A/V Geeks 16mm Lunch 5-5-2026

Añadido: 2026-05-12

[00:00:07]Hello everybody. This is Skip Alzheimer.

[00:00:09]Welcome to the AV Geeks lunchtime streaming show. This is a safety show uh in case I don't make it back in time to do the show live um as I am getting will be getting an MRI. Supposedly it's going to be short, but who knows, man.

[00:00:29]Who knows? So, anyways, here's films about magnetism and magnets.

[00:00:36]Enjoy.

[00:00:44]During the summer of 1973, three astronauts, Alan Bean, Jack Lousma, and Dr. Owen Garriot were launched into space where they spent almost two months in the Skyab spacecraft, the world's first laboratory in space.

[00:01:13]The crew performed a variety of scientific experiments and demonstrations.

[00:01:18]The onboard scientist, officially designated a scientist pilot, was Dr. Owen Garriat, a former professor of electrical engineering and an astronaut since 1965.

[00:01:32]During the mission, Dr. Garri conducted a number of demonstrations specifically for the use of science students.

[00:01:39]Back on Earth, Dr. Garri discusses some of his Skyab experiences with high school students.

[00:01:46]Skyab is perhaps the most unusual laboratory that you can imagine. Living in weightlessness meant that you didn't walk from point A to point B. Instead, you floated. And to do that, you simply pushed yourself off and drifted over to the spot that you were headed for.

[00:02:03]Now, here you see some of our weightless girrations that we went through during a little time we took away from our work schedule.

[00:02:13]Now this weightlessness also made it possible for us to conduct some science demonstrations that we simply couldn't do here on earth because of the strong influence of gravity. Now one of our demonstrations involved the earth's magnetic field and this completely surrounds the earth making in effect uh the earth itself a giant magnet. One of the many things that this field does is to protect us from dangerous solar particle radiation coming not only from the sun but also from the outer reaches of our galaxy.

[00:02:43]Skyab moves through the steady magnetic field of the Earth making almost 16 revolutions every 24 hours. As you see, Skyab's orientation with respect to the nearby magnetic field is constantly changing.

[00:02:57]The uniqueness of the Earth's magnetic field has led some to the conclusion that this field may have been a vital factor even in the development of life here on Earth. We also know that even minute changes in the Earth's magnetic field, both its strength and in its direction can seriously interfere with some aircraft instruments and also ship's compasses. And this can lead to endangering lives when their navigation is performed by reference to the strength and direction of the Earth's magnetic field.

[00:03:26]This field also is basically related to the appearance of aurora or northern and southern lights as we call them and interacts with the solar wind also which is a new discovery of our space age.

[00:03:39]Let's take a look at some of our TV footage and see a skyab demonstration in which some of the effects of the earth's magnetic field are demonstrated.

[00:03:49]>> I'm sure most all of you at least at one time or another in your past have had the occasion to use a compass. It's of course made from just a little magnet and the north end always points toward the north pole and you can use it to determine for example your way out of the woods or your way on a hike and that sort of thing. Well, perhaps many of you haven't ever thought about whether or not the Earth's field extends far out into space. For example, here we are up in Skylab some 270 mi above the surface of the Earth and indeed the Earth's magnetic field does extend out this far and in fact a good deal further. But we can demonstrate that as well. And perhaps you would uh enjoy seeing such a demonstration. Uh here's a group of little magnets that I have. Uh these are about 2 in long and about 2/10 of an inch in diameter. And when we put these uh just floating out in midair like I'm doing here, you can see that they take on a very definite orientation. For example, there is one. I've released it right out in the center now. And you can see it is settling down to a specific orientation.

[00:04:47]That's the direction in which the Earth's magnetic field is running here, right along parallel to this little bar magnet. And we can of course use this to determine the direction of the Earth's field right up here at the location Sky Skylab is is at. Now, as we travel around the Earth, if we could watch this for a longer period, we would see this little magnet make several rotations as we circle the Earth because the direction of the Earth's magnetic field is changing. Now, here's another thing that we can do. You see, we must not get too close to it with another magnet because when we do uh the two of them interact and they very much perturb uh and influence the other one. Now, uh let me just make a couple other little uh demonstrations here for you. First of all, we found you see that one magnet align very nicely with the Earth's field. And if you watch very carefully, you can see this dipole oscillate back and forth. And I'm removing all the other magnets so that they do not influence it. Now perhaps you'd find interesting in one of your science classes to try to compute the period of that uh uh little uh compass. Or another way to do it, you can measure the period from this oscillation and compute the strength of the Earth's magnetic field right up here at the Skyab orbit. Uh to do it, you'll need to know a little bit about this magnet. uh but you can uh estimate that approximately from the fact that it's a 2-in long 210 inch diameter magnet and make a calculation to determine what the period of that oscillation should be. Now, here's another little question that uh might uh be appropriate for some of the younger uh members of our audience. Suppose I take two of these magnets and I put them together like this. Now, if I put these out here and let them float, you see they have almost no tendency at all to line up with the Earth's field. There's just a little bit remaining but almost no tendency at all until perhaps you can explain that or if you can't you can ask your science teacher and uh he or she can tell you why it is with two magnets like this uh we seem to lose that tendency to line up with the earth's field.

[00:06:50]Well, we saw the period of one oscillation a minute ago. Let me uh try it uh with uh two magnets end to end like this. Now you see they also oscillate. It's not like the two side by side. Two end to end will still oscillate and maybe you can see that and uh you can measure this period and from that period uh you could again calculate the earth's field or you can determine something about the inertia of these uh combination of two magnets and I'll leave that for another calculation for your science class.

