A knowledge of the rate of change of frequency of a supply system is a useful addition to the information generally available in control rooms. At times when the available generating plant is inadequate to meet the prospective peak-demand, it assists in the recognition of critical conditions, and thus enables the necessary action to be taken to meet these conditions.The instrument described in this paper consists essentially of a sensitive frequency-meter: changes in frequency over successive fixed timeperiods are used to obtain values for the mean rate of change of frequency over these periods.The frequency-meter section of the instrument consists of a frequency-sensitive bridge network which is made continuously selfbalancing by means of a servo motor. The servo motor is controlled by the out-of-balance voltage from the bridge and itself controls the variable-impedance bridge elements necessary to maintain balance.The rate-of-change-of-frequency display unit depends for its operation on the measurement of the angular rotation of the servo-motor shaft in successive equal intervals by rotary voltage-dividers driven by the motor.The main design features are described, together with details of operation and performance of the instrument.