May/June 2018
Has anyone tried to revive permanent magnets that have weakened over time? What is the best method? Can they be made “good as new” or is it better to just replace them?
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I am not sure that time is the cause of loss of magnetism of permanent magnets. They normally lose their magnetism by three methods.
I have two large toroidal magnets which sit on a tube with opposite poles facing each other. The top magnet ‘floats’ about 2cm above the bottom one. They have been like this on my workbench for at least 10 years and there is no discernible difference in the distance between them so I don’t believe that they have lost any of their magnetism.
The magnets weigh about 100g so the force required to hold the floating magnet against gravity is about 1 newton. I have no idea where the energy to do this for 10 years has come from and I have not found a physicist who can provide a satisfactory answer. I will keep asking!
Unless there’s something special about the old magnet it’s usually easier and cheaper to replace them. A case I’m familiar with where it’s worth “recharging” old magnets is in old motorcycle magnetos.
In this case we put a coil with a core inside the magnets and then pass a high current for a short period of time. The field from the coil has to be much stronger than the original magnet but doesn’t need to be maintained for very long. Discharging a capacitor into the coil is a good way to achieve this.
To little field and nothing happens, to high a field does no harm. Make sure the field you generate is in the same direction as the magnet or you’ll wipe out any remaining magnetism and possibly reverse it’s polarity.
There are a few ways to partially revive some types of magnets. However, the more powerful the magnet was originally, the more “brittle” is the arrangement of domains, and the harder it is to realign them.
For that reason, remagnetization is most useful for antiques, ornaments or teaching devices, made of materials such as carbon steel. Don’t expect a ferrite or neodymium magnet to gain back much strength.
First, determine the alignment of the original magnet. Rubberized magnetic backing, for example, often has stripes of opposite polarity on one side; these would be impossible to repair. A horseshoe magnet, often found on hand-cranked generators, or a bar magnet, is easier to fix.
Simplest is to put the old magnet in contact with a strong permanent magnet, e.g. a large rare-earth magnet, letting it align by greatest attraction (North touching South). Tap the old magnet a few times with a hammer to help rearrange domains. More effective is to wind wire around the magnet and pass a large pulse of DC current through the coil. Capacitive discharge magnetizers are simple to build, but involve high currents and voltages and present some safety hazards. See https://www.rcgroups.com/forums/showthread.php?307133-Inexpensive-magnetic-flux-(gauss)-meter/page3&highlight=capacitor%20discharge%20magnetizer for more details.
I would try making and using an electromagnet to re-magnetize your magnets. Works well for bar magnets, for other shapes it might be a bit more involved.
Since many “standard” type magnets are made from magnetic metal placed into a magnetic field to begin with I would say if this can be done for your magnets you stand a good chance of making them every bit as good as they were originally.
