An alternating-current analogue of the Kelvin double bridge

In connection with a project for an absolute determination of the ohm, the author has developed an a.c. 4-terminal resistance bridge. This new bridge is analogous to the Kelvin double bridge, but resistive ratio arms are replaced by transformer ratio arms and inductive dividers. Besides the benefits of impedance matching and ease of amplification which the use of alternating current confers, the replacement of resistive ratios by inductive devices brings great advantages in accuracy and stability of ratio. In the bridge described here, the ratio is believed to depart from the nominal value of 10: 1 by little more than 2 parts in 107 over the frequency range of 330 rad/s (52.5 c/s) to 33000 rad/s (5.25 kc/s).