In a pipe of rectangular cross-section a × b there is a gas of density ρ. The vertical walls of the pipe are insulators, and the horizontal walls are electrodes. A discharge is ignited at one end of the pipe, after which the current I is kept constant. The resulting area of discharge combustion is pushed into the pipe by magnetic forces, "raking" the gas in front of it. Determine the steady-state velocity of the plasma "plug", assuming that it is always greater than the speed of sound in the gas. The magnetic field of induction B is perpendicular to the vertical walls of the pipe.