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The measurement of small currents of the order of 10-12ampere by thermionic electron tubes requires special considerations not ordinarily encountered with larger currents. Electrometer tubes are especially designed for this service. The necessary differences between ordinary electron tubes and electrometer tubes is discussed from a practical viewpoint. Plate current and plate voltage are much lower in electrometer tubes, and this presents problems both in tube and circuit requirements. Low grid current and high leakage-resistance techniques are presented. Characteristics of such tubes are given in terms which are generally applicable.