Funding for this program was provided by the annenberg/cpb project when I set out to prepere these lectures on megnetism for you, I went to the library to get afew books and find out what the subject was about. Our library isn't up-to-date on the subject of magnetism. The only book I found--the most recent I could find-- Is this one... Which came out in 1600. It's calledilde magnete, It's written by William Gilbert. William Gilbert was born in 1544 and lived to 1603. That makes him a contemporary of kepler and galieo. He was physician. He got his M.D.degree in 1569. At the university of cambridge. He became a promise london physician. Eventually,in 1600,He was named personal physician to gueen elizabeth I. He apparently wasn't good at it. She died almost immediately. However,when he was not busy applying leeches to unfortunate sovereigns, he spent his time studying magnetism. The fact is that the only personal bequest that queen elizabeth made when she dide was a sum of money for willam gilbert so that he could go on studying magnetism. Gilbert,as a matter of fact,discovered that you could destoy the magnetism 　of amagnetic material by he heating it up. Gilbert made a number of other important discoveries about magnetism. He found out how you could take a permanent magnet and make it even stronger by stroking it with another magnet. He found that if you kept an iron bar strictly aligned for a long time, it would gradually become magnetized. Perhaps his most important discovery was that the earth behaves like a giant magnet. A few more facts about magnetism have been discovered since the time of gilbeat. That't my topic for today. Of all the magnets on earth, none is quite so grand as the earth itself. In his elizabethan study, Dr.William Gilbert had discovered that fast. Since then,scientists have discovered that most of the planets have magnetic fields. So do the stars and,especially,the sun. In fact,much of the universe is like an extended family of magnets. As in any family, each member can have a significant effect on the others. Whether at the north pole or here at the south, nowhere else is the earth's magnetism felt or studied with as much intensity. This can be brutal environment, but it's not without certain rewards, such as meeting the native inhabitants... Or experiencing the aurora, which just may be the grearest show on earth. It's produced by charged particles from the sun and directed by the earth's magnetic field. But exactly what is magnetism? Everybody has played with magnets at some time, so you probably already know a lot about magnetism. You probably know you can use a magnetism to pick up a paper box, provided it's full of steel paper clips. I imagine you also know you can use a magnet won't pick up aluminum or copper. In other words,some materials are magnetic, and others are not. One thing you may not know is that sometime magnetic can have its magnetism destroyed just by heating it. For example,this is an iron wire. You can see it's magnetic because it's being held up by this bar magnet. I'm going to destroy the magnetism of that iron wire by heating it. I'll do that by passing an immense current through it. As it warms up,the wire will start to sag. But I'll have to get it almost red-hot before it will fall off the magnet. Now watch. There it goes. When it got hot enough, it destroyed its magnetism and fell off the wire. Now that I've disconnected the current,it's cooling down. When it's cool enaugh, it should stick back to the magnet again... Like that. Magnetism can be destroyed by heating. It's restored when the material cools down again. That's not true only of iron. That's not true of other materials as well. I have here something that looks like ordinary iron, but it isn't. This is a very rare metal known as gaddlinium. That's to say,it will not stick to this bar magnet. That's because room temperature is already so hot for gaddlinium thatits magnetism has been destroyed. If I cool it a litte below room temparature, it will become magnetic again. I can do that... I can cool it down by dipping it in this liquid nitrogen. watch. I just have to hold it inhere long enough for it to get cold enough to become magnetic. Let's try it now. You can see now it's magnetic. It'sactually magnetic only on one side. Only one side is cold. If I try the other side like this... It won't stick to the magnet. Only the cold side will stick, Like that. So magnetism can be destroyed by heating. It can be restored by cooling again in magnetic materials. Hot or cold, in the form of a simple magnet, or the spectacular aurora, what is the essential nature of magnetism? The equation for the force between magnetic poles resembles the equation for the force between electric charges, or the force between gravitational masses. Like electric charges, magnet poles come in two kinds called north and south. Opposite poles attrast. Like poles repel. Unlike electric charges, magnetic poles always come in equal and opposite paris. If you try to cut a magnet in two to separate the two poles, the cut-end points create their own poles. So each fragment still has a north and south pole. It's that difference between electric charges and magnetic poles that lead us to the question-- Can a single magnetic pole exist all by itself anywhere in the cosmos? Accoding to certain theories,it can. Theoretically,some single magnetic poles called magnetic monopoles do exist, and they have existed ever since and as a result of the big bang. But if magnetic monopoles came into being then, where in the world are they now? A number of scientists have been searching for magnetic monopoles, but none have been found as yet. Inthe meantime, there are still plenty of magnetic dipoles to go around. Magnetic dipoles are two poles