With the recent advances in photoelectric cell technique a number of experimenters have endeavored to devise a method of using vacuum tube amplification for the measurement of weak currents and small emf's. Among these may be mentioned Wynn‐Williams1and Eglin2who have developed a balanced bridge amplifier of high sensitivity; also Razek and Mulder3 4who have devised two single‐tube circuits. The single tube circuit does not possess the sensitivity of the balanced bridge circuit but has certain advantages in the way of simplicity of operation and control. Under proper conditions it may be used in a multi‐stage amplifier, using the principle of resistance‐coupling.