Magnetic fields are invisible, so there is no way to tell if a magnet is good or bad just by looking at it. There are a variety of tools for testing available, but one of the simplest and most popular is a Helmholtz coil. Connected to a fluxmeter, you can use it to measure the magnetic moment or dipole moment of permanent magnets.
How it works:
A Helmholtz coil captures the magnetic field lines from a magnet, similar to how a butterfly net is used.
Just about any wire wrapped as a coil can be used to capture and measure the fields produced by a magnet, but to maximize sensitivity and usability, a special arrangement of two works best:
This arrangement was first mathematically described by the German physicist Hermann von Helmholtz, and the coil arrangement has been named in his honor. A Helmholtz coil contains two identical magnetic coils that are placed concentric along a common axis. There is one coil on each side of the experimental area where each sample magnet is placed. The amount of magnetic field lines produced and captured by the Helmholtz coil is directly proportional to the strength of the sample magnet. Since the volume and the material are fixed properties, capturing the magnetic field lines tells one if the magnet is properly magnetized.
How to use it:
For a Helmholtz coil measurement, the coil must be minimum of three times larger than the magnet. The coil is connected to a fluxmeter. The magnet is placed in the center of the coil, the fluxmeter is zeroed out, and the magnet is pulled straight out of the coil. The fluxmeter displays how many of the magnetic field lines were captured by the coil. Generally, a minimum acceptable value is calculated beforehand.
Consistency and speed:
One of the many advantages of the Helmholtz coil measurement is its tolerance for variability. User A will obtain virtually the same readings as User B or User C. Once setup is complete, the measurement only takes a few seconds, lending itself to use in a high quantity production environment.
For more information about measuring magnet characteristics, check out our blog covering How to Measure a Magnets Strength, How a Gaussmeter Works, & How to Optimize Results, or our post about testing the angular direction (ϕ, θ) of magnetization using our m-axis testing equipment. And be sure to think of Adams for everything magnetic!