Transformer bridge for the high-accuracy comparisons of standard resistors at low frequencies

A transformer bridge has been developed at IMM for comparing low value standard resistors (0.1 Ω-100 Ω) at frequencies from 0.5 to 20 Hz. Two current sources and two resistance standards are connected in series in one current circuit. The potential leads of standards are respectively connected through the differential current transformer. The null-detector consists of two identical current amplifiers with a low input resistance. Each amplifier has a Schottky noise as the dominant component. The differential current transformer produces the inverse current pulsations in the input winding of another current amplifier. At 1.2 Hz, the current noise of the null-detector is equivalent to 1 pA in 100-Ω resistor (or 10 pA in 1-Ω resistor), i.e., a relative sensitivity below 0.001 ppm at room temperature. The bridge has no resonant circuits and can operate using a square-wave current