Coercivity and the Horseshoe

Magnets have been in use for ages and currently the shape that is most recognized and is used to represent magnets is the horseshoe. Why is it that magnet are usually made in this shape and why does it provide an advantage over the other shapes?

Many people say that magnets are designed like horseshoes because putting two poles together brings about much greater force and this increases the strength of the magnet’s hold. This is true but it may not be the only reason. You can still be able to put two poles together if you place two magnets side by side. Think about the horseshoe and specifically the circular part. This part does not get used and may be viewed as extra material. This therefore disqualifies this theory as being the only reason for the horseshoe shape being used in magnets.

Coercivity

A magnet is usually considered strong depending on the pull force experienced when it is in use. A magnet’s strength can also be measured by finding out the magnetic field strength that will be able to demagnetize it. This is what is known as coercivity.

 Earlier magnets had such a low coercivity that their own magnetic fields were able to demagnetize them. These magnets were mostly made of iron. More powerful magnets have been developed over time and this includes an increase in coercivity as well. Neodymium magnets are stronger than most magnets and they also have a higher coercivity and this allows them to maintain their magnetism over time.

Significance of the horseshoe shape

The shape of a magnet influences how much magnetization is lost. A long cylindrical magnet will stay magnetized longer compared to a thin disc magnet. It takes a little more to demagnetize a horseshoe because it is like a stretched version of a cylindrical magnet.

The permeance coefficient (Pc) of a magnet is defined as how difficult it is for a magnet’s field lines to move from the north pole to the South Pole. This means that taller magnets will have a higher Pc and shorter magnets a lower Pc. A horseshoe has a high Pc as well. This prevents it from easily getting demagnetized. This is further increased by using a steel keeper.

This explains why neodymium magnets are not made into the horseshoe shape. Neodymium is a very strong magnet that does not demagnetize at all. In the event that a neodymium magnet gets demagnetized, you will find that the magnet was probably very thin or it was exposed to very high temperatures.

You can even place two neodymium magnets together without causing them to demagnetize each other. Their coercivity is very high which makes them more flexible for use in different applications. Neodymium is also a hard and brittle material. Shaping it into horseshoes would cause them to break very easily.

This shape is therefore applied to magnets with low coercivity like Alnico and iron. They will be able to keep their magnetization for longer with this shape.

For more information about various magnets, please visit http://www.samaterials.com/214-neodymium-magnets.