[en] The measurement of plasma diamagnetism by a simple diamagnetic loop can be significantly altered by the presence of a conducting wall and its associated eddy currents. These effects can be significantly reduced by the use of compensated diamagnetic loops. It is shown that the wall effects are of order β compared to the diamagnetic term in a uniform magnetic field with field inhomogeneities, however, the error terms may be larger than the diamagnetic term for compensated loops. Field distortion limits are discussed in relation to plasma induced distortions and vacuum field distortions due to a slowly varying magnetic field. (author)