[The Philpott]

If the horseshoe type magnet has deteriorated over the decades by, say, 1%, all the voltage and current readings given by the meter will be slightly low. it is generally possible to modify the magnetic field by swinging the shunt arm around- up to 45 degrees away from it's minimum position. Any more than 45 degrees and the effect reduces. If not enough increase is available it is possible to experiment by removing this arm altogether.

Dave