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I have seen somewhat similar things with very old meters. It seems the magnets lose their strength over so many years. I think the only options are to recharge the magnet or replace the unit. Well, you can get fancy I suppose and build a little amplifier to convert 100 microamperes to 400 or so. A current gain of 4 is easy, and the unit can be powered with a lithium cell or a couple of alkalines.
The magnetic flux probably decreases exponentially with time, meaning that the change is slowing down and it probably will be consistent for quite a while.
One more option is to recalibrate the scale and only use the first quarter of the pointer travel.
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