Consider a rectangular current-carrying conductor with electrons as charge carriers. When a magnetic field is applied perpendicular to the plane of the conductor, the electrons moving in a straight line drift due to the deflecting magnetic force. The electrons accumulate on one side of the conductor, leaving excess positive charges on the other side. This charge separation creates an electric field within the conductor, which exerts an electric force on each electron opposing the magnetic force. At equilibrium, the magnitude of electric force balances the magnetic force, stopping further drifting of the electrons. The potential difference between the two sides of the conductor is the hall voltage. By expressing the drift velocity in terms of the current, the Hall voltage can also be used to measure the number density of the charge carriers. Hall probes measure the magnitude of an unknown magnetic field from the Hall voltage and the conductor's known parameters. A significant application of this discovery is that the polarity of the Hall voltage implies whether the charge carriers in the conductor are negative or positive.