  We develop an effective low-frequency theory of the electromagnetic field in
equilibrium with thermal objects. The aim is to compute thermal magnetic noise
spectra close to metallic microstructures. We focus on the limit where the
material response is characterized by the electric conductivity. At the
boundary between empty space and metallic microstructures, a large jump occurs
in the dielectric function which leads to a partial screening of low-frequency
magnetic fields generated by thermal current fluctuations. We resolve a
discrepancy between two approaches used in the past to compute magnetic field
noise spectra close to microstructured materials.
