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Magnetism now and then - Part I

Sunday, 20 August 2017

Magnetism is not a single phenomenon, but rather an entire class of properties subsumed under one of the four forces. That force used to be the magnetic force. Back then there were only two other forces: electricity (dynamic and static) and gravity. Then magnetism was merged with electricity (by a physicist called Maxwell) to become the electromagnetic force. That meant there were only two forces. But then two others were discovered: the strong and weak forces. So that meant there were four. But recently, physicists have merged electromagnetism with the weak force. So now there are three again: gravity, the strong force and the electroweak force!

Magnetic fields can be caused both by electric currents flowing through a medium and by what are called magnetic moments. Moment in this case, refers not to  a moment in time, but is related to momentum - and specifically angular momentum. In other words, it is related to the spin of a charged particle or even the orbit of electrons around an atomic nucleus. When charged elementary particles spin or orbit a system, they create magnetic fields. These fields in turn interact with other currents and magnetic moments.

Most people know about magnetism from ferromagnetic materials. These are either natural magnetic materials,or materials that have been magnetized. Regardless of whether are natural or man-made, ferromagnetic materials are strongly attracted by magnetic fields. Technically, a ferromagnetic material is any material that can be magnetized to become a permanent magnet. But there are very few truly ferromagnetic substances in nature. Iron is the most abundant of them, followed by nickel and then cobalt, which is highly radioactive.

The word ferro - used as a prefix - is derived from the Latin word ferrum, which means iron. The reason for this is that magnetism was first observed in iron. This is only natural when you consider that iron is the second most abundant metal in the earth's crust and the fourth most abundant element there beaten by oxygen, silicon and iron. It is in fact the most abundant element on earth, because the planet's entire molten core is made of iron. And that is why the earth has magnetic poles: because the molten core spins inside the earth, but not at the same rate as the surface.

But I digress. The point I was making is that although ferromagnetism occurs in several elements, it was originally observed in a form of iron that was known as lodestone. This was actually an iron-rich ore, also known as magnetite.

But it is important to bear in mind that although ferromagnetism is the type of magnetism that we usually "see" in real-wold situations, there is a whole world of magnetic effects that we don't usually see with our eyes and yet are ever present. For example, there is a whole other class of magnetism called paramagnetism and a matching class of materials called paramagnetic. These include those elements mentioned before that are even more abundant in the earth's crust: aluminium and oxygen. These are weakly attracted when a magnetic field is applied.

Then there is another class of materials called diamagnetic.These include other abundant materials like carbon (the main building block of life) and copper (the metal that use to conduct our electricity). But unlike, paramagnetic materials, their diamagnetic counterparts are weakly repealled. There are even antiferromagnetic materials that also interact weakly with magnetic fields. For this reason, paramagnetic, diamagnetic and anti-ferromagnetic materials are all referred to in everyday speech as "non-magnetic" - even though this is scientifically incorrect.

But magnetism is itself not a fixed property of a material regardless of its phase (solid, liquid, gas) or even its temperature. Quite the contrary, temperature, pressure and an external magnetic field, all affect the magnetic characteristics within an atom, molecule or atomic lattice. And as the variables change, so might the behaviour of the material itself.

Magnetism is of course nothing new. It was discovered independently by many ancient civlizations. This was usually in the form of loadstones. The Ancient Egyptians, Babylonians, Chinese, Greeks, Summarians and Assyrians all discovered it. Aristotle wrote about it - attributing the discovery to Thales. The magnesian stone (named after a district of ancient Greece that was in turn named after the Magnetes tribe) gave its name to the modern phenonmenon. In ancient India, a pioneer of surgery, Sushruta, used magnetism in medicine.

The ancient Chinese wrote about the earth's magnetism in a work written in the fourth century Before the Common Era. But it was not until the second century BCE that they wrote about lodesones. And a hundred years later, they combined the two and wrote about lodestone needles pointing in the direction away from where the sun reached its peak in the sky. They had discovered the compass. And although they were never a great sea-faring nation - they didn't need to be, sitting bang in the middle of a major land-trade route - they used it for navigation.

By the 12th century, the Europeans had got in on the act. One Alexander Neckam described the compass and how it could be used. Then, in 1269, a certain Peter Peregrinus de Maricourt described magnets in some detail in his work Epistola de magnete. And in the middle east, a Yemini scholar by the name of Al-Ashraf described magnets and the navigational compass in 1282. We will discuss this more in the next article.