A loadstone will attract steel staples.
Technorama Forum Lecture
Presented by Paul Doherty, 18 October 2001
The legend has it that before 1 AD a Greek shepherd boy named Magnus from the Greek region of Magnesia noted that certain stones attracted iron nails. These stones we now call magnetite, they are made of iron and oxygen with the chemical equation, Fe3O4. Lucretius wrote that these stones attracted and repelled each other.
A loadstone will attract steel staples.
Then an amazing thing happened for the next 1000 years, at least amazing viewed from our modern perspective, nothing. After all that time in China it was found that if loadstone were carved into a spoon shape the handle of the spoon when placed on a smooth surface would point south.
An activity for the audience, reproduce the great Chinese discovery of the magnetic compass.Throughout the audience I have placed magnets on strings. Hang them from the backs of the chairs in front of you. Notice that they all hang pointing in the same direction.
Paul Hangs a magnet from a string and shows that it points north
Chinese refer to compasses as "south pointers." South was an important direction in the Chinese practice of Geomancy. It is now still important in Feng Sui.
It is hard to tell when the compass was invented because there is another "south pointer" in Chinese, the emperor's chariot. The chariot was geared so that as it twisted and turned in its progress through a city the emperors throne always pointed in the same direction, south. It is difficult to determine when the words "south pointer" stopped meaning the emperor's chair and started meaning a compass but it was sometime before 1100 AD.)
Meanwhile back in Europe...
In 1265 a warrior scientist named Peter Peregrinus wrote about his experiments with magnets. He noted that they had two special ends which he named "polus" or poles, and that when the magnets were floated on water the poles lined up so that one pointed to the north star. The Europeans used the compass to navigate and so gave great importance to the north pointing end, the end that showed them the location of the north pole star during the day. The end that pointed to the north pole star was named the "north seeking pole." Later this was shortened to the north pole.
Aside: Since navigators were lead by the stones they were called leadstones which has become lodestones in modern English. (By the way, the name of magnetite stones in French is appropriately "loving stones," since the stones attract and "kiss.")
It was known that like poles repelled and opposite poles attracted. For example, north poles repelled north poles and attracted south poles.
Why did the north seeking pole point north?
300 years later William Gilbert discovered why the stones pointed north, the earth itself was a magnet. Unfortunately, this meant that the names that Peter Peregrinus had chosen were confusing. The north pole of a magnet pointed to the north geographic pole of the earth because it was attracted to the north geographic pole which meant that the north geographic pole was a south magnetic pole, the confusion this caused has remained until the present day. On all maps the south pole of the earth magnet is named the "north magnetic pole."
Audience Experiment, repeat William Gilbert's experimentIn the audience there are magnetic globes with staples on them. These staples make a pattern. they stand up perpendicular to the surface at the two poles, north and south, they lie down flat along the circle of the magnetic equator. And they incline at in-between angles at other latitudes.
A rubber earthglobe with a magnet inside allows me to repeat William Gilbert's experiment.
This can also be shown on an overhead projector where a neodymium magnet inside a plastic lid from a peanut butter jar, about 8 cm in diameter holds staples at the correct angle.
It is also shown in the illustration drawn by Gilbert himself.
The Curie Point
Gilbert thought that the earth was an iron magnet. While the core of the earth is made of iron, it cannot be magnetic because it is so hot. What Gilbert didn't know is that when iron is heated up hot enough it loses all of its magnetism, the temperature of a material where it loses its ferromagnetism is known as its curie point. For iron the curie point is 770 °C. The iron inside the earth is much hotter than 770 °C and so cannot be an iron magnet.
I have several rods with a curie point of -5 °C. I can attract these out of the ice using ferrite magnets. When the rods warm up they lose their magnetism and drop off the magnet.
When rods start above the Curie point and are cooled below it in the presence of a magnetic field such as that of the earth they become magnetized. The Chinese used this to make compasses out of iron. Lava includes hot, iron-containing minerals such as basalt, which can be magnetized. As the lava cools it locks in the direction of the magnetic field of the earth. This locks in the latitude at which the lava cooled. On Aneroid peak in eastern Oregon there are lava layers which preserve horizontal magnetic fields. This means that those lavas cooled near the equator. Continental drift has moved them far north to where they are today in Oregon.
Magnetic Bacteria
There are bacteria which use the magnetic field of the earth. Inside these bacteria are grains of magnetite. The bacteria are anaerobic, i.e. killed by oxygen. They use the magnetic field to sense "down" which is the direction away from oxygen and toward life. In the northern hemisphere these bacteria swim towards a south magnetic pole, in the southern hemisphere they swim toward north poles.
Image of magnetic bacteria with magnetite grains.
