In some cesium frequency standards and clocks, the uniform, static magnetic field (C field) is generated by an assembly consisting of four conducting rods carrying equal currents and symmetrically placed at the corners of a rectangle inside concentric cylindrical magnetic shields. The spacing of the rods and their location within the shield to give minimum variation of the magnetic field within an area of rectangular cross section close to the axis of symmetry are calculated for the case of the National Research Council cesium clocks CsV and CsVI.