Thermal magnetic noise generated by an eddy current shielded room

Using a SQUID-based second-order gradiometer, we have measured the thermal magnetic noise generated by a thick aluminum plate and an eddy current shielded room used for biomagnetic measurements. The measured noise agrees with the theoretical prediction for the thermal noise fields generated by infinite conducting slabs. The noise is substantial near the walls of the shielded room, but is predicted to be less than 0.02 fT/√Hz at 1 Hz at the center of the room. This is too small to influence the biomagnetic measurements usually taken at this location