I have a supposedly strong magnet, probably alnico, that has been stored without its keeper, so all the magnetism has fallen out. :-) If I wind a big coil round it, and pass a large current through it, i can remagnetise it, this much I know. The magnet has 2 iron bars fitted, so it is in a "U" shape. Should I add a keeper bar, and try to magnetise it with the keeper in place, to make a closed ring, or removed?
Martin L
Firstly, yes, a keeper will help a lot.
Next question: how long is the magnet itself (length measured in the direction the magnetism goes)?
If the magnet is Alnico, you will need about 200 000 A/m (the A is the current multiplied by the number of turns) to saturate it which you must do to magnetize it fully. This doesn't need to be d.c., it can be a pulse say from discharging a capacitor. This means that the wire used can be much thinner in relation to the current. Pulse length should be around 20 ms. If the magnet is ferrite (a sort or rusty brown ceramic - like a 'fridge magnet) pulse length can be shorter.
About 12cm, so I only need to get 12,000 A, a mere 100A up a 120 turn coil will do. :-)
Definitely alnico, I need a boxful of 400V electrolytics, and charge them from rectified mains, then switch them to the coil. (wear goggles for this part children)
I think I might have trouble getting 100A into a 120 turn coil though, have to suck it and see, a project for later on.
(To get the polarity right, I can use less capacitors, or a lower voltage, and magnetise a file, then compare the poles with those of the magnet in question and a compass.)
Cheers
Martin L
The fuse blowing magnetising method is much simpler and cheaper. It's described in the Model Engineer book Electric Motors in the Home Workshop. Checkout your local library.
Jim
P.S. I was greatly cheered tonight to find the newsgroup messages not dominated by unproductive diatribes between well known characters. Please don't feed the trolls - whoever you may consider to be the guilty troll!
12 cm is quite a long Alnico magnet. Maybe it is 35% cobalt steel. That would be (slightly) easier to magnetize.
For a one-off, think about a car battery. In good condition (and with a low resistance) these will deliver 500 A briefly. Then you only need a 24 turn coil, but with a resistance of 0.024 ohm - not a problem with thick wire.
Don't worry about it. The largest magnet I've ever had to magnetized weighed over a ton, and need a quick flash from a 2MW supply - and we managed to short that out!
When my childhood magnets wore out Dad used to take them to work and hang them on the arms of the spot welder for a while. A few cigarette machine cabinets later and they were restored.
Dad sold up a couple of years ago so I can't offer this service...
Regards,
Steve, Salisbury
Some info here:
Lucas: "Remagnetising a Magneto".
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