How is the strength of a magnet measured?
When considering the ‘strength’ of a magnet there are several key measurements that all contribute to a magnet’s strength in different ways, which can be a little confusing.
Also, when trying to determine or measure a magnets strength the answer will be dependent on what is meant by strength be it either the pulling force or magnetic field strength, which are both often referred to as strength.
So below we will describe each attribute that contributes to a magnets performance and in turn answer the question how is the strength of a magnet measured?
Maximum Energy Product
The first contributing attribute is a magnets maximum energy product, this is measured in Mega Gauss Oersteds (MGOe).
This is the primary indicator of a magnet’s ‘strength’ as usually the higher the maximum energy product value, the greater the magnetic field generated by the magnet.
To calculate the maximum energy product, take a magnet’s remanence (Br) and multiply it by its coercivity (Hc).
The remanence is described as the magnetism that is left in the magnet after the removal of external magnetic force applied to magnetise it, this is measured in Gauss.
When a material has been magnetised, it will have remanence, as the magnetism has previously been induced by an external magnetic field.
The remanence value of a magnet is the flux density held by the magnet whilst in a closed circuit
The coercivity of a magnet is the energy which is required to reduce the magnetisation of a magnetised object to zero, which had previously been magnetised to the point of saturation.
Essentially, it measures a magnetic materials resistance to demagnetisation and is measured in Oersteds.
Each factor can only be measured with a hysteresis graph testing machine which plots a second quadrant hysteresis curve.
Open Circuit Flux Density
The intensity of a magnetic field, measured in Gauss or Teslas (10,000 Gauss = 1 Tesla), is also a very common measurement of a magnets strength as it is the representation of the density of a magnetic field produced by a magnet, known as flux density.
A magnetic field can be visualised as magnetic lines passing through a magnet along the direction of magnetism, the field strength relates to the density of the lines over a given area and the total number of lines is the magnetic flux density.
The remanence value of a magnet is the flux density held by the magnet whilst in a closed circuit, but after being removed from the testing machine it is no longer in a closed circuit and is in an open circuit. The magnetism will fall instantly to a lower level which will be dependant on the ratio between the surface area and its relative magnetic length.
For example, magnets with small poles and long lengths have a much higher open circuit flux densities in comparison to magnets with large poles and small magnetic lengths.
Neodymium magnets are very commonly used, and manufacturers and suppliers will provide strength for each of their magnets to show how much weight a magnet can hold.
The pull strength is the highest possible holding power of a magnet, measured in kilograms. This is the force required to prise the magnet from a flat steel surface when they are in direct surface-to-surface to contact.
The grade of the metal, surface condition and angle of pull will all have an impact on the pull strength of a magnet.
Pull Gap Curve
A pull-gap curve plots the pulling power of a magnet in direct contact with a thick and flat piece of steel and then through a steadily increasing range of air gaps. All magnets can be tested over a variety of air gaps using a pull-gap testing machine.
So, when it comes to selecting a magnet the key things to look out for are the maximum energy product value of a magnet, the open circuit or surface gauss value and the maximum pull strength. These attributes give you the opportunity to compare individual magnets.
If you have any questions or queries then be sure to get in touch with our team of experts who are available on 0845 519 4701 or firstname.lastname@example.org and are always happy to help.