[00:07:30]Now we'll give you a close-up look at the oscillation of one magnet.

[00:07:36]Put it back in the center.

[00:07:43]And now we'll use two to oscillate.

[00:07:50]And you see the period of this oscillation is a good deal more slow when we have two of them together.

[00:07:59]And we can extend that even to three three rides if we like.

[00:08:12]And there the period is very clear and can be measured rather accurately. Now, if I put uh three side by side again, see, they'll have a different period.

[00:08:26]Now, they moved a little too fast there.

[00:08:29]There we go.

[00:08:33]Before resuming our Skyab demonstrations, I'd like to give you an opportunity to measure the period of the oscillations of the different combinations of magnets that we've just been looking at. Now, after you've measured these periods and the film has ended, your instructor will supply you with the other data that are needed to calculate either the moment of inertia or the strength of the Earth's magnetic field from our Skyab location up in space. Now, here's how I would suggest that you make the measurement of the period of these oscillations. This little pin that I'm going to show you here will represent the dipole that you've just seen. And in my other hand, I've got a stopwatch. And so the way I would measure that period is to wait until a dipole has tipped to one extreme of its motion and more or less stopped, ready to turn around. And at that point, start your stopwatch and then measure at least one or if there is time perhaps two or even three oscillations of the dipole and then stop the stopwatch. Now, I've just simulated there, for example, measuring three of the oscillations, and my stopwatch reads 10 1/2 seconds. And so, the period dividing 10 1/2 by 3 gives us 3 1/2 seconds per oscillation of the dipole. And then you'll want to go back, of course, and measure that period for each of the various combinations of magnets that we've just been looking at. First, let's make a dry run. Start your stopwatch precisely when you see the dot and hear the tone.

[00:10:09]Count the oscillations.

[00:10:11]One, two, and then stop the watch when you see the dot again and hear the tone.

[00:10:20]Okay.

[00:10:23]Now, we're going to repeat the footage for the single magnet so you can make that measurement.

[00:10:32]When you're through, write down the measurement on your paper.

[00:10:35]And then we can measure the period of the oscillations for two magnets.

[00:10:49]Now let's measure the period of three magnets side by side and write that number down.

[00:11:08]Then finally, we'll measure the period of the three magnets when they're placed end to end in a long single dipole.

[00:11:28]When you've completed that, we'll have four measurements recorded on our paper, and we'll use these measurements in our calculations when the film is ended.

[00:11:37]Now, in another demonstration, we will observe the effect of a magnet attached to a nut similar to this one that we caused to spin like a top in space.

[00:11:48]Now, we're going to tape a little magnet to the nut just like this. Use a little piece of gray tape just the way we did on board the spacecraft. And we're going to observe the effect of this magnet on the spinning of the nut. Note that the polarity of this flat magnet is not along its long axis as it is with the little bar magnets. Instead, the north and south poles are located on the flat sides.

[00:12:13]What we'll see is first of all when we launch it in an orientation parallel to the earth's magnetic field that the spin is fairly stable. You see, once again, it continues to spin pretty nearly stably about the direction in which it was launched.

[00:12:29]>> But what happens if we allow the nut and the magnet to float freely in space?

[00:12:34]Let's take a look at the case in which the attached magnet has an orientation or a polarity in which its dipole is not in the same direction as that of the Earth's magnetic field. But the next thing I want to show is uh when I tip these in a direction perpendicular to that, if it's not spinning, watch what it does. You see it tips right over and turns parallel to the Earth's field so that it spin axis or its axis through the disc is oriented with the Earth's field just the way it was before.

[00:13:04]That's its normal tendency is to tip right over and align its own dipole with the direction of the Earth's field.

[00:13:12]Now I'm going to spin it about this direction and we'll see what its reaction is.

[00:13:25]All right. Now it's more or less spin axis perpendicular to the Earth's field at this point.

[00:13:32]We may have to watch it for just a minute here. But the thing that I want you to observe is the fact that its spin axis is tipping over in another direction. You see, now it's now tipped over to the point where it's almost facing the camera. And now it has it's tipped about 90°, but not toward the Earth's field, which is off to my right.

[00:13:51]Instead, it is tipped over towards the camera. Now, this is called precession.

[00:13:56]And we'll go back and check the Earth's field once more. We'll see whether or not it's changed.

[00:14:03]Take another of our little magnets.

[00:14:09]It may have changed a little bit, but not a great deal in the time that we've been uh talking.

[00:14:15]But these magnets are already influencing each other as you can see and they'll align themselves just like that in the direction of the Earth's field.

[00:14:31]>> Now, this concludes our demonstrations of magnetic effects in space. We've observed first of all the oscillations of various combinations of little bar magnets. And then next we've seen the precession of a spinning object. In this case, a small gyroscope represented by this spinning nut with a magnet attached. After you've completed the suggested calculations, you will know that the Earth's magnetic field extends far out into space above the Earth, and you will have calculated its magnitude and observed its changing direction.

[00:15:03]We've also seen the very first demonstration of precession ever observed in this marvelous new laboratory that we call space. And in this laboratory, of course, gravitational forces no longer perturb our experiments. Perhaps your own curiosity and imagination will suggest even more sophisticated experiments to you which can be demonstrated on later flights.

[00:17:08]Heat. Heat.

