[en] Analytic formula for the multipole coefficients of a single current filament between two infinitely-permeable iron plates are obtained. These are applied to the eddy currents generated in a two-dimensional conducting surface, such as a synchrotron vacuum chamber, when it is exposed to a rapidly varying magnetic field. We find that the sextupole component, b₂, is insensitive to the chamber shape and depends only on the gap between the plates, if the width of the chamber is much larger than the height. Higher multipoles, b_n decrease rapidly with increasing chamber width. The radius of convergence for the multipole expansion, at least up to n = 10, is about 80% of the width of the chamber. 3 refs., 1 fig., 2 tabs