Hello, I'll try to present you today some results about the Earth's core and the Earth's magnetic field. What we know of this field and how we think it is generated. We all know that the Earth has a magnetic field. We know this because we've used our phone, which has a magnetometer inside that orientates itself and gives direction or more pragmatically, we've used a compass needle. The compass needle does orientate itself in a magnetic field and that's a bit mesmerizing. It's an interesting experiment to do. It's surprising that when I approach a magnet, the compass needle changes direction and orientates itself in the new magnetic field due to this magnet and as you can notice, that it wobbles. It oscillates around its direction and it does so more rapidly when the magnet is closer than when it's away. This rapid oscillation is an indication of a rapid force that restores the direction of the needle. That's a sign that the magnetic field is stronger. This oscillation gives us an indication of the strength of the magnetic field, in pretty much the same way as the oscillation of a pendulum gives us an indication of the strength of gravity. Because gravity is constant at a given height on Earth, the oscillation has always the same period for a given pendulum provided the amplitude of the oscillation is not too large. That's a property that was used for decades to build clocks. People were building clocks on this property that the period here is pretty much constant, independent of the amplitude when it's small enough, but depending on the local gravity, which is the same at the surface of the Earth. That's a way to measure the gravity magnetic field. We can perform the very same experiment, either with a magnet or with a natural magnet. This is magnetite. It's an oxide of iron which is naturally magnetic, and that's probably the very first magnet that was discovered by mankind. We don't know exactly who was the first man to play with a magnet. We have no idea of that. However, we know that there are indications of things that probably are magnets in China, quite a while ago. Historians dispute exactly which is the first quote, which is clearly an evidence of magnet. This is a replica for a Han era magnet. That's roughly 200 years BC and this is a spoon which is magnetized and orientates itself in a magnetic field with the handle pointing south. We shouldn't be afraid to play with a magnet. In fact, it's an important part of science to do experiment and be amazed by what's occuring. Do you recognize this man? You probably know him. It's Albert Einstein. He's a world famous physicist and when he was five years old, he was sick at the hospital and his father gave him a compass as a present. That's an unusual present for a kid, but he said later in his life, when he wrote his memories much later in his life, he explained that this really changed his way of seeing the world and that he was amazed by this experiment and playing with the compass and the needle. The magnetic field has a strength and has a direction. It's quite surprising really because we can't see it. We're a bit amazed by this field that acts as a distance without touching anything. It's able to move objects without any contact, where in fact, we're only surprised because we discovered that magnetic field very late in our life, gravity is just the same thing. It also has a strength, it also has a direction and when I jump, I fall back on Earth without any contact. It's just a question of how early in our life or late in our life we discovered this field that really surprises us. In order to see the magnetic field, you can use iron filling and then you see the field line around a magnet. The first documented text on the magnetic field dates back to 1269. It was written by Petrus Peregrinus de Maricourt. He was a French working in Italy as a soldier and he was belaying siege of a city in Southern Italy, the city of Lucera. You know what belaying siege is like? It's waiting for people to be enough hungry so that they go out and the outcome of that is that he had plenty of time and he used this time to study magnet, split magnet in two. So that when you split a magnet in two, you get two magnets, each with two poles and study magnetic attraction. That's really the first text where scientific question is asked about the magnet. His real motivation was to solve the problem of perpetual motion. He was trying to perform a device that could move forever. This was quite a hobby at the time. Of course, we know nowadays that there's no hope in that. The magnetic field has a direction. It doesn't point exactly to the North, it points slightly off shift to that and that's what we call the declination. The compass needle does not indicate the axis of rotation of the planet, but something which is slightly 10 degrees, 11 degrees off this axis. We don't know who discovered that for the first time. We know who discovered the inclination. The inclination is the fact that the needle also dips within the vertical plane, it's not purely horizontal, it also has an angle with the horizontal plane. That's Georg Hartmann in 1510, who wrote this in a letter to the Duke of Prussia. Suddenly for him, this letter stayed unnoticed. The same thing was rediscovered years later by another compus maker, Robert Norman, who was very carefully designing compass in London. He was making sure that his needles stayed exactly horizontal until he'd magnetized them with a magnet and then they started to dip. He discovered that the field really had a direction in 3D. It does live in the 3D space. To study this, he performed a beautiful experiment with a needle perfectly balanced with a cup of water that would float, not at the surface, but stay at the same position and inclinate with the magnetic field. The real breakthrough on the Earth's magnetic field came in 1600 with William Gilbert was the personal physician of Queen Elizabeth the first. He was also very interested in science and he wrote the De Magnete which is considered by many as the first real scientific book where everything is tested with an experiment. He introduced a spherical loadstone or a magnet, which he christens the Terrella. He explained that all the measurements to date, were compatible with the Earth being a big spherical magnet. It's represented in this figure extracted from his book. Horizontally, you see that at this time, the axis of rotation of the Earth was very often represented horizontally so that the equator is like this, and North and South Pole are horizontal. That's another view of the same Terella that Gilbert introduced, also compatible with all measurements at the Earth's surface. The Earth is a big magnet, that's Gilbert's theory. Well, the first crack in the theory came only 32 years later. Henry Gellibrand professor at the Gresham College in London found a problem. He was measuring this declination, this angle between the actual north and the north as pointed by the compass needle in London and he found that this angle was varying with time. It's interesting because in order to do that, he had to rely on the measurements done by his predecessors, so that means he had a great trust in all the professors before him that measured this angle. He wrote a book explaining that this angle was varying. So the Earth can't really be just a big magnet. It's too simple. Another setback will come many years later with the thesis of Pierre Curie. Pierre Curie, also a very famous physicist known for the study of radioactivity, made a breakthrough during his thesis. He discovered what is called the Curie temperature, which means that if you heat the material, it will lose its magnetic property past a given temperature. When it comes to iron, that's only a few 100 degrees. We now know, as you have seen in this course, that the Earth is a structured planets, its differentiated. That means that the light elements, like rocks are on the top, the first 3,000 kilometers below our feet. Whereas in the interior of the Earth, it's mainly liquid iron. Then as you go deeper and deeper, the pressure increases and you end up with solidified iron, almost pure iron at the center in the inner core. Well, the temperature also increases as you go down and at the depth of the Earth's core, it's a few thousands of degrees, far enough for the Curie temperature to apply. There's no permanent magnet at such temperatures.