[00:17:34]America's space station Skyab afforded scientists the world over their first large-scale opportunity to conduct experiments in the weightless vacuum of space.

[00:17:54]One series of demonstrations explored the sometimes mysterious, often useful, but always fascinating phenomenon of magnetism.

[00:18:23]I'm Owen Garriot, the scientist pilot for the second man's mission to Skylab.

[00:18:28]While living aboard the space station, we had an opportunity to perform several interesting demonstrations of magnetic effects. But before we see these, let's look at some more familiar applications.

[00:18:43]Magnetism works in our homes for holding and closing things, in the doorbell, and in every electric motor and generator.

[00:18:58]Automobiles are full of magnets.

[00:19:03]Computers use magnetic cores in their memory banks. In fact, magnetic effects are woven into the very fabric of our technological society.

[00:19:27]Sometimes large electromagnets are used to move tons of metal and much smaller ones are even used to record my voice which you hear now.

[00:19:39]Today the compass is perhaps the most familiar of all applications of magnetism.

[00:19:45]But a thousand years ago, it was the only application and magnetism was highly mysterious.

[00:20:01]By the early part of this century, the role of magnetic forces in nature had been understood both experimentally and in precise mathematical terms.

[00:20:12]All magnetism depends on the flow of electric currents.

[00:20:16]Whether on atomic dimensions or as large as the coils of a huge fusion test device in which plasma is contained magnetically and heated to millions of degrees.

[00:20:31]In today's avalanche of scientific and technical wonders, we may have forgotten a childhood fascination with magnets.

[00:20:40]My own memory was refreshed by some demonstrations with magnets in weightless conditions in the zeroravity environment of the orbiting Skyab space station.

[00:20:52]I'd like to show you just how magnets behave in the freedom of zerog and why their motion can resemble that of a simple pendulum.

[00:21:04]First, a little review.

[00:21:07]Permanent magnets like this compass needle behave as if they had two strong poles called north and south.

[00:21:15]Opposite poles attract and like ones repel each other as we can see in this demonstration.

[00:21:24]Now an electromagnet affects the compass in the same way but its magnetic field is produced by an electric current flowing in a wire coil. As we can see when I close this electrical circuit, the field of a permanent magnet like this one is also caused by electric currents. In this case, due to the spin and orbital motion of electrons in materials like iron, these atomic currents can be lined up by an external magnetic field and then it will remain aligned for a long time after the external field is removed.

[00:22:01]The Earth is also a magnet with north and south magnetic poles. The explanation of terrestrial magnetism seems to require a dynamo effect driven by the rotation of the Earth. The experts still don't understand exactly how electric currents in the Earth's core generate its magnetic field. But the measured pattern resembles that of a giant bar magnet located at the center of the Earth.

[00:22:27]But the magnetic field strength at the Earth's surface is thousands of times weaker than that near strong magnets.

[00:22:35]Nevertheless, the Earth's magnetism can easily influence a delicately suspended compass or these magnets that we see floating on the water. When I turn a compass away from the north south direction, the Earth's field twists it back. In other words, the Earth's field exerts a torque on a magnet to align it.

[00:22:56]But since the compass needle swings freely, it first overshoots and then oscillates until friction damps out the motion.

[00:23:06]This bar magnet hanging by a thread can act the same way just like a magnetic pendulum.

[00:23:16]And now you may begin to understand the behavior of magnets in Skyab.

[00:23:21]Here we have an ordinary pendulum.

[00:23:24]No magnet, just gravity tugging on it, producing a torque which tends to restore the pendulum to its equilibrium or rest position aligned with the Earth's gravitational field.

[00:23:38]Magnetic torque performs the same function for the magnetic pendulum. You may know that a longer pendulum swings more slowly. To be precise, the swing period varies as the square root of the length as shown by this equation.

[00:23:56]The magnetic pendulum has an equation with the same form. It will swing more slowly if it has a larger moment of inertia. That is if the magnet is longer or heavier.

[00:24:09]On the other hand, the more strongly it is magnetized, the faster it swings because the Earth's field B will exert a stronger torque on it.

[00:24:23]Suppose we conduct some of these demonstrations in the weightless conditions of space.

[00:24:30]We can't show a simple pendulum because the effects of gravity cannot be experienced when the spacecraft is in a freely falling condition.

[00:24:40]Magnetic effects were apparent because the Earth's field extends far out into space, well beyond the 270 mi altitude of Skyab.

[00:24:51]But as Skyab moves in its orbit around the Earth, its orientation to the magnetic field slowly changes.

[00:25:00]>> Let's watch the TV transmission from Skyab for a few minutes.

[00:25:04]>> Demonstration. Uh here's a group of little magnets that I have. Uh these are about 2 in long and about 210 of an inch in diameter. And when we put these just floating out in midair like I'm doing here, you can see that they take on a very definite orientation. For example, there is one. I've released it right out in the center now. And you can see it is settling down to a specific orientation.

[00:25:27]That's the direction in which the Earth's magnetic field is running here, right along parallel to this little bar magnet. And we can, of course, use this to determine the direction of the Earth's field. Right up here at the location Skyab is is at. Now, as we travel around the Earth, if we could watch this for a longer period, we would see this little magnet make several rotations as we circle the Earth because the direction of the Earth's magnetic field is changing. Now, here's another thing that we can do. You see, we must not get too close to it with another magnet because when we do, uh, the two of them interact and they very much perturb, uh, and influence the other one. Now, uh, let me just make a couple other little, uh, demonstrations here for you. First of all, we found, you see that one magnet aligns very nicely with the Earth's field. And if you watch very carefully, you can see this dipole oscillate back and forth. And I'm removing all the other magnets so that they do not influence it. Now, perhaps you'd find interesting in one of your science classes to try to compute the period of that uh uh little uh compass.

