The strength of a magnet is most commonly measured using a magnetometer, also known as a gaussmeter, which are used to measure everything from the Earth’s magnetic field to small magnets. A magnetometer consists of a small conductor or semiconductor at the tip of a probe through which an electrical current is passed. The effect of the magnetic field on the electrons in the conductive material can then be measured. The International System unit for measuring magnetism is the tesla which measures something called the magnetic flux density, but teslas are only really useful for measuring very large magnetic fields.
A more suitable way to measure smaller magnetic fields is to use the unit gauss. One tesla equals 10,000 gauss. The Earth’s magnetic field is about half a gauss, a fridge magnet about 100 gauss and a large electromagnet like that in a MRI could be up to 1,500 gauss. There are several factors that will affect the results of any measurement such as the distance from the magnet the reading is taken, the magnet’s size and whether or not the magnet is attached to anything. A more practical measure of a magnet’s strength is to measure how much weight it can lift.
Connor Skates, Science Museum