Two magnetic amplifier circuits for automatically controlling a constant temperature hot wire vacuum gauge are described. The differential circuit senses temperature variations by means of voltage and current control coils which respond to the ratio of voltage to current in the hot wire. The bridge circuit senses temperature variations by unbalance in a Wheatstone bridge circuit. In either case, the sensing signal is amplified and fed back to the hot wire in such a direction as to very nearly restore the original temperature. Experimental data are given showing good gauge sensitivity up to 50 mm. Circuitry is simple and, since the magnetic amplifier is inherently a stable device with no perishable components, long trouble‐free life can be expected.