A moving-coil voltmeter gave accurate reading when it was new but, after some years, all its reading

Question

A moving-coil voltmeter gave accurate reading when it was new but, after some years, all its readings are found to be $9 \%$ too low. This could be because

Professor PC Thomas Classes · Answer

A moving coil voltmeter's reading decreases if the magnetic flux density of its permanent magnet decreases with age.