A tuned bridge detector with a constant band width of 5 cps for use at audiofrequencies is described. The center of this band is clamped electrically to the output frequency of the associated signal generator and deviates less than ½ cps from it. This makes manual tuning of the detector unnecessary. The narrow pass band of the amplifier reduces the noise signal to a very low value and allows detection of a ½‐&mgr;v signal across a one‐megohm input for a continuous range of frequencies from 100 cps to 20 kcps. The indicated noise level is of the order of ½ &mgr;v for the one megohm input; this is about the theoretical thermal noise value (0.41 &mgr;v for 5 cps, 1 megohm). The final detector signal can be easily used to activate mechanical drives for the two bridge controls in order to maintain balance of the bridge when the measured impedance changes or to reset to balance from an unbalanced condition.