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Gonzo Gizmos

Projects & Devices to Channel your Inner Geek

Chicago Review Press Incorporated

MAGNETISM

We've 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 electronics and toy stores you can buy flexible plastic magnetic strips that can be cut into shapes with scissors. You can also buy inexpensive (and brittle) ceramic magnets, stronger AlNiCo magnets, and even the new super-strong rare-earth magnets. These are made of neodymium-iron-boron or samarium-cobalt, and are very powerful.

Through mail-order surplus houses, you can buy large neodymium-iron-boron magnets that are incredibly strong. These magnets, which cost about five dollars each, can hold paperback books onto your refrigerator, or drag one other around a two-inch-thick table, one on top of the table and one underneath. I once entertained my guests and several waiters at a restaurant by mysteriously moving the stainless flatware around the table. Most people are not familiar with the properties of powerful magnets and are amazed at what they can do.

Because of their high strength-to-weight ratios, neodymium-iron-boron magnets appear to be little affected by gravity. Small ones can be placed on either side of your nose, and will stay there until you laugh so hard they slide upward, against gravity, and snap together. They can also be used as temporary earrings, but be sure to handle the larger magnets with care, since they will pinch hard enough to cause blisters if they are separated by only a small bit of skin. They can also easily erase magnetically stored information on credit cards, computer floppy disks, and cassette tapes, so take care where you place them.

A Few Simple Magnet Experiments

If you are showing magnets to a very young person for the first time, choose large, cheap, ceramic magnets that will not be easily lost or swallowed. Then try a few tricks. Show how they attract and repel each other. Have the child slide donut-shaped magnets over a pencil, stacking several so that each repels the one below it, to form a magnetic spring.

SHOPPING LIST

* Cheap magnets

* Sand Sticky tape

* Sheet of paper

TOOLS

* Clear plastic bag

* Paper clips, pins, key chains, and other bits of ferrous metal

* Pin or screwdriver

Next, put several cheap magnets into a clear plastic bag and drag it through sand at a playground or beach. The magnets will attract black iron ore from the sand. (The plastic bags keep the ore off the magnets, keeping them clean and making it easier to remove the powder. If the powder does get onto the magnets, use some sticky tape to remove it.) Sprinkle this ore onto a sheet of paper with a magnet placed underneath, which will create arcing lines of powder that trace out the magnetic lines of force. A magnetic compass placed near the magnet will align itself with the arcs of powder, following the curve as you move it around.

Place several magnets under the paper to create interesting shapes in the iron ore above. Paper clips, pins, key chains, and other bits of ferrous metal placed beneath the paper will alter the shape of the ore powder as they become temporary magnets under the influence of the permanent magnets.

Stroke a bit of iron or steel (such as a pin or a screwdriver) across the magnet. This makes a new magnet that will attract other bits of steel or the iron ore. By placing this new magnet under the paper with the ore powder, you can demonstrate that this magnet is not as powerful as the original. Heat up the pin or screwdriver to show how heat destroys the magnetism.

Place two magnets under the paper, separated by a paper clip or other small piece of iron. Make sure each magnet touches the paper clip. Now when you sprinkle the ore on the paper, it forms a much smaller arc of powder. This shows the effect of a magnetic flux concentrator (the paper clip), which narrows and thus strengthens the force between the two magnets.

These simple experiments and observations form the backdrop for the rest of this chapter. In it you will see effects that are not so simple or obvious, but yet are simple extensions of the same properties of magnets shown above.

Magnetorheological Fluids

A magnetorheological fluid is a liquid that hardens near a magnet but becomes liquid again when you remove the magnet. These fluids are simple to make in your kitchen after a trip to a sandbox.

SHOPPING LIST

* Iron filings or beach sand

* Paper plate

* Strong magnet

* Vegetable oil

TOOLS

* Plastic bag

* Plastic spoon or other nonferrous object, such as a popsicle stick

As described earlier, mine iron ore from playground or beach sand using magnets and plastic bags. You will probably spend a while at the beach because you will need a large handful of ore. To save time, you can also purchase a vial of iron filings from a scientific supply store.

The ore you collect will likely have quite a bit of sand entrained in it. You can remove the sand with some additional refining. First, be sure the ore is dry. Spread it 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 this process until no more ore rises to the magnet. Don't let the bag get too close to the plate, since there are many sand grains with ore stuck to them. You want to keep only the ore that is not stuck to grains of sand. The ore on the second plate should be visibly darker than what is left on the first plate. If you can see a lot of sand on the second plate, repeat the process using a third plate.

Put the refined ore into a small plastic cup. The cup should be small enough so that the ore fills at least a third of it. Add a little vegetable oil to the ore, stirring the mixture with a plastic spoon or another nonferrous object, such as a popsicle 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 a magnetorheological fluid.

Now you're 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 it 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; the solid mass slumps back into the cup and the spoon falls over.

Put some of the magnetorheological fluid into a plastic bag, and stick a magnet to the outside. Now you can mold the fluid into shapes by pressing the bag. The fluid will act like clay and hold its shape. When you eventually remove the magnet, the shape will slump into a puddle.

Why Does It Do That?

Iron ore in oil reacts pretty much the way it reacts without the oil. Only one thing is different: the oil allows the powder to slump more easily than it can when dry. This is because of the oil's additional weight, its lubricating ability, its viscosity, and the fact that the iron ore is more buoyant in oil than in air.

Saying that iron ore in oil behaves like dry ore doesn't really answer the question unless you know why dry ore acts the way it does. If you look very closely at the ore with a magnifying glass or a microscope, you will notice that the pieces are slightly longer than they are wide, like small footballs. Shapes like this do interesting things in a magnetic field.

