where Ψ(x) is the electron wave-function, expressed as a function of distance x measured from the emitter's electrical surface, ħ is the reduced Planck constant, m is the electron mass, U (x) is the electron potential energy, E n is the total electron energy associated with motion in the x -direction, and M (x) = [ U (x) − E n ] is called the electron motive energy. M (x) can be interpreted as the negative of the electron kinetic energy associated with the motion of a hypothetical classical point electron in the x -direction, and is positive in the barrier.