Result (23) can be given a simple and useful physical interpretation by referring to Fig. 1. The electron state at point "F" on the diagram ("state F") is the "forwards moving state at the Fermi level" (i.e., it describes a Fermi-level electron moving normal to and towards the emitter surface). At 0 K, an electron in this state sees a barrier of unreduced height φ, and has an escape probability D F that is higher than that for any other occupied electron state. So it is convenient to write J 0 as Z F D F, where the "effective supply" Z F is the current density that would have to be carried by state F inside the metal if all of the emission came out of state F.