The conducting slab has a thickness b and a width a. The magnetic field is constant and points out of the paper. The voltmeter is disconnected from the circuit. There is a steady flow of a horizontal current I moving from left to right, or a steady flow of electrons from right to left with an average drift velocity vd. In the direction perpendicular to the drift velocity, there is a magnetic force on the electrons that must be cancelled out by an electric force. What is the magnitude of the electric field that produces this force