Life on Mars
Grains similar to those made by bacteria have been found in a meteorite from Mars. This meteorite was found in the Allan Hills region of Antarctica and is called ALH 840001. It is one bit of evidence that there were at one time bacteria on Mars.
The Connection between Electricity and Magnetism
In 1820 a physics lecturer, Hans Christian Oersted, was passing an electric current from a voltaic pile through a wire in an experiment which showed that the wire became hot.(I will repeat this experiment soon.) Nearby was a compass. When the current started to flow through the wire, Oersted noted that the compass needle moved. This was the first time electricity and magnetism were shown to be connected. An electric current creates magnetism.
The Magnetic Field
In England, Michael Faraday made a great advance in magnetism by using field lines to understand magnetic experiments. Perhaps you have sprinkled iron filings over a magnet, they line up around the magnet in a pattern. Faraday hypothesized that they lined up with a force field made by the magnet.
Here is a display of lines around a magnet shown by the alignment of many iron rods.
The Magnetic field of Sunspots
The sun also has a magnetic field. Indeed sunspots are magnetic and come in pairs one north pole spot paired with a south pole spot.
You can "see" the solar magnetic field above sunspots in this image from the TRACE satellite. The magnetic fields are followed by glowing ionized gasses. (These ionized gasses happen to be iron with 8 electrons removed, but any ionized gas would do, it doesn't have to be iron.)
A simple Experiment
A simple experiment can be done which cannot be
understood without the concept of field lines.
This is a dangerous experiment, I'm going to use magnets which are so
big and strong that they can leap several inches and crush the bones
of my fingers between them as they smash together.
I start with a pile of three large flat magnets, one on top of the other. The north pole of one next to the south pole of its neighbor attracting the two together. When I pull the top magnet to the side it is pushed up in the air by the same magnet that had been pulling it down! I work hard to pull the top magnet to the side to emphasize the danger. The field lines from the central magnet point up above the magnet and then point down on the sides of the magnet. This changes the direction of the force on the other magnets from a downward attraction when one magnet is above the other to upward repulsion when one magnet is pulled to the side.
It is incongruous to see a magnet leaning on the air. It is even more interesting to give the leaning magnet a downward push and watch it bob up and down for a long time. Put one magnet to either side and start one oscillating. The magnetic fields will push on the distant magnet and start it oscillating too. Tune the oscillations so that they have the same frequency and the magnet you started will come to rest while the other magnet steals all of its motion. The process then continues as all the motion is returned.
Light is Electromagnetic Radiation
Faraday's discoveries provided the basis from which James Clerk Maxwell created the theory of electromagnetism and discovered that light was electromagnetic radiation.
Motion from Magnetism
Faraday also discovered that magnets could push on current carrying wires. This lead to the invention of the electric motor which has revolutionized our lives. Think of all the electric motors that surround you: powering refrigerator compressors, blowing air in hair dryers, rolling down car windows and many many more.
Here is an exhibit which shows that magnets push on current carrying wires. This is a wire from a toaster.
When I put a current through the wire it heats up until it glows. If I blow on the wire it dims briefly and then resumes glowing as it heats up.
When I bring a magnet near the wire, the wire begins to oscillate back and forth. The current I am putting through the wire oscillates, it is AC at 50 hertz. The magnet pushes on the AC current with a force that drives it into resonant oscillation.
Ferrofluid
One great modern material is ferrofluid, an oil full of suspended magnetic particles.
You can see some great ferrofluid demonstrations
here at Technorama.
I have some homemade ferrofluid here.
Notice that when I hold a strong magnet above it the fluid begins to bulge upward and then leaps up against gravity to reach the magnet. This ferrofluid is made by burning steel wool to make iron oxide, and then grinding the iron oxide to a fine powder with a mortar and pestle. It is then mixed with cooking oil.
Magnetic Levitation
To see a fluid leap upward against gravity makes most people laugh, but it would be even better to suspend material in mid air. To make matter fly.
Unfortunately, a scientist named Earnshaw once showed that there was no stable levitation possible using static electric and magnetic fields. After Earnshaw, physicists didn't even try to make levitated object using static magnets. However, other experimenters found a way around Earnshaw's theorem.
For example I can use electrostatics to levitate a shredded plastic ribbon. I get around Earnshaw's theorem by using a time varying, i.e. non-static feedback system, me.
Earnshaw's theorem doesn't apply if other stabilizing forces are present such as those from a pencil. Slide two donut magnets onto a pencil and one will levitate above another.
It pays to watch children. They will do things that physics professors would never think of doing. I did this pencil levitation trick for 40 years before a 7'th grade boy showed me a neat trick. Using a pencil with a steel eraser band you can hold the pencil point between two fingers and the lower magnet will stay on the pencil as it is attracted to the steel. Raise up the top magnet and drop it. The upper magnet falls and seems to pass through the lower magnet. This is an illusion resulting from an elastic collision in which the upper magnet collides with the lower one. The upper magnet stops and is caught by the eraser band while the lower one pops off at the same speed as the upper one.
