Already know the sensitivity (250 microamps) but thanks for the tip
about peeling pole piece plating, don't think it's that though because
the coil/needle rotate freely. Reason I suspect the o/c is because
there's a 'green, furry blob' (due damp storage sometime in the past ?)
halfway along the fine coil connection wire, possibly surrounding a
joint. Job for a fine brush & steady hand I think !!