Thats what I was going to suggest too. Re-magnatise them with a strong magnet.
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You can't heat magnets.the only way you are going to solve this problem is by lazering or PUK welding the jump ring.it's a simple job to do ,if you have the tool that is.
Neodymium magnet
As found in hard disc drives, BTW - if you ever need a source of cheap, dangerously strong magnets. Note the lower Curie temperature of them versus ferrite magnets - so they'll lose their magnetism at temperatures as low as 310C (depending upon the exact type).
Oh, and remagnetising them requires some fairly serious kit - you might be able to do it with an MRI scanner, but a) I wouldn't want to run the risk of trying and b) there tends to be a bit of a queue waiting to use them...
Physics is of no interest to me unless I've got an application to go with the theory... ;)
Ahh, now I really do see! Thanks Peter :cool: