Resistance Measurements at Frequencies below 10 Hz

Several electronic circuits are described which increase the input resistance of inductive voltage dividers up to 109 Ω at frequencies below 10 Hz. One of the circuits has been used to build an isolating inductive voltage divider consisting of 8 decades and having an input resistance of 109 Ω. The in-phase errors are less than 2 parts in 10+8. This inductive voltage divider is the main part of an ac potentiometer for the precise measurement of low value resistors with comparatively high lead resistances. Lead resistances of 50 Ω generate errors of only 1 part in 107. Apart from the preceding application, the inductive voltage dividers with high input impedance may also serve as a component of a voltage comparator. Such a comparator, for example, permits the calibration of a resistive voltage divider which is used for maintaining the unit of voltage via the Josephson effect and which, for such a purpose, is kept at a temperature of 2 K. An uncertainty of 5 parts in 10+10 is achieved for the measurement of a 1:1 ratio, even if the ratios differ by a few parts in 103 from 1:1.