How to Shield Magnets

Magnetic attraction

Magnets have the ability to attract and repel other magnets and ferromagnetic materials. A magnetic field surrounds every magnet no matter its size. This magnetic field causes unwanted interference. That is why it becomes necessary to shield a magnet.

Instructions

1. Find out the composition of the magnets. Different materials produce magnets of differing strengths. Neodymium boron magnets, for example, are the strongest in terms of magnetic attraction and repulsion, and may require several layers of shielding material.

2. Map the magnetic field. Use the gauss meter--an instrument for measuring the magnetic field strength of magnets--to get accurate readings. The magnet field strength decreases with increasing distance from the magnet. Move the gauss meter in a systematic fashion around the magnet, till you have mapped out boundaries where the reading is 2.5 mG (milligauss) or less. To get an accurate picture, take the measurements under varying conditions and find out if there are other electromagnetic sources nearby.

3. Measure the area you have mapped. This tells the amount of shielding material you will need. Remember that the field strength gets concentrated at the edges of the shield. If people will be in the area you might want to increase the shield size, so the edges fall away from where people are. A good rule of thumb is to shield at least two feet around the magnet.

4. Decide how much reduction of the magnetic field you want. This will determine the size and material of the shield. Try for attenuation to 2.5 mG or less in living/office spaces, or anywhere people spend time.

5. Experiment to see if you will need more than one shield. Sometimes you require several layers of the shielding material. Many types of shielding materials are available-- from paper-thin rolls you can cut and shape yourself to thick plates that the seller cuts to your specifications.

Tips & Warnings

• The greatest shielding occurs at the center.

• A spherical shell shield is the most efficient shape. If that is not practical for your project, consider a cylindrical shape with a 4-1 ratio of length to diameter. Putting end caps on the cylinder increases its effectiveness.

• If a box is the only shape that will work, try for the largest bend radius.

References

• Acorn NMR Inc.: Magnet Shielding
• LessEMF: Magnetic Field Shielding