[00:26:32]Or another way to do it, you can measure the period from this oscillation and compute the strength of the Earth's magnetic field right up here at the Skyab orbit. Uh to do it, you'll need to know a little bit about this magnet. Uh but you can uh estimate that approximately from the fact that it's a 2-in long 210 in diameter magnet and make a calculation to determine what the period of that oscillation should be.

[00:26:54]Now, here's another little question that uh might uh be appropriate for some of the younger uh members of our audience.

[00:27:01]Suppose I take two of these magnets and I put them together like this. Now, if I put these out here and let them float, you see they have almost no tendency at all to line up with the Earth's field.

[00:27:12]There's just a little bit remaining, but almost no tendency at all. And so perhaps you can explain that or if you can't, you can ask your science teacher and uh he or she can tell you why it is with two magnets like this uh we seem to lose that tendency to line up with the Earth's field.

[00:27:30]Before we see our next demonstration performed on Skyab, I'd like to show you how to measure the period of an oscillation of either our magnet or of the pendulum. To start with, I'll give our magnet a small oscillation. And to time it, I'll wait until the motion is at one extreme and then start my stopwatch. We'll time one or even two or three periods.

[00:27:54]And when it reaches the other extreme again, I stop the watch. And that was just exactly 8.1 seconds. And when I divide three into this number, it gives 2.7 seconds for the period of each oscillation.

[00:28:09]You can use this technique to measure the period of any oscillating object, including our magnets in space. To make it a little easier, we'll mark the extremes of motion with a red flash in the next scenes.

[00:28:23]>> Well, we saw the period of one oscillation a minute ago. Let me uh try it uh with uh two magnets end to end like this. Now you see they'll also oscillate. It's not like the two side by side. two end to end will still oscillate and maybe you can see that and uh you can measure this period and from that period uh you could again calculate the earth's field or you can determine something about the inertas of these uh combination of two magnets and I'll leave that for another calculation for your science class.

[00:29:02]Now, we'll give you a close-up look at the oscillation of one magnet.

[00:29:09]Put it back in the center.

[00:29:16]And now we'll use two to oscillate.

[00:29:23]And you see the period of this oscillation is a good deal more slow when we have two of them together.

[00:29:32]Now if I put uh three side by side again see they all have a different period.

[00:29:42]Yeah they moved a little too fast there.

[00:29:45]There we go.

[00:29:47]Now in another demonstration we will observe the effect of a magnet attached to a nut similar to this one which we cause to spin like a top in space. Now I'm going to tape this magnet to the top of the nut with a piece of gray tape just like we used in Skyab. And we'll observe the effect of this magnet on the spinning of the nut. We'll want to keep in mind that in this case the polarity of this magnet is such that one face is a north pole and the opposite face of the magnet is a south pole.

[00:30:23]But the next thing I want to show is uh when I tip these in a direction perpendicular to that, if it's not spinning, watch what it does. You see it spins right over and turns parallel to the Earth's field so that it spin axis or its axis through the disc is oriented with the Earth's field just the way it was before.

[00:30:43]That's its normal tendency is to tip right over and align its own dipole with the direction of the Earth's field.

[00:30:51]Now I'm going to spin it about this direction and we'll see what it's reaction is.

[00:31:03]All right. Now it's more or less spin axis perpendicular to the Earth's field at this point.

[00:31:11]We may have to watch it for just a minute here. But the thing that I want you to observe is the fact that its spin axis is tipping over in another direction. You see now it's now tipped over to the point where it's almost facing the camera. And now it has. It's tipped about 90° but not toward the Earth's field, which is off to my right.

[00:31:30]Instead, it is tipped over towards the camera. Now, this is called precession.

[00:31:36]Now, I'd like to show you a few of the more unusual effects of the Earth's magnetic field.

[00:31:43]Giant eruptions on the Sun throw out clouds of electrically charged gas particles.

[00:31:50]They traversed the solar corona in a few hours and crossed the 93 million miles to Earth in several days.

[00:32:01]Near the Earth, some of them are accelerated and then guided by the Earth's magnetic field into our polar regions.

[00:32:10]The particles funnel down into the upper atmosphere where they excite neutral atoms and molecules producing the visible lights known as aurora.

[00:32:20]A number of fine auroral displays were photographed from Skylab starting about 2 and 1/2 days after a large flare on the sun.

[00:32:31]Higher energy charged particles also reach the earth's upper atmosphere. Here they collide with air molecules and their energy is transferred to new kinds of particles. They come not only from the sun but from across the gulf of interstellar space. These particles we call cosmic rays. Here we see their tracks in a cloud chamber on the earth's surface.

[00:32:57]Trapped within our Milky Way galaxy by chaotic magnetic fields. These subatomic messengers are the only sample of matter we have from beyond the solar system.

[00:33:10]Magnets are essential for sorting out cosmic rays according to their mass, charge, and energy while they are still above the Earth's atmosphere and have not yet been transformed by collision.

[00:33:28]With instruments such as this, we hope to discover the origin of cosmic rays and learn something about the birth of our universe.

[00:33:40]When the magnet is cooled to a mere 4° above absolute zero, it can operate indefinitely without external power.

