OK - so how I came up with this topic. I wanted to bring some magnets to work, but how? In one pocket is my keys (with the transponder thingamajig). In the other pocket is my phone. In the brief case - wallet (with magnetic strip cards) and a computer. OK - I’m probably paranoid, but I won’t risk putting magnets near any of these. So I got to thinking - what causes that kind of concern for magic items?
First thought - lead, but that’s not right. Lead just blocks or resists magic, it doesn’t mess it up. So unless we’re going to think of the lead as draining the magic out of the items, this won’t work.
So what are magnets? They are electricity caught up in an object. And what is our tech? Electricity and magnetism. So magnets mess up our technology, because they are our technology. So the analogy would have to be that magic is what messes up magic. Hmmmmm.
So I am still trying to think through this. These thoughts are at best half baked. I think there would have to be some manner of “opposite” magics that would start to interfere with each other. Example - If you have a sword with magical poison on it (assuming death magic) and armor that gives you regeneration (life magic), then at some point, they would interfere with each other. A flaming mace and a ring of fire resistance. Tunic of flying and bow of petrifying other people (assuming elemental air and earth here).
So if you drag a ring of fire resistance across a flaming weapon, does it automatically wipe it out? No, I don’t think so. So when? Well, I think you would need to store them separately. I think if the two objects were left in a treasure chest for centuries, the two would both be ruined by the time they were rediscovered. On a more immediate case, I think it would somehow take a fumble or the equivalent to have the two items either wipe each other out or have one wipe out the other.
So if a fumble can wipe out your magic items, then you might want to think about not carrying off-setting items. You might want to hope you can get your magic to overwhelm your enemy’s when the two weapons clash (probably a critical). I’m still working this out, but you get the idea. What are you thinking now?