The second derivative of the current‐voltage relationship of an ideal plane diode is calculated from the Epstein‐Fry‐Langmuir analysis. From this analysis it is shown that for the large saturation emission characteristic of oxide‐coated cathodes at normal operating temperatures, the current at the inflection point (i.e., the zero of the second derivative) is a sensitive measure of the saturation emission and is unaffected by the existence of a linear interface impedance. It is also shown that the general features of the results apply to cylindrical diodes.