In electronic measuring systems the accuracy of measurement is affected by drift not only of the zero but of the gain due to temperature changes, aging and supply conditions. The invention provides an automatic gain correction means so as to maintain an accurate relationship between the magnitude of the characteristic being measured and the value displayed or recorded. The measuring system is such that a train of pulses is counted over a predetermined time interval with a train of pulses representing, by its pulse repetition rate, the magnitude of the characteristic. A reference count is provided and means are provided for generating a pulse train simulating a predetermined magnitude of the characteristic being measured. Means are provided for counting the pulses of the generated pulse train and means are provided for comparing the reference count with the count of the generated pulse train to determine any error in the gain. Means are provided for automatically adjusting the predetermined time interval to correct the effect of the determined gain error.