The logarithmic region of the forward characteristics of p-n silicon junction diodes at low currents is utilized to give accurate logarithmic conversion of current into voltage over a range of eight decades at room temperature. A chopper type d.c. amplifier is used to measure the logarithmic voltage without drawing an appreciable current. It consists of a specially developed low-leakage balanced diode chopper, a high-gain a.c. amplifier and a demodulator. The voltage output of the demodulator is fed back to the chopper. Temperature compensation is achieved by a thermistor element in the feedback loop and the resulting thermal drift is less than 0.005 decade per degree centigrade over most of the amplifier range. Many of the available types of diodes were found to exhibit irregular and considerable deviations from accurate logarithmic relationship.