An integrator is described which operates on the Miller principle. It is bipolar and has a minimum integrable voltage of less than 0·25 mV and a temperature coefficient of 11 μV degC-1. The input d.c. voltage is converted into a pulse rate and the pulses accumulate in an electromechanical counter. Two input connections are available with different input impedances and dynamic ranges; in one of them the maximum counting rate for 1% accuracy is 220 counts per minute, corresponding to an input of 130 mV, and in the other this maximum counting rate occurs at 92 counts per minute with an input of 55 mV. The power source consists of four 6 V dry cells, which must supply a steady current of 1·2 mA and peak currents of 150 mA.
