The triode‐connected 5889 electrometer tube was found to be useful as a logarithmic element over a grid current range of 10−14to 10−6amp. The relationship of plate current to grid current was carefully studied over a span of three and one‐half decades as a function of the plate supply potential, the plate circuit load resistance, and the filament voltage. A circuit was developed for use in an ionization chamber survey instrument. Most 5889 tubes will work in the circuit after a 150‐hr aging period. Over‐all accuracy of ±10% of the reading is feasible.