The Shielding of Electric and Magnetic Fields

An experimental investigation of the shielding of electric and magnetic fields is reported, for both constant and changing fields. The effect of using iron shells, or sheets, for shielding against the fields of permanent magnets, as well as those set up by electric currents, is considered; the best form for the iron sheets is deduced and an expression for a measure of the shielding action is suggested. The reason for the leakage of magnetic and electric fields is shown to be due to differences of magnetic or electric potentials in the circuit in which the fluxes are being set up; several cases are cited in which no external fields are set up, as the circuits exhibit no differences in potential. An expression for the shielding effect of a short-circuited coil is deduced and experimental verification is offered for frequencies between 10²and 10⁶cycles per second. Finally the shielding effect of metal sheets against changing magnetic fields is analyzed and experimental results are given to show how the action depends upon the characteristics of the material of which the shielding plate is made, its thickness, and upon the frequency being used. The effect of slits in the metal sheet, and the value of wire mesh, is indicated.