A simple method of measuring low currents from current sources (transducers) is described. This method has a noise equivalent to an input current of 5 fA (5 × 10−15A). A possible modification for 1 fA noise input is given. In this method of measurement, a charge generated by a current source is collected on a capacitor during a fixed time interval. In discharging the capacitor, an ac pulse is produced which is then amplified, and the required information extracted.