Ferromagnetism

The video below shows a brief history of Magnetism

Permanent Magnets - Story Time

The first recorded examples of permanent magnets can be found in the writings of Thales of Miletus in Greece in approximately 600 BCE.

In the (what is most likely an apocryphal) story, Thales of Miletus was out walking in the hills around the village of Miletus when he came across a large rocky outcropping and as soon as he stepped onto the rock, he tripped over. He turned to see what he had tripped over to find his boot "glued" to the rock. When he pulled his boot free, it wasn't sticky and neither was the rock! But as soon as he put his boot down, it again stuck to the rock!

But why were his leather boots sticking to the rock? He took off his leather belt and placed it on the rock, but nothing happened...

It was then that Thales realised that it wasn't the leather that was sticking to the rock, but the Iron nails in the boot's soles! Thales named the effect of the rock after the region of Anatolia he lived in - Magnesia.

The above story is most likely just that - a story, but it does show a clear example of a type of natural source of Magnetism, in this case a type of rock called a Lodestone.

Image result for magnetite

Lodestone is a naturally occurring magnetic rock, found throughout the World. It is the ancient name for a type of Iron Ore called Magnetite (Fe3O4). Magnetite is found in many different types of rock throughout the Earth's crust (the image above shows Magnetite crystal in a matrix of Feldspar), but only a small fraction of this is naturally magnetised, forming a Lodestone.

The image below shows a piece of magnetised Magnetite, with various iron and steel objects attached to it :-

Image result for lodestone

Lodestones have been used since the 11th Century (first use in China) to create simple compasses for use in navigation.

The image below shows a replica of the 11th century compasses built by the Chinese. The "spoon" is made of Magnetite, and the handle has been fashioned so that it will always point South :-

Image result for lodestone compass

Navigation in the Ancient World - The Vikings ( Non-Examinable )

The Vikings also developed the use of Lodestones as a way to navigate. Instead of using a spoon that rotated to show south, the Vikings used a much smaller (and easier to use aboard a ship) system, using a floating needle. The needle was made of Iron and was magnetised by drawing it across a Lodestone several times. This needle was then floated in a bowl of water and if left to move freely, would rotate such that the needle lined up North-South. This can be seen in the image below :-

DIY Compass 2

As good as this compass was at showing the direction of North, it did not show how far North you were. This is very important when in the open ocean, as once outside of land, it was impossible to know if you were following a true West/East course. In order to do this, the Vikings appear to have used a technique known as Latitude Sailing. By using a device known as a Sun-board. A Sun-board worked by setting sailing along a specific Latitude, and using the Sun at noon to set a shadow position on the board. If when sailing, at the noon the shadow as too short, then the ship had strayed South or if it is too long, then the ship has strayed North.

This would allow a sailor to follow a set East/West Course, and is thought to be how the Vikings navigated easily from Scandinavia to Scotland, the Faroe Islands and Greenland.

The Vikings also appear to have used a technique using Polarisation of light to find the Sun on overcast days to aid navigation, using a type of rock known as Iceland Spar which is found throughout Scandinavia. Iceland Spar is a transparent form of Calcite (CaCO3) which shows very strong birefringence. The Polarisation of incoming light when viewed through the rock would allow the observer to locate the Sun to within a few degrees (not great but good in a pinch), through completely overcast skies.

The video below shows a scene from The Vikings, with the main character Ragnar explaining the use of the Sun-board and a Sun-stone

Ferromagnetic Domain Theory

In the above examples from the ancient World, naturally occurring magnets were used. it was only on the advent of generation of electricity that Magnetism could be generated artificially. It was research from Scientists such as Hans Christian Orsted that led to the development of Magnetic theory beyond permanent magnets.

The understanding of why some materials were magnetic and others were not was an area of great Scientific research. It was found that only certain elements could show magnetism. These magnetic Elements are :-

1. Iron

4. Rare-Earth Metals ( Such as Neodymium )

These Elements are referred to as Ferromagnetic Elements. The group is named after the Latin name for Iron - Ferrum.

But what is it specifically about these Elements in particular that make them magnetic ? It is to do with their location within the periodic table. All these Elements are found in the middle of the table, and as such have incomplete outer orbitals. These incomplete orbitals allow for microscopic magnetic effects due to the presence of unpaired electrons (beyond this course level, see University-level Physics for further explanation here).

It is how these magnetic effects on an atomic scale combine together that gives the magnetic effect of Ferromagnetism. The explanation of how these combine is known as Domain Theory.

Within all Ferrous metal, there are regions where all the atomic scale Magnetic fields are aligned with each other. These regions are known as Domains. Throughout the metal, there are lots of these Domains, each with a random orientation. This random orientation of the Domains means that the Magnet has no overall magnetic field, as the Domains cancel each other out.

If a large external Magnetic field is applied across the metal, however, the Domains can be "forced" to align in one direction. This means that their individual atomic Magnetic fields all align, giving the Ferrous metal an overall Magnetic field.

The diagram below shows a graphical explanation of Domain Theory :-

Once the Domains are aligned and a permanent magnet created, it is possible to reverse the process and randomise the Domains again, in effect demagnetising the metal.

The following methods will demagnetise a magnet :-

Raise its temperature above its Curie Temperature - the increased Ek within the crystal structure fully randomises the Domains.

Apply a high frequency alternating magnetic field across the metal - the high frequency alternation causes the Domains to randomise.

"Hammer" the Metal - The mechanical jarring can randomise some of the Domains.

The video below shows a simple summary of Magnetic Domain theory

Magnetic Fields

The following diagrams show the shape of the magnetic field lines around several types of magnet :-