The sensitivity to voltage, measured in volts per mm scale deflection, of a direct‐current thermionic amplifier (simple circuit) is taken as inversely proportional to the mutual conductance divided by the plate current of the valve employed. The mutual conductance is found to decrease less rapidly than the plate current, as the filament and plate potentials are lowered. The most sensitive operating value of the plate potential of a UX 222 is found to be 1.5 volts giving a voltage sensitivity of 8×10−4volt/mm in the simple circuit with no plate current compensation. By suitable use of the screen grid, a grid‐filament resistance of 1014ohms may be obtained giving a current sensitivity of 8×10−15amp./mm with an input resistor of 1011ohms.