A. Errors with both D.C. and A.C.
i. Errors due to hysteresis. This source of error is due to hysteresis in the soft iron moving part, due to which too high values are recorded by the instrument, when the current is increasing and too low readings are liable to be indicated when the current is decreasing.
ii. Errors due to stray fields. External stray magnetic fields are liable to affect adversely the accurate functioning of the instrument. Magnetic shielding of the working parts is obtained by using a covering case of cast iron.
B. Errors with A.C. only
Errors may be caused due to change in frequency because change in frequency produces
(i) change in impedance of the coil and
(ii) change in magnitude of eddy currents. The error due to the former is negligible in ammeters, as the coil current is determined by the external circuit and the error due to the latter can normally be made small.