A null instrument for accurate measurement of small a.c. currents

Summary An apparatus and a method are described that are based on a.c. null balance, which may be used for measuring 50 Hz currents of 2 × 10⁻¹¹ A, or higher, with an accuracy of 0.1 per cent. The apparatus, which has been applied in experiments of different kind in our laboratory, is fed by a.c. line voltage. The main part is an a.c. amplifier with 30 MΩ input impedance, provided with a variable phase-shift. The amplifier is essentially, followed by a synchronous meter and is connected to a stabilized and calibrated 50 Hz reference source. The design, adjustment and performance of the apparatus are treated in §§ 1–3. Also the use of a linear rectifying meter instead of a synchronous meter, in direct-reading measurements, is discussed with respect to the signal-to-noise ratio. In § 4 the application of the described a.c. null balance method to photometry will be reviewed, with special regard to the attainable accuracy.