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Q & A: making magnets

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Most recent answer: 10/22/2007
Q:
Re. Making permanant magnets. I would like to make some smallish perm magnets from steel. I read that the way forward is to make a solenoid, insert the material to be magnetised inside and apply a pulsed dc current through the solenoid winding. 1:-Is this correct? and 2:- Do you have a circuit diagram for the "pulser" ? Regards.
- David Miles. (age 21 years plus a lot)
grantham lincolnshire england
A:
It sounds like you heard right. There's a reason for using pulsed fields. You want the field as big as possible in order to flip the magnetic domains into lining up with the applied field. For small applied fields, the domains stay stuck. However, running lots of current through your coil to make a big field will make it heat up and ultimately melt or start a fire. So you run a big current for a short time. The domains that don't respond to a pulse of a few milliseconds mostly wouldn't have responded on longer times anyway. In other words, it's more effective to have a big field a small fraction of the time than a small field all the time.

You might experiment with having the steel at different temperatures. The hotter it gets, the easier it is to get the domains to line up during the pulse, at least until they get so hot that they start to lose their magnetism altogether. However, the hotter it gets the easier it is for the domains to relax back to the un-aligned state after the pulse ends. There may be a best temperature a bit warmer than room temperature.

Unfortunately, I don't have a circuit diagram of an automated pulse circuit available. Maybe you could just make pulses with a manual switch, say by discharging a capacitor through the coil.

I just tried magnetizing a paperclip by very brief contact with a permanent magnet. It worked ok, and didn't seem to fade over the weekend. So maybe you won't need many pulses.

Mike

(published on 10/22/2007)

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