forever bound togather into a single magnet. They go all the way back to William Gilbert. William gilbert,the man who founded the study of magnetism, characterized the earth as a giant magnet. The earth dose have an intrinsic magnetic field. It very much is that of a giant magnet. The earth's magnetic lines of force leave the surface in one hemisphere and return in another hemisphere. The pattern of those lines of force is very similar to that of a bar magnet. The magnetic field of a magnet, with its two poles, is similar in form to the electric field of two equal and opposite charges. A circular loop of electric current also creates a magnetic field of this form. So does every proton,every neutron, and every electron in the universe. The earth itself has a dipolefild that happens to point south, which is why compass needles on earth point north. That point ccan be very useful at times, especially at a time like this. These marines are using a conpass to find their way back to camp soldiers as well as sailors have long used the compasss to get themselves from here to there, over a hillside or around the world. And to the extent they've succeeded, It's been because the magnetic needle of a compass always points north. But how does a compass work? Or in other words, What actually happens to a magnet in a magnetic field? In any magnetic field, a magnet experiences equal and opposite forces At its poles, so that it tends to line up with the field. In other words, the field exerts a torque that tends to make the north pole point in the direction of the field. In fact,that's how the direction of the field is defined. ページ７ 　 no matter the destination, the magnetic needle of a compass can point the way because it lines up with the field of the earth at least it does so here on the surface but what hapness farther afield above the surface of the earth? how far does the earth's magnetic field extend? originally, people thought that the earth's dipole field extended indefintely into space, but now we know that's not the case. it's confined in the solor direction by the solor wind from the sun that impinges on it continuously. it confess it to a distance of 40,000 kilometers in the solor direction. in the opposite direction, the tail dirction, it extends for millions of kilometers, as far as we know. the earth's tail, which is not unlike a comet's, is actually made up of magnetic flux. magnetic flux is defined in perfect analogy to electric flux as the flow of the field through any surface. the electric flux though a small element of surface is equal to the area times the component of electric feild perpendicular to it. the total flux is the sum, or integral, of all the flux trough the surface. the flux through any closed surface. is equal to a constant times the change inside, sometimes written as /epsilon subzerd. this is called gauss' law. for an electric dipole, gauss' law applise bybalancing outward flux ページ８ from the positive charge... against inward flux toward the negative change. so the total flux, like the change, is zero. magnetic flux is defined in exactly the same way. in concept, flux is a mesure of the total number of lines of force passing through any surface. but sins\ce all magnets are dipoles, the total magnetic flux through any cllosed surface is always zero. the ooutward flux from the north poles is always cancelled by inward flux toward the south poles. this is gauss's law for magnetism, asfor any other magnet, then, the total magnetic flux out of the earth exactly equals the flux into the earth. but just where does the planet's flux come from? the earth's main magnetic feild we know to be caused by the flow of large-scale electric currents deep inside the earth. there are other components to the feild, but the main feild-- the dipolar component-- is associated with these currents. we don't know exactly how the magnetic feild is prpduced. we don't have an accept model in detail for the production mechanism. we know it's somehow related to the motion of these molten conducting materials in the rotation of the earth. those materials are\\especially in the deep interior-- primaly molten nickle and iron. because the earth's magnetic feild stems from the rotation of the earth ページ９ it more or less lines up with that rotation. in reality, the whole feild wobbles around once each day because it's off by about 11 1/2 degrees. that difference has been know since antiquity. explorers have always had to compensate for it... some more succesfully than others. but even for the best sailors, the earth's magnetic feild doesn't quite stay put. the earth's magnetic feild is continuously changing. as a matter of fact, the navigational charts, the things that we use to determine our direction by using mesurements with compasses, have to be changed every so often to be sufficiently accurate for navigational changing. over the feild is constantly changing. over the scales of millions off years, it changes much more dramatically than that. in fact, the polarity actually reverses, not in aperiodic way, but in an aperiodic time. from a geologic pooint of view, this flipping of the earth's magnetic poles-- north for south-- takes place fairly often, about every half million years or so. compare that cycle to how often the sun changes polarity. the feild of the sunchanges direction every 11 years. this is aremarkble process which starts with the sunsports. the magnetic polarity of sunsports is different in each successive 11-year cycle. a sunspot is adark region that we see on the surface of the sun, which, we fined, is the result of intense magnetic feilds. thesunsports always ページ１０ occur in pairs because there are no magnetic monopoles. so i always have to have a north and a south or a plus or a minus polarity