The magnetic distribution of the single-coil and dual-coil magnetic rheological dampers with the same effective piston length was analyzed by using the finite element ANSYS software.

By assuming the weak lens focal length expression to be the first term in a power series a simple and accurate formula can be derived for the objective focal length of a magnetic electron lens.

Then the curves of magnetic strips field ( B) with length ( L ) and magnetic roller field( B) with its angle (DEG) are described by printer.