This paper examines analytically the field between equal stator and rotor ducts when displaced from each other and evaluates, for any relative position, the total flux crossing the air-gap and the amount entering the sides of each duct. The flux entering the sides of the stator ducts in a.c. machines varies at supply frequency and produces eddy currents the path of which is not restricted by the direction of the laminations and which consequently give rise to considerable loss. With relative displacement of the ducts the fluxes entering the two sides of a duct become unequal and produce an axial magnetic force of engineering importance.Numerical values are given for the variation of gap permeance and magnetic centring force, and are plotted in curves directly applicable to design calculation for ratios of duct to gap width varying from ½ to 5 and for any relative displacement.