together. i tend to see two big sunsports, and the magnetic lines of force will go from one sunsport into another. we see sunsports come up as though a big tube of this tube with the surface as avery strong sunsport. very strong sunsports create very strong consequences. among the most spectacular are the solor flares. there's also the solar wind, not as obvious, but equally evident when it whips out to play havdc with the earth's magnetic feild. the sun is the origin the solar wind, ahigh-speed outflow of all the material in the atmosphere of the sun. this happens in a curious way. the sun's outer atmosphere, the solar corona, is very hot. that's what we see around the sun in a total solor eclipse. it's a million degrees in temperature. when the gas gets that hot, it becomes easier for it to escape from the sun. because it's so hot, the conductivity is high. the solar wind's tempereture stays high a long way out. even at the earth, it's quite high. consequently, the gas can escape from the sun. it flows outward under control of the magnetic feild. the solor wind drags out the lines of force, ページ１１ then the lines of force control the material. the sun is responsible for the solor wind by heating the material in the corona. if i have a gas of charanged particles threaded by a magnetic feild, the charged particles can move easlity along the lines of force. but when they try moving across the lines of force, they spiral around it. that spiraling is produced by what we call the lorentz force. a magnetic feild applies no force at all to an electric charge at rest. however, if the electric charge is in motion, there's a magnetic force called the lorentz force. it's perpendicular to both the feild and to hte direction of motion of the change. the force is conveniently using the vector cross product. because the magnetic force on a moving electric charge is always perpendicular to the velocity, the force does'nt speed it up or slow it down. instead, charges then to curve around the feild in circulur or helicial paths. in nonuniform magnetic feilds, electric charges can be trapped, as they are, for example, in the van allen radiation belets. near the polar regions, charged particles sometimes get close enough to strike the atmosphere, giving off light like electrons striking a television screen. that's the origin of the spectacular aurora at the north and south poles. ページ１２ in general, however, the effect of the magnetic feild is to keep charged particles away from the earth. the earth's magnetic feild acts as a shield to us, protecting us from energetic particles from the sun and cosmic rays. we're fortunate to have the earth's magnetic feilds. it promotes life on earth as we know it. some other planets do have magnetic feild. jupiter, saturn, and uranus have magnetic feild. people used to call venus "earth's twin" because they were similar in size and almost the same distance from the sun. now we know that's not the case. venus is like an earth that's gone bad because it doesn't have a magnetic feild to protect it the way we do on earth. when william gilbet claimed that the earth behaved like a giant magnet, little did he realize that he had hit upon one of the critical ingredients that make life on earth possible. we really have learned a lot about the nature of magnetism. but we shouldn't forget that it all started with this book by william gilbert. this book is actually exetrmely rare. there are thought to be something like 60-odd first editions of william gilbert in the entire world. this particular copy is very special because an inscription on the front page is thought to have been written by gilbert himself. if it was, then this is the only know example ページ１３ of gilbert's handwriting. however... rare and valuable as this book is, it isn't the oldest thing we have written about magnetism. in fact, there's a manuscript about magnetism that came more than 300 years before gilbert's book. also know as peter peregrinus. that means pilgrim. so you can think of him as pete the pilgrim. let me tell you everything that's know about this shadowy figure. his manuscript is called an epistola, which is the latin word for letter he wrote. the title of the letter is "letter on the magnet of peter peregrinus of maricourt to sigirus of foucauoourt soldier" period end of title. the letter describes the behavior of magnet, various things about magnetism, and tells nothing at all about peter himself. at the end of the letter, the conclusion says, "completed in camp "at the siege of lucera, on the year of out lord, 1269, 8th day of august." and that is everything that is known about peter peregrinus. based on that information, historians have told us that he was an engineer in the army of charles of anjou, king of sicily, that he was of noble birth, that he came from maharicourt in picardy, that he was atheologian and even a franciscan. how do historians know all that? ページ１４ 　 i thik it's called the scholarly imaginations. scholars aren't the only ones with imaginations. pierre himmself, peter peregrinus, had an imagination. he was the first person to invent a macnetic perpetual motion machine. it was the first of many. when it didn't work, he was the first one to blame it on the guy who made it for him. but his shrewd observations about the behavior of magnets made his manuscuript exetremely valuable. there are 31 known copies of that manuscript. and by the standards of the middle ages before the invention of the printing press, that means that his manuscript was asmash best seller. at least one of those copies fell into the hands of william gilbert because gilbert dren on and cited heavily this work, this letter, that came to us from the mists of time. speaking of time, we're out of it. i'll see you next time.