The sensitivity and the signal‐to‐noise ratio of electrically deflected measuring instruments, such as magnetometers or galvanometers, can be increased significantly by reversing the exciting current periodically at the fundamental resonance frequency of the suspended system and observing the peak amplitude of the oscillatory dynamic deflection. If these current reversals are made to occur exactly at the midpoints between successive zeros of deflection, the oscillations of the suspended system will be stably maintained at the resonance frequency with a ratio of dynamic to static deflection of approximately 4∕δ, where δ is the logarithmic decrement for free oscillation. This note describes electronic apparatus devised for controlling automatically the required quadrature timing of the current reversals. The circuits comprise a photocell‐actuated Schmitt trigger, a bistable flip‐flop, a timing capacitor, and a voltage comparator. Small timing errors due to finite slit width and hysteresis in the voltage comparator are of opposite sense and can be made to cancel.