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Measuring Permanent Magnet Characteristics with a Helmholtz Coil and Fluxmeter

The magnetic moment is a property that we use to verify the quality of our magnets. It is a way to measure the strength of a magnet and is widely used throughout the industry. Moreover, it is an easy, quick, and repeatable measurement.

The magnet grade and magnet volume define the magnetic moment. Testing for it confirms:

  1. the correct magnet grade was chosen,
  2. the magnet has the correct dimensions & orientation, and
  3. the magnet was fully and properly magnetized.

The magnetic moment is a measurement of the overall magnetic output of a magnet. Many illustrations of magnets show ‘lines of flux’ going from one magnetic pole to the other:

Magnetic lines of flux from a cylinder-shaped magnet

So, the magnetic moment is the summation of all those lines. Assuming the magnet is fully and properly magnetized, the two main factors determining the magnetic moment are the Type/Grade of magnet material and the Volume of a magnet.

  • The stronger the magnet grade, the larger the magnetic moment
  • The larger the magnet volume, the larger the magnetic moment

As a rough approximation, the four major types of permanent magnet material can be ranked lowest to strongest:

  1. Hard ferrite/Ceramic
  2. Alnico
  3. Samarium Cobalt
  4. Neodymium Iron Boron

For the same size magnet, here is an illustration depicting the relative magnetic lines of flux:

In conclusion, here are some examples:

  • The Earth’s magnetic moment is 8 x 10 22 Ampere-meter2
  • A bowling ball made of Neodymium Iron Boron (strongest grade) would have a magnetic moment of 5937 Ampere-meter2
  • A Neodymium Iron Boron (strongest grade) disc the same size as a US Penny would have a magnetic moment of 0.4824 Ampere-meter 2
  • A Neodymium Iron Boron disc in a typical mobile phone has a magnetic moment of 0.007909 Ampere-meter2

 


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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:

helmholtz coil

Photo credit: Lakeshore

Photo credit: Magnet Physik

 

 

 

 

 

 

 

 

 

 

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.

Further reading:

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!


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