For 10 points, name this effect in which a magnetic field perpendicular to a current induces a voltage difference perpendicular to both.
In one version of this phenomenon, its namesake conductivity takes on values equal to the elementary charge squared over Planck's constant, a quantity which is multiplied by either a fractional or integral value. Another version of this phenomenon can be used to measure the charge carrier density, since it arises as a consequence of the Lorentz force law applied to those particles. Its namesake quantity is proportional to the product of current and (*) magnetic field, divided by the thickness of the plate and charge carrier density. For 10 points, name this effect in which a magnetic field perpendicular to a current induces a voltage difference perpendicular to both.
In one version of this phenomenon, its namesake conductivity takes on values equal to the elementary charge squared over Planck's constant, a quantity which is multiplied by either a fractional or integral value.
Hall effect