Electromagnetic screening by metals

To dispel a widespread but erroneous belief among physicists that the penetration of ac magnetic fields into normal metals is determined by the usual skin depth δ alone, a simple analysis is presented of two problems in each of which a different length scale determines the effective screening. For a cylindrical can of thickness d≪δ and radius R≪λ, where λ is the wavelength, it is shown that the critical thickness for effective screening is dc =δ2/R. For a planar film with thickness d≪δ, dc =c/2πσ, where σ is the conductivity. An exact analysis is also presented of the screening for a cylinder of arbitrary thickness, as well as an analogy between screening by normal metals and screening by superconductors.