[00:33:49]Low power consumption is a necessary refinement for space flight or even balloon flight.

[00:33:56]Such powerful magnets and their electronic particle detectors have not yet been flown in space. But numerous balloon flights have proven the feasibility of doing so.

[00:34:10]One peculiar problem which must be solved first is the alignment of the magnet with the Earth's own field. In balloon flight, this is an advantage.

[00:34:21]The magnet stabilizes the balloon gondola until it is turned off by radio command at the end of the flight.

[00:34:28]Finally, the payload is separated from the balloon and returned to Earth by parachute for further use.

[00:35:04]But in the space shuttle flights of the 1980s, a strong magnet trying to align the whole spacecraft with the Earth's field could be quite a nuisance.

[00:35:17]What can be done to keep the magnet from controlling the spacecraft attitude like the proverbial tail wagging a dog?

[00:35:27]The solution adopted by the designers can be guessed from this shot which you saw earlier in Skyab.

[00:35:37]The answer, two magnets, a pair of opposing coils with their current flowing in opposite directions. This nearly cancels the magnetic field far away from the coils and minimizes the alignment problem.

[00:35:54]Space shuttle will allow countless new experiments in basic and applied science. Many of these will involve magnetic effects since electromagnetism is one of the fundamental forces of nature. And I expect that some of you will have the opportunity to work on experiments in this new weightless laboratory in orbit above our Earth.

[00:37:44]In the filmon transfer of kinetic energy, professor Freriedman predicted and measured the heating of a copper target when bombarded by electrons.

[00:37:54]In this, he used the same arrangement as in the melican experiment, two parallel plates 3.1 mm apart and a hot filament to supply the electrons.

[00:38:06]Here you see the thermouple being inserted into the copper target to read its temperature.

[00:38:15]The thermouple is connected to this meter and it reads 28 divisions before the current is turned on. Here's the experimental setup that was designed and built by Dr. Madson who collaborated with Professor Freriedman. There's a current of 2 milliampers.

[00:38:32]The temperature of the target starts to rise and the run lasts for about 20 seconds.

[00:38:40]The total meter deflection is 24 divisions and this is a measure of the total energy dissipated during the run.

[00:38:51]Today we're going to continue this story and in particular we're going to ask what is the source of energy that ultimately appears as thermal energy of the target and secondly how does one describe and measure the energy output of such sources. Now professor Freriedman calculated the total kinetic energy of the electrons using this formula. The force per elementary charge was obtained from data of the Milican experiment using three batteries. The plate separation D was 3.1 mm. He measured the number of electric elementary charges per second with an ammeter and the time in seconds.

[00:39:37]He also used twice the number of batteries that is six.

[00:39:43]This doubled the force per elementary charge. He then h haveved the current, the number of elementary charges per second.

[00:39:54]At the start of this run, the meter was set at 20 divisions and he measured the identical heating of the target in 20 seconds.

[00:40:07]The thermalouple meter again showed a deflection of 24 divisions.

[00:40:14]Furthermore, it was indicated that one could do other experiments varying the time, the current, the force per elementary charge, but with his apparatus, the one thing he couldn't change was the plate separation D.

[00:40:29]Today, we're going to start by repeating his experiment with a tube that has a different plate separation. Here's the tube.

[00:40:39]And if I hold a centimeter scale up, you see that the separation of the plates is about a centimeter.

[00:40:47]And that's a good three times the separation of the plates in the Milican apparatus.

[00:40:53]Now, let's do the experiment.

[00:40:56]Here's the thermalouple meter measuring the temperature of the target. And you see it reads 20 divisions. To start, we throw the switch 2 milliamp and we're going to let it run for 20 seconds there. mm coming up to 44 divisions.

[00:41:33]That means a deflection of 24 divisions.

[00:41:36]And this is just the deflection that Professor Freriedman and Dr. Madson found in their experiment. So we found the same energy dissipation for two different values of D, the separation of the plates. If you look at our formula for kinetic energy, D has changed. And certainly something else must have changed to keep things equal. I'll let you figure out what that is. Now, the equality of heat dissipation for two different separations of the power plates suggests that maybe this dissipation doesn't depend at all on plate separation. And to make sure of this or in fact to find out if this is really true, we must do other experiments. Now let me show you a tube which allows us to do this experiment at different separations.

[00:42:30]You see that the separation of the plates is small actually 3.1 mm the same as Professor Freriedman used. I can release one of these plates.

[00:42:41]Slide it along.

[00:42:46]Lock it into position. And there it is at 1 cm. The separation we've just used.

[00:42:54]I release it again.

[00:42:56]Slide it still farther.

[00:42:59]Lock it. Now the separation of the two plates is 1.5 cm.

[00:43:05]And if I release this electrode and move it once again, I get a 2 cm separation.

[00:43:12]Now, we've done the experiment with this tube at all four separations. And each time you get the same heating. So that we can conclude with reasonable certainty that the energy dissipation at this copper target does not depend on the separation between parallel electrodes.

[00:43:31]Maybe the energy dissipated at the copper target doesn't depend on the use of parallel plates. To find out if this is so, we'll do an experiment with a different type of vacuum tube. Here's one with a bare filament. Electrons can come out in all directions.

[00:43:50]This copper target is identical with those that we've used before. You'll also notice these fine wires running parallel to each other and surrounding completely this filament target structure. These wires are connected to the filament and become negatively charged. The negative charge on these wires then repels any electrons from the filament which otherwise might hit the glass. After all, we want to make sure that those electrons which we measure with this meter have come directly from the filament to the target without a chance of losing any of their energy.

