  The experimental problem of the calibration of magnetic field in large iron
detectors is discussed. Emphasis is laid on techniques based on ballistic
measurements as the ones employed by MINOS or OPERA.In particular, we provide
analytical formulas to model the behavior of the apparatus in the transient
regime, keeping into account eddy current effects and the finite penetration
velocity of the driving fields. These formulas ease substantially the design of
the calibration apparatus.Results are compared with experimental data coming
from a prototype of the OPERA spectrometer.
