High resistance measurement at NIM

An automatic ratio bridge based on BVD is used to take measurement up to 1 G/spl Omega/ at NIM. Adopting a "Virtual Null" mode efficiently reduces effects of insulation of bridge and offset current of detector. The measurement of a 100 M/spl Omega/ Hamon resistor shows an agreement of 1 parts in 10/sup 6/ at ratio 1 G/spl Omega/:100 M/spl Omega/.