[00:44:34]All right, now we're prepared to do our experiment. will keep all the conditions exactly as they were before.

[00:44:45]Our thermalouple meter reads 20.

[00:44:50]Here we go.

[00:44:58]2 milliamp is again. We'll let this run for 20 seconds.

[00:45:10]I'll need a stove climbing.

[00:45:13]There it is. About 44. Once again, the heating of the copper plate is just about the same as it was in all those experiments we did with parallel plate electrodes.

[00:45:25]This means that the energy dissipated at the target does not depend on the particular geometry of the tube I use.

[00:45:32]What then does it depend on? Let's do an experiment now in which we vary the number of batteries. We're going to use three times the original set of three batteries. And here on the table, you see them set up for me using the same tube as I just used. We'll now run at 1/3 the current, that is 2/3 of a milliamp here, and see what sort of heating we get.

[00:45:58]Our thermalouple meter reads 20 again.

[00:46:02]And here we go.

[00:46:06]There it is climbing.

[00:46:24]That's about it. A deflection of about 23 to 24 divisions. So once again we find the same heating using three times the number of batteries and only one/ird the current. Now it's reasonably clear that this total heating is proportional to the number of electrons which bang into the target. If we double that number, we'll double the heating. As a consequence, the energy per elementary charge which is transformed into thermal energy at the target depends only on the number of batteries and in fact is proportional to that number.

[00:47:01]This energy per elementary charge doesn't depend on how big the force per elementary charge is in the tube. It doesn't depend on the direction in which this force points. Nor does it depend on the particular path that any electron follows going from the filament to the target. Maybe it doesn't depend on the fact that we've used a vacuum tube in each of our experiments. Let's find out by doing a different experiment without a vacuum tube. Now, here is a heating element.

[00:47:35]This consists of a lot of little resistors which are connected in series and mounted between these two very thin metal plates. The plates are cemented together around the edge to form this button. When an electric current flows through it, it heats up and we have the problem of measuring the thermal energy dissipated in this button.

[00:48:00]To do this, we'll use the following apparatus.

[00:48:10]This is the equipment Professor Freriedman used to determine just how many jewels were necessary to produce the now familiar 24 division deflection of the thermouple meter.

[00:48:24]This is the way it was done. The loss of potential energy of a falling body was transformed by friction into thermal energy of these copper cylinders.

[00:48:39]The meter deflection of 24 divisions corresponded to an energy dissipation of about 11 jewels for each cylinder.

[00:48:50]Now we're ready to connect up our heating element.

[00:49:06]by connecting this lead to this switch.

[00:49:11]I'm using these three batteries. And when I throw the switch, you'll notice that 4 milliampers flow through the heating element. That's just twice the value of two that we used in our vacuum tube experiments. Finally, we connect the heating element to this apparatus by inserting it between the two copper cylinders.

[00:49:38]Let's get it lined up and holding it tightly pressed with the help of this spring.

[00:49:47]Now, the thermouple meter must be connected to the thermouple in that copper cylinder. And we shall do so disconnecting the meter from the vacuum tube and connecting it to the apparatus we're using now.

[00:50:05]Now we're just about ready to try our experiment. But before I throw this switch, let's take a look at the construction of the heating element.

[00:50:17]Here you see the resistors as they are actually mounted.

[00:50:21]This is the inside of a button similar to the one that we're going to use. Now, the average motion of the electrons in a resistor such as this one is directed along its length. And this is also the direction of the force per elementary charge on each electron in that resistor.

[00:50:42]So if we start here, the force per elementary charge inside the resistors runs up let's say through this one then down through this then back up through this and so on.

[00:50:56]This field pattern is much more complicated than anything in the vacuum tube experiments.

[00:51:03]And furthermore, the detailed processes by which energy is dissipated in such a resistor are quite different than in the vacuum tube. Now let's see what happens when we perform this experiment.

[00:51:19]First check our thermalouple meter. It's on 20 as before. Here we go. 4 milliamp.

[00:51:27]There goes the meter. Now because the dissipated energy is divided equally between these two identical copper cylinders, we use twice the current of 2 milliampers which we used in the vacuum tube with a single copper target.

[00:51:48]There it is settling down a deflection of just about 24 divisions again.

[00:51:55]And this is just what we found in all our other experiments.

[00:51:58]Now let's see what all this means. In the first place, the thermal energy of these copper cylinders that we've been measuring is supplied from the stored chemical energy of the batteries we use.

[00:52:13]But ever so much more important is the fact that the amount of energy that each battery delivers per elementary charge that flows through it doesn't depend on anything but the chemical nature of the battery.

[00:52:28]This energy per elementary charge is called the electromotive force or emf of a battery.

[00:52:36]Professor Freriedman showed that the energy delivered by one set of batteries and a definite number of elementary charges was equal to that delivered by two sets of batteries and half this number of elementary charges. Also we saw that using three sets of batteries and one-third this number of elementary charges we got the same energy.

[00:53:00]Therefore two sets of batteries deliver twice the energy for each elementary charge as does one and three sets of batteries three times this energy. In other words, the emf of two batteries connected in series is twice the emf of one. And the emf of three batteries in series, as you see here, is three times the emf of one.

[00:53:27]Now, don't get the idea that emf is something that's stored somehow or other in these batteries. It isn't. What's stored is chemical energy. And emf measures the amount of this chemical energy which is transferred per elementary charge to the circuit to which the batteries are connected. EMF is really a work per elementary charge.