To see why, put 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 parallel to a bar magnet if you hold them side-by-side. But as you move the nail toward one pole of the magnet, it will rotate so that the point of the nail points toward the pole. Eventually, when the nail is above the pole, it will point directly at the pole.

There are two ways to think about what is happening. Pretend the attraction of the magnet is like the gravitational attraction of the Earth and that 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 straight up again. Physicists 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 a domino to lose this energy by lying down. It has much less of a tendency to spontaneously gain energy and stand back up. Similarly, the nail tends to "fall down" so that it aligns with the magnetic lines of force that surround the magnet.

The second way of thinking about the nail and magnet requires 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. You might expect the magnet to attract the second nail, just like the first nail did, but instead you'll find that the two nails repel one another. If you 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 bar magnet. 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, but the nails repel each other because like poles repel. 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, and its south pole points to the magnet's north pole. When the nail is beside the magnet, it rests parallel because these two attractions are equal. When the nail is closer to the magnet's north pole, the nail's south pole attracts and the north pole repels, so the nail rotates.

Magnets repel one another when their matching poles are side-by-side. They attract one another when their opposite poles 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 other, they will stagger so the poles of one string will be next to the center of the other string. In this arrangement, like poles are as far apart as possible. These strings will still repel from one another slightly, which is exactly the behavior of grains of iron ore sprinkled on paper above a magnet.

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 of its neighbor. The strings thus formed repel each other, and the powder expands. 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.

A Magnetic Heat Engine

SHOPPING LIST

* Canadian nickel or rare-earth magnet

* Stiff copper or brass wire

* Large ceramic magnet

* Small candle or alcohol burner

TOOLS

* Wire coat hanger (optional)

* Needle-nosed pliers

A simple Magnetic Heat Engine demonstrates how heat causes magnetic material to lose its ability to be magnetized, and how cooling that material allows it to regain its magnetic property.

I originally built this device using a Canadian nickel. Canadian nickels are made of pure nickel, unlike U.S. nickels, which contain so much copper that they are not magnetic. You can build this device with the nickel or with a rare-earth magnet. A rare-earth magnet will work a little better because it loses its magnetic properties at a lower temperature, and thus the engine can use a candle instead of an alcohol burner for its heat source.

The pendulum on the Magnetic Heat Engine should only swing back and forth in one direction, so it needs two wires to suspend it. Cut about a foot of stiff copper or brass wire and wrap the center of the wire around the large ceramic magnet. Then, using needle-nosed pliers, twist both ends into small loops. Bend the loops up to form the two supports for the pendulum on page 10.

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. Demagnetizing the rare-earth magnet will prevent it from jumping onto the large ceramic magnet while you adjust the pendulum.

Wrap another foot of wire around the nickel or the rare-earth magnet that will act as the pendulum weight. Bend the two ends of the wire to hang on the loops of the pendulum support, making 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 demagnetized. Cut off the remaining wire.

Suspend the Canadian nickel (or rare-earth magnet) at the end of a pendulum. Place a large magnet near the pendulum, so that the nickel sticks out toward the large magnet. The magnet should be close enough that the nickel hangs at an angle toward the magnet, and not straight down, at the bottom point of the pendulum's swing.

If you have chosen to use the Canadian nickel, you will need a better heat source than a 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 the lamp.

Place the alcohol lamp or candle under the nickel, so the flame just touches it. The flame will heat up the nickel until it loses its ability to be magnetized. Gravity will eventually pull it away from the large magnet (and thus away from the flame). The nickel will then cool down once it is away from the flame, regaining its ability to be magnetized. The large magnet will then pull it up into the flame, and the whole process will repeat.

If the weight still touches the flame when it has fallen away from the magnet, adjust the pendulum's supports so that the weight rests a little farther away. Be careful when adjusting the supports, since they may be quite hot. (Also, be careful to move the heat source so as not to burn yourself on the flame.) 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. 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 flame burns.

Why Does It Do That?

A 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 material.

The Curie temperature for iron is about 800° Celsius (C). 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 affect them. The Curie temperature for the Canadian nickel is lower, about 631°C. This temperature is within range of an alcohol lamp, and almost possible with a candle. The Curie temperature for a rare-earth magnet is 310°C, 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 surface area.

I have tried Ronson lighter flints, which also have a Curie temperature within easy range of a candle flame. The combination of their small size and low Curie temperature makes them stay above their Curie point too long. The magnet and the flame have to be close together for the engine to work. When the flints are close enough to the magnet to overcome gravity, they will be close enough to the flame to rise above their Curie point.

Other heat engine designs are possible, involving placing the flints on a wheel and using a soldering iron as a heat source. A magnifying glass could also be used to focus the radiant energy of the sun on a flint when it is touching the magnet, but not heating the flint when it falls away. Experiment with other designs; there are many possibilities.

MORE ABOUT MAGNETS

(The Scientific Part)

It is fairly simple to visualize nails becoming magnets and aligning themselves in the magnetic field. It is also easy to visualize them as dominoes that "fall" in line with the field. But what is really happening?

Magnetic fields exist because somewhere electrons are moving. In the nails, the electrons in the iron atoms are orbiting the nuclei, and each electron is also spinning on its axis. Most materials are not strongly magnetic because their magnetic poles (caused by spinning electrons) are either randomly oriented or they pair in opposite directions.

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Excerpted from Gonzo Gizmos by Simon Quellen Field. Copyright © 2002 Simon Quellen Field. Excerpted by permission of Chicago Review Press Incorporated.
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