The apparatus for measuring the Hall effect is shown above. Consider
a charge carrier (to keep you from having to mess with the
negative charge on the real charge carriers - electrons) moving
through the apparatus in a material with an unknown charge
carriers per unit volume. Derive an expression for , given
, , , , and . Note that I'd have to consider you
moderately insane to have memorized this result (I certainly haven't)
but by considering the strip to be a region of self-maintaining
crossed fields and relating the current to the drift velocity you
should be able to get it fairly easily.