Recall , where the sign shows that the charge carrier is negative. ( is the electron density, electron charge, speed of light)

The Hall effect has to do with a bar of metal placed in a magnetic field. An electric current runs through it perpendicular to the field direction. However, the magnetic field term in the Lorentz force creates a force perpendicular to both the magnetic field and electric field. This ``pulls" the current astride from its original straight path. Charge starts accumulating on one side of the slab from this force, even as current continues to flow perpendicular to the accumulation direction. In equilibrium, the magnetic force balances the electric force from this charge accumulation. Apply some basic EM theory, and one can derive the Hall coefficient () defined above.