The Range and Validity of the Field Current Equation

Field currents from cold cathodes under high vacuum conditions were investigated over a range from ${10}^{-17\mathrm{}}$ to ${10}^{-5\mathrm{}}$ ampere. Electrometer measurements of currents below ${10}^{-10\mathrm{}}$ ampere showed no deviation from the empirical equation of Millikan and Lauritsen. Precise measurements in the higher range exhibited a consistent deviation from the Millikan and Lauritsen equation, proportional to the fourth power of the field. This deviation is shown to be not necessarily in disagreement with the basis of the Fowler-Nordheim theoretical field current equation.