A new type of thermionic tube recently described by Metcalf and Thompson has made it possible for the first time to construct d.c. amplifying circuits with a current sensitivity exceeding that of any form of electrometer except the Hoffman; and with a ruggedness and dependability unattainable with any form of sensitive electrometer. The circuits are easily constructed and simple to operate. Three types of circuits have been tested out and are described. (1) A simple single-tube circuit with a Type R galvanometer has been found satisfactory for measurements of currents as small as 10^-14 amp. (2) A two-tube bridge circuit gives greater stability and will easily measure currents of 10^-16 amp. It has been found capable of detecting currents of 5×10^-18 amp. (3) A two-stage circuit, using one of the new tubes and one of the UX-112A type, will amplify currents of 10^-14 amp to such a value that they may be read on a microammeter.