of an issue than it used to be.
If you are showing magnets to a very young person for the first time,
there are several tricks you will not want to forget.
Use some large cheap ceramic magnets from Radio Shack or a toy store
so they will not be easily lost or swallowed.
Show how they attract and repel each other.
At Radio Shack you can get some donut-shaped magnets that can slide over
The child can stack several so they each repel, forming a magic spring.
A magnetorheological fluid is a liquid that hardens near a magnet, and
becomes liquid again when you remove the magnet.
They are simple to make in your kitchen after a trip to a sandbox.
In the introduction to this section I described
how to mine iron ore
the sand from playgrounds or the beach.
You may want to spend a while at the beach, because we will need a good
handful of ore.
The ore that sticks to the magnets in the plastic bags has quite a bit of
sand entrained in it.
We can remove the sand by some additional refining.
Make sure the ore is dry.
Spread the ore out on a paper plate, and hold the bag with the magnet over
the plate until a small amount of ore jumps up to the bag.
Put this ore onto another plate, and continue until no more ore rises to the
magnet in the bag.
Don't get the bag too close to the plate, since there are many sand grains
with ore stuck to them.
We wish to keep only the ore that does not stick to grains of sand.
The ore in the second plate should be visibly darker than what is left in
the first plate.
If you can see a lot of sand in the second plate, repeat the process, using
a third plate.
Put the ore into a small cup.
Soft plastic cups work nicely.
The cup should be small enough that the ore fills it at least a third of the
Add some vegetable oil to the ore, and stir with a plastic spoon, or another
nonferrous object, such as a popsickle stick.
Keep adding oil until you get a thin black paste.
Now gently place a strong magnet on the side of the cup.
It should stick to the side as it attracts the ore.
The ore should become quite stiff.
Tip the cup over another cup to let excess oil and ore pour off.
What remains in the first cup is our magnetorheological fluid.
We are now ready for the fun part.
Hold the cup upright, and remove the magnet.
Stir the liquid with the plastic spoon.
It may be a little stiff at first, but will soon stir easily.
Tip the cup a bit to the side, and bury the bowl of the spoon in the liquid.
Now place the magnet on the side of the cup to stiffen the goop.
The spoon will now stand upright when the cup is righted.
The cup can even be inverted without losing any fluid, although a little oil
may still drip out the first few times.
Set the cup upright again, remove the magnet, and the solid mass slumps back
into the cup, and the spoon falls over.
Put some fluid into a plastic bag, and let a magnet stick to the side of the bag.
Now you can form the fluid into shapes by pressing the bag.
The fluid will act like clay, and hold its shape.
When you remove the magnet, the shapes slump into puddles.
Why does it do that?
Iron ore in oil reacts pretty much the way it reacts without the oil.
Only one thing is really different.
The oil allows the powder to slump more easily than it can when dry.
This is because of its extra weight, its lubricating ability, its viscosity,
and the fact that the ore is more buoyant in oil than in air.
Saying that it behaves like dry ore doesn't really answer the question unless
we know why dry ore acts the way it does.
If we look very closely at the ore with a magnifying glass or a microscope,
we will notice that the pieces are a little longer than they are wide.
They look like small footballs.
Shapes like this do interesting things in a magnetic field.
Take a small iron nail in one hand, and a magnet in the other.
Move the nail around the magnet, holding the nail loosely so it can move under
the influence of the magnet.
The nail will align itself with a bar magnet if you hold the two of them
parallel to each other.
As the nail moves toward one pole of the magnet, it will rotate so the point
of the nail points toward the pole.
Eventually, when the nail is above the pole, it will point straight at the pole.
There are two ways to think about what is happening.
Pretend the attraction of the magnet is the attraction of the Earth, and the
nail is a domino standing up on its end.
A slight push makes the domino lie flat, and it takes a larger push to make
it stand up straight again.
We say that the domino has more potential energy when it is standing up than
it has when it is lying down.
There is a tendency for dominos to lose this energy by lying down.
They have much less of a tendency to spontaneously gain energy and stand up
The nail tends to "fall down" so that it aligns with the magnetic lines of
force that surround the magnet.
To illustrate the second way of thinking about the nail and the magnet we
need a second nail.
Hold the first nail parallel to the magnet, about half an inch to the right.
Bring the second nail parallel to the first nail, a little to its right.
We might expect the magnet to attract the second nail, just like the first nail.
Instead we find that the two nails are repelling one another.
If we lower the second nail so its top is near the bottom of the first nail,
it now attracts the first nail.
The nails seem to have become magnets themselves while in the presence of the
Their poles repel when they are parallel, and attract when they align vertically.
If the bar magnet has its north pole facing away from you, the nails will have
their south poles facing away from you.
The nails attract the magnet because unlike poles attract each other.
The nails repel each other because like poles repel each other.
It is now easy to see why the nail follows the magnetic lines of force as it
moves around the magnet.
Its north pole points toward the magnet's south pole.
Its south pole points to the magnet's north pole.
When the nail is beside the magnet, it is parallel because the attractions are
When it is closer to the magnet's north pole, the nail's south pole attracts
and the north pole repels, and it rotates.
Magnets repel one another when they are side by side.
They attract one another when they are end to end.
The natural state of a collection of magnets will thus be a string of them
stuck end to end.
If there are two strings next to each another, they will stagger so the poles
of one string will be next to the centers of the other.
