Current density is best measured in A/m. The total current density emitted from a small uniform region can be obtained by integrating the total energy distribution j (ε) with respect to total electron energy ε. At zero temperature, the Fermi–Dirac distribution function f FD = 1 for ε <0, and f FD = 0 for ε >0. So the ECD at 0 K, J 0, is given from eq. (18) by