[00:53:51]Now we could have done any of these experiments with devices other than batteries such as electric generators.

[00:53:58]Any such device which supplies energy with the help of electric charges is called a seat of emf. And this is a dangerous use of words, but remember what I said about EMF not being stored anywhere.

[00:54:15]If we use an electric generator, then energy isn't stored at all.

[00:54:24]Here you see Mr. Henry replacing one set of three batteries by a generator.

[00:54:30]This circuit is the one which draws 2 milliampers from this set of three batteries.

[00:54:40]Now there's no energy stored in this generator.

[00:54:43]You've got to drive it to get electrical energy.

[00:54:55]Notice how the current gradually increases to a value of 2 milliamp.

[00:55:01]When we get 2 milliamp from this generator, its emf is just equal to that of the three batteries.

[00:55:10]Now, in the experiments we've done, we've drawn different currents from our batteries and we found that the emf of the battery was unchanged. This constancy of emf with current holds over a very wide range for batteries. In fact, until the chemical constitution of the battery is appreciably changed, it is true. And when it is changed, we say the batteries run down for other devices such as a generator. This wide range of constancy of emf with current is no longer true. Not in general. Generators can be made to deliver a wide range of EMFs. For example, when Mr. Henry speeded up his generator, its EMF started at zero and build up until it was equal to that of three batteries, at which point 2 milliampers flowed through our circuit.

[00:56:04]But in any case, whether we use batteries or generators or any other equivalent devices, the concept of EMF, the energy delivered per elementary charge is exceedingly important. We'll find that out when we face up to the problem of accounting for and measuring the various energy conversions that happen in electrical circuits.

[00:57:42]It all started one morning when Michael was eating breakfast. Suddenly, he saw his mother do something that looked like magic.

[00:57:50]>> Say, "Mom, how'd you do that?"

[00:57:53]>> This It's my new bulletin board. Dad brought it to me last night.

[00:57:59]>> Yeah, but how does it work?

[00:58:00]>> With these little magnets. See?

[00:58:03]>> Can I try it, Mom? Can I play with it?

[00:58:05]>> Of course you can after school. But hurry now and finish your breakfast.

[00:58:09]You'll be late.

[00:58:12]>> That was Michael's introduction to the magnet.

[00:58:16]>> Right after school, he made a beline for that bulletin board.

[00:58:21]Strange. It doesn't stick to the wall.

[00:58:24]Although it sticks to the bulletin board.

[00:58:28]It sticks to the kitchen cabinets, but not to the milk bottle. Wait a minute.

[00:58:34]The cabinets are metal and the bottle isn't. Maybe that's the answer.

[00:58:42]Yep. The bulletin board seems to be made of metal, too. Maybe that's it.

[00:58:48]But no, it doesn't stick to this metal.

[00:58:52]What could it be?

[00:58:55]Michael set out to keep track of the things the magnet sticks to and the things it doesn't.

[00:59:00]By evening, he had two pretty large piles.

[00:59:04]>> Dad.

[00:59:05]>> Yes, Mike.

[00:59:06]>> Why is it the needles are attracted by the magnet and not the pins?

[00:59:12]>> Take a look at the box that pins came out of, son.

[00:59:16]>> What does the box have to do with it?

[00:59:19]Oh, is it because they're made of brass?

[00:59:21]>> Right.

[00:59:22]>> And the needles, they're made of steel.

[00:59:25]>> Right. Again, >> everything in this pile is either iron or steel, >> right?

[00:59:34]>> Nothing in this pile is iron or steel.

[00:59:39]>> Yes, Michael has found the answer.

[00:59:41]Magnets pull or attract only things made of iron and steel. They don't attract things made of anything else in the house.

[00:59:50]Look, Mike, I hate to interfere with your experiments, but I have to have that magnet for my bulletin board.

[00:59:56]>> A just when I was getting started.

[00:59:58]>> I'm sorry.

[01:00:00]>> Tell you what, son. Tomorrow, if I can remember, I'll bring you home some magnets of your own to experiment with.

[01:00:06]>> Oh, Dad, you will. You won't forget.

[01:00:08]>> I'll try not to, son.

[01:00:10]>> Oh, boy.

[01:00:19]Michael's father didn't forget them. In fact, he brought home four magnets.

[01:00:24]There were two U-shaped magnets called horseshoe magnets and two straight magnets called bar magnets.

[01:00:34]And for the next several days, Michael tried all kinds of experiments. He found out everything he could about magnets.

[01:00:41]To help him understand, he asked questions and he read up on magnets, too.

[01:00:47]He had so much fun, he wanted to share it with some of his friends. He decided to put on a magic magnet show.

[01:00:56]>> Ladies and gentlemen, watch carefully, please. I will now make this sailboat move in any direction at my command. Right.

[01:01:08]Left.

[01:01:10]There's a storm at sea.

[01:01:14]G.

[01:01:15]>> I will now show how the force of this wonderful magnet will go through other material.

[01:01:21]>> It will go through cardboard to pick up this key.

[01:01:24]>> Now watch it.

[01:01:28]>> See, it will go through glass to move things on top.

[01:01:37]It will go through water to pick up nails.

[01:01:47]>> This magnet is a horseshoe magnet. Now, let me show you another magnet. This magnet is a bar magnet. Where's the magnet the strongest?

[01:02:01]>> Let's do an experiment to see where the magnet is is the strongest. You see these tags?

