Magnetic force on the walls of a square coaxial transmission line

There is magnetic force acting on the wall of a square coaxial transmission line when TEM wave propagates in it. The self-inductance can be determined based on a conformal transformation for the field region, which demonstrates that the inductance only depends on the side length ratio of the two walls instead of the length of the walls. The magnetic force on each side of the walls is then calculated by employing the principle of virtual work. It is proved that the magnetic force has the same magnitude and opposite direction to the electric force. And then the electromagnetic force on the wall is zero. The general meaning of this conclusion for all kinds of coaxial transmission line is proposed.