Provided that the conversion factor β is independent of voltage, the parameter f has the alternative definition f = V / V φ, where V φ is the voltage needed, in a particular experimental system, to reduce the height of a Schottky–Nordheim barrier from φ to zero. Thus, it is clear that the factor v (f) in the exponent of the theoretical equation (30) gives rise to additional V -dependence in the pre-exponential of the empirical equation. Thus, (for effects due to the Schottky–Nordheim barrier, and for an emitter with φ =4.5 eV) we obtain the prediction: