This paper discusses the usefulness of common tube types for small current measurement and, after presenting the results of a detailed study of the three types 38, 954, and 959, offers recommendations of operating conditions for their use. The observations reported consist primarily in data showing the dependence of grid current upon applied potentials, and secondarily in information about the corresponding plate current characteristics. Grid currents less than 10−15ampere in the 959 tube are reported, associated with stability comparable with that of special electrometer tubes.