A new device capable of measuring minute dc voltages is described, and a simple theory which describes the operation and predicts the performance is presented. Experimental amplifiers have demonstrated dc sensitivities which closely approach the maximum attainable by any device. Their performance characteristics agree satisfactorily with the theory. Sensitivities of 4×10−14V with a source resistance of 4×10−7&OHgr; and a time constant of ≈ 1 sec have been obtained at 4.2°K. The ultimate sensitivity appears to be limited only by the noise present in the source itself.