Like poles are thus as far apart as possible.
The strings will still repel from one another slightly.
This is exactly the behavior of grains of iron ore sprinkled on paper above
When a magnet is placed on the side of a jar of iron ore powder, the powder
arranges itself into strings.
Each grain of powder becomes a magnet and attracts the opposite pole if its
The strings thus formed repel each another, and the powder
If the powder has been mixed with oil, the oil wicks into the spaces created
by the expansion, and sticks there by surface tension.
The result is a dry appearing solid that does not leak oil.
To learn more about magnets, skip ahead to the
Building a magnetic heat engine.
I originally built this toy using a Canadian nickel coin.
Canadian nickels are made of pure nickel, unlike U.S. nickels, which
contain so much copper that they are not magnetic.
You can build the toy with the nickel or with the Radio Shack rare-earth magnet.
The rare earth magnet will work a little better because it loses its
magnetic properties at a lower temperature, and thus the toy can use a
candle instead of an alcohol burner for its heat source.
Some particularly nice tiny rare-earth magnets can be found
in our catalog.
This heat engine is very simple.
We suspend a small piece of magnetic material at the end of a pendulum.
A large magnet is placed near the pendulum, so that the small piece of
material sticks to the large magnet.
The magnet should be close enough that the material never rests at the
bottom of the pendulum's swing, but instead jumps up to the magnet.
A candle is placed under the material, so the flame just touches it.
The candle flame will heat up the magnetic material until it loses its
ability to be magnetized.
Gravity will then pull it away from the magnet (and thus away from the flame).
The magnetic material will cool down a little bit once it is away from
the flame, and regain its ability to stick to the magnet.
The magnet will then pull it up into the flame, and the whole process repeats.
All the parts can be found at Radio Shack, but if you want to build the
engine using a Canadian nickel, any hardware store will have the other
parts you need.
You will need some copper or brass wire, a large ceramic magnet (the cheap
kind that Radio Shack sells for about a dollar), and a candle.
We want the pendulum to swing back and forth only, so we use two wires to
hold it up.
Cut about a foot of wire and wrap the center of the wire around the large magnet.
Then form the two ends into small loops and bend them up to form the
support for the pendulum.
If you are using the rare-earth magnet for the pendulum's weight, it
helps at this point to demagnetize it by holding it in a candle flame.
You can stick it onto a coat hanger and hold the magnet in the flame
until it falls off.
This will prevent the magnet from jumping onto the large ceramic magnet
while we adjust the pendulum.
Wrap another foot of wire around the nickel or the rare-earth magnet
that will form the weight for the pendulum.
Form the two ends of the wire into loops that slip into the loops of
the pendulum support.
Make sure that the pendulum weight is just close enough to the magnet
that it rises to it when the pendulum is vertical.
The wires of the pendulum and its support should be long enough that
the weight can fall away from the flame and hang vertically when it is
With the pendulum stuck to the large magnet, position a short (lighted)
candle so that the flame just touches the weight.
You may need to shield the flame from drafts so it remains steady.
The flame will heat the rare-earth alloy until it loses its ability to
stick to the large ceramic magnet.
It will then fall away, and swing a few times as it cools.
When it is cool enough to be magnetized again, it will rise and stick
to the magnet, where the flame will again heat it up.
If the weight still touches the flame when it has fallen away from the
magnet, adjust the pendulum's supports a little so that the weight rests
a little farther away.
If the weight is so far away that the magnet cannot pull it back up once it
is magnetized, adjust the supports to bring it closer.
Be careful when adjusting the supports, since they may be quite hot.
Also be careful to move the candle so as not to burn yourself on the candle
When the engine is adjusted just right, it will settle down to a predictable
swing, often taking only one swing to cool enough to stick to the magnet again.
It will run as long as the candle burns.
If you have chosen to use the Canadian nickel, you will need a heat source
better than the candle.
A small alcohol lamp or fondue pot burner will do nicely.
You may have to make the pendulum support wires longer to make room for
Why does it do that?
The heat engine works because of something called the Curie effect.
The Curie effect describes how a magnetic material loses its ability to
stick to a magnet when heated above a certain temperature.
This temperature is called the Curie temperature, and varies with the
The Curie temperature for iron is about eight hundred degrees Celsius.
The Curie temperature for the inexpensive ceramic magnets is also quite
high, which is why the candle flame or even the alcohol lamp does not
The Curie temperature for the Canadian nickel is lower, about 631 degrees
This temperature is within range of the alcohol lamp, and almost possible
with the candle.
The Curie temperature for the Radio Shack rare earth magnets is 310 degrees
Celsius, and the candle can reach this easily (not only because of the
lower Curie temperature, but because the magnets are so much smaller
than the nickel that they heat up faster, and have less unheated
I have tried Ronson lighter flints, which also have a Curie temperature
within easy range of a candle. The combination of their small size and
low Curie point makes them stay above their Curie point too long.
The magnet and the flame have to be close together for the engine to
When the flints are close enough to the magnet to overcome gravity,
they are close enough to the flame to rise above their Curie point.
Other designs have been tried successfully, involving placing the
flints on a wheel and using a soldering iron as a heat source.
A magnifying glass could be used to focus the sun on the flint
when it is touching the magnet, without heating the flint when
it falls away.
Experiment with other designs.
There are many possibilities.