An Audio-Frequency Four-Terminal Resistance Bridge

The design and performance of an audio-frequency four-terminal resistance bridge is described which compares resistors from 1 to 0.001 Ω over a frequency range of 50 Hz-10 kHz. Current scaling establishes equal voltage across the resistors, permitting the standard to have the larger value and most optimum design while dissipating less power than the unknown. An amplifier-aided two-stage current transformer forms the ratio arms. Ratios of 1, 2, 4, 5, 10, and 20 are available, and a maximum applied current of 200 A is accommodated. Bridge ratio errors are less than 2 × 10-6 in both magnitude and phase at frequencies up to 1 kHz, increasing to 2 × 10-5 at 10 kHz. Simple transformer scaling techniques are described for producing accurate and stable resistance standards at the 0.001-Ω level.