[01:02:07]>> Yes.

[01:02:08]>> I spread the tags on the cardboard.

[01:02:17]>> Now I place the magnet on the T.

[01:02:21]See where they stick?

[01:02:24]>> That's right. The ends are called the poles. That's where the force of the magnet is the strongest. This force is invisible, but it's very real. I can make the force visible. Do you want me to?

[01:02:37]>> Yeah, sure.

[01:02:40]>> All right. I put the magnet down and cover it with a piece of cardboard.

[01:02:46]In this salt shaker are iron filings I collected. I sprinkle the iron filings over the cardboard and the invisible lines appear.

[01:02:57]>> Yes, they are appearing. No.

[01:03:01]>> Yes. These iron filings show the path of the magnetic lines of force. See how they are heaviest around the poles of the magnet.

[01:03:10]>> Now, who would like to help me with a little experiment?

[01:03:13]All right, Susan.

[01:03:17]Now, hang this bar magnet up here.

[01:03:22]Notice that the two magnets attract each other.

[01:03:25]>> There's a trick going on here. Watch.

[01:03:29]Now you try it, Susan.

[01:03:34]It doesn't work. This magnet pushes that magnet.

[01:03:38]>> I saw you, Michael. I saw you. You switched when you handed it to Susan.

[01:03:42]That's what you did.

[01:03:48]>> Yes, Michael played a trick on Susan.

[01:03:50]>> The pools on this magnet look the same, but they are different. One is marked S and the other is marked N. They're called opposite poles.

[01:04:00]>> Let's see what happens when the S pole of one magnet is brought near the N pole of the other. It is pulled toward or attracted.

[01:04:09]The rule about magnets is that opposite poles attract each other. Another rule about magnets is that two poles which are alike push each other away like this. This is called repels.

[01:04:23]One S pole repels the other S pole.

[01:04:27]But an N pole and an S pole attract each other.

[01:04:34]>> And now I'm going to show you how to make a magnet. You see this needle? It is not a magnet. It will not pick up the other needle.

[01:04:48]Notice how Michael is stroking the needle on the magnet always in the same direction toward the pole.

[01:04:55]>> This needle did not pick up the other needle before. Now watch.

[01:05:02]>> See, it has become a magnet.

[01:05:06]>> Now I'm going to make a compass which points to the north.

[01:05:10]I take this needle which I've made into a magnet and put it in the paper. Now I put it in the pan. Watch. See it turn.

[01:05:20]Which way is it pointing?

[01:05:22]>> It's pointing north. Now see, we've made a compass out of a plain old needle.

[01:05:30]>> The Earth itself affects magnets so that a magnet which can turn freely will turn its end pole toward the north. That's why we call it the N pole. N stands for north seeking.

[01:05:42]What does the S on the other pole stand for?

[01:05:48]>> So, Michael has shown his audience something really important that magnets do for us. Just look around and you'll see all kinds of uses for magnets.

[01:05:57]In the kitchen, for instance, magnets are used in many ways.

[01:06:02]They hold the knives in place.

[01:06:06]They are sewn into hot pads to hold them to the side of the stove.

[01:06:11]They are attached to can openers to hold the lid of the can so that it won't drop inside.

[01:06:18]But magnets have more important uses, too. There is a magnet in every radio and television speaker and in every telephone receiver.

[01:06:31]Every electric motor has a magnet in it too. an electromagnet which is different from the permanent magnets we have been talking about.

[01:06:39]So the things that Michael showed his friends in his magic show are all important in our everyday life. Remember what these things were. First that magnets attract only iron and steel.

[01:06:52]Second that magnetic force goes through other materials.

[01:06:58]Third that the magnetic force is strongest at the poles.

[01:07:03]Fourth, that opposite poles attract like poles repel.

[01:07:10]Fifth, that you can make a magnet by rubbing a piece of steel on a magnet.

[01:07:17]And last, that the Earth itself is a magnet so that the magnetic needle of a compass is made to turn toward the north.

[01:07:25]Why don't you get a magnet to experiment with and try some of these things for yourself?

[01:07:51]This process might have been the origin of the solar system.

[01:07:55]>> Yes, gentlemen.

[01:07:58]We're witnessing the secret of creation.

[01:08:07]Mr. Mayor, the city must go down to supply the power we need.

[01:08:10]>> And after 11 hours, what then? Then we'll need the entire power of Boulder Dam to feed it.

[01:08:15]>> We must make preparation to evacuate the city.

[01:08:18]>> Looking like creatures from another planet.

[01:08:22]These two scientists risked their lives to move the new Titanic element to the one place where they might fight it.

[01:08:31]>> I'm going to set the machine and leave in time. That's all the men. But she isn't built to take such a load. She break up.

[01:08:37]>> Dr. Benton, our only hope is that she'll break that element before she breaks herself.

[01:08:43]>> This one man stood between the earth and doom.

[01:08:51]Only he dared face the terror of the monstrous thing that had suddenly come alive. A cosmic Frankenstein that threatened to engulf the world and hurl it into outer space.

[01:09:03]>> I'm now going to set the Deltatron at its maximum output and close the floodgates. I want you all to leave.

[01:09:10]You've got about 7 minutes to reach the surface.

[01:09:19]C the magnetic monster battle its deadliest enemy, the giant Deltatron.

[01:09:29]See the last desperate chance they took to check its appalling power.

[01:09:36]See it shatter the steel walls of its mammoth prison beneath the sea.