Thermionic constants determined from retarding‐field data

In the method described here, the intersection point of the ’’Davisson lines’’ determines the emission constant A, the anode work function (wf) φa, and the cathode wf φ (T). The Davisson lines are defined by ln(ir/Tκ) =lnA+e (V−φa)/ kT, where ir is the current density in the retarding‐field range, T is the absolute cathode temperature (parameter), κ=2 for metals, κ=5/4 for oxide cathodes, and V is the anode‐cathode voltage (independent variable); at the intersection point V−φa=0, ln(i/Tκ) =lnA. At the knees of the i (V) curves, V=Vs(T) =voltage at ir=is0=zero‐field saturation current density, such that φ (T) =φa−Vs(T). Shelton’s method [Phys. Rev. 107, 1553 (1957)] is shown to be equivalent to the Davisson‐line method. The method is applied to experimental data for a W emitter (κ=2) and for an oxide cathode (κ=5/4). The advantage of the method is that A, φa, and φ (T) result directly and correctly for any φ (T). The large A values resulting for the oxide cathode (A?45 000) remain to be interpreted.