The Type 707-A Cathode-Ray Null Detector is a sensitive and highly selective null detector of the visual type for use in balancing an impedance bridge. It indicates, separately, the effect of adjusting the reactive and resistive balancing controls of the bridge. It also indicates whether one of these controls, selected as desired, is off-balance in a positive or a negative direction. It employs a tuned degenerative amplifier which is entirely free from any electromagnetic or electrostatic pickup; the instrument does not in itself broadcast any appreciable electromagnetic field.