Fig. 4 shows a simple model in which a phase diagram is shown at the bottom. In the space inside the magnetic shell material, the magnetic field is the resultant of the superposition of the impressed magnetic field and that originates from the magnetization of the shell material. Due to the close resemblance in magnitude and phase as shown in Figs. 2 and 3, one can imagine that an active compensation system can be based on the feed-forward scheme, if the amplitude and the phase of the externally impressed magnetic field can be monitored correctly and the phase difference can be properly adjusted. In the experiment, an optimum point for placing a monitoring fluxgate magnetometer was searched; the position on the outer surface of the shell yielding the minimum interference from compensation coils is observed. Fig. 5 shows results of the scan along a line parallel to the axis for 2 and 50 Hz in which data points for the two almost overlap. The point on the surface 10 cm from the end coil was found to be an optimum point for observing the impressed magnetic field. At that point, the influences from the end coil and the center coil cancel each other. The frequency characteristics of the impressed magnetic field seen at the observation point are shown for the range 2–50 Hz in Fig. 6. It is found that the amplitude is very flat and attenuated to 17% of the amplitude