Earnshaw's theorem can also be bypassed by using spinning magnets which are stabilized by their gyroscopic action. This was used in the toy known as a Levitron. Martin Simon of Los Angeles and Ortwin Schenker of Germany have recently built the spin stabilized magnetic levitator with the highest levitation ever achieved.
Diamagnetism
Earnshaw's theorem also disallowed levitation using a weak form of repulsive magnetism known as diamagnetism
In the 1980's very strong magnets were invented by a team at General Motors Research Laboratory that included a student of mine. These magnets are made of neodymium, iron, and boron. They are strong enough to reveal the weak magnetism of everyday objects, called diamagnetism, which causes some materials to be repelled by both poles of a magnet.
I can show diamagnetism using a torsional pendulum made of two grapes. The grapes are repelled by both poles of a strong neodymium magnet.
With even stronger magnetic fields from electromagnets wound from superconducting wire an entire frog can be suspended in air against gravity. No harm comes to the frog. At the moment no one can conceive of a magnet strong enough to levitate a person. However it would certainly be a great feeling to be able to fly.
Image of a frog levitated by magnetism.
AC Levitation
A pulse of AC magnetic field can be used to accelerate a conducting ring of aluminum and so shoot it into the air. This is shown by the exhibit, "Magnetic cannon," at Technorama. I have that exhibit here.
Show that the magnetic cannon shoots an aluminum ring into the air.
Aluminum is not magnetic yet the AC magnetic field induces an alternating electric current in the aluminum this current is then repelled by the AC magnetic field. This is what is happening inside the grapes, except that the electric current is inside the atoms and molecules of water and sugar inside the grapes.
This behavior is described by Lenz's law which says that when a magnetic field through a conductor changes, currents will flow in the conductor that make a magnetic field which opposes the change.
If we can increase the electric current in the aluminum ring we can increase the magnetic force on the ring and increase the height to which it is shot. I can do this by making the aluminum a better conductor. This is done by cooling the aluminum in liquid nitrogen. Cold aluminum conducts electricity better than room temperature aluminum. Besides the cold aluminum condenses a trailing cloud from the humid air of the room and looks cool
Chill the aluminum ring in liquid nitrogen and shoot it out of the magnetic cannon. It goes twice as high.
It's a good thing that I tested this experiment earlier. Materials also shrink when they get cold. When the aluminum ring was chilled it shrank so much that it would not fit over the glass tube of the magnetic cannon any more. ( We removed an inner ring of Teflon from the aluminum before it would fit.)
Superconducting Levitation
In 1911 Kammerlinge Onnes in Norway discovered that some materials lost all resistance to electrical current when cooled. These materials are called superconductors. In the 1980's superconductors were found which worked at temperatures of 70 K, below the boiling point of liquid nitrogen. I have a disk of this material known as 1,2,3 compound because it is made from Yttrium barium cupric oxide with an equation Y1Ba2Cu3O8. This material will levitate against gravity when placed over neodymium magnets. And, in accordance with Newton's third law of action and reaction, neodymium magnets will also levitate above the superconductor.
Levitate a disk of superconductor above a railroad track of Neodymium magnets. This particular type of superconductor has been processed so that it follows the track of superconducting magnets. It exhibits "flux pinning."
Recently very pure graphite crystallized in the right orientation has been found to show enough diamagnetism to levitate above a track made of neodymium magnets. I'd like to show you the first demonstration of this discovery tonight.
Levitate a graphite rectangle above the neodymium railroad track.
The graphite also follows the magnetic railway track.
Black graphite levitates a red paper train car above a
neodymium magnet track.
An experiment at Technorama recently showed that graphite cooled in liquid nitrogen levitates even higher above the neodymium track.
And so I've taken you across 2000 years of magnetic history, but I've left out the most important part of the story.
It turns out magnetism can be used to record music on a magnetic tape recorder and to create music in what I think is the pinnacle of magnetic technology...
The electric guitar.
The electric guitar is made from a 2 meter long dowel, steel piano wire stretched between eye bolts, a magnetic coil, and neodymium magnets.
In this my privately designed "Bender" electric guitar, so named because by bending it I bend the notes, you can see how the magnets magnetize the music wire. The wire moves and makes a changing magnetic field in the coil of wire. This makes an electric current which is amplified. The current changes in the same way that the wire moves, musically.
And thus magnetism reaches its highest form helping us to make music.
And so ends my tale of human progress in understanding magnetism.
Who knows what will come next?
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Scientific Explorations with Paul Doherty |
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28 October2001 |
