Child–Langmuir law

The experiments of Langmuir on the Edison effect (aka thermal emission) showed that the amount of current drawn between a planar diode is related to the voltage on the anode and the anode-cathode (AK) separation. Langmuir himself considered how the relation was modified for current flow between coaxial cylinders and concentric spheres. The problems are incredibly rich, and the physics deceptively subtle, but the caveats and special conditions even for the simple diode problem led van der Ziel to advise about Child's so-called law. Such a history of a law has more of Creon in it than Zeus, but referring to the limits placed on electron emission by cathode geometry and anode potential as the "Child-Langmuir limiting relation for particular circumstances" lacks both panache and gravitas compared to referring to it as a "law".