The Campbell-Shackelton Shielded Ratio Box

The Campbell-Shackelton shielded ratio box is intended to serve as the nucleus of a one-to-one bridge for the comparison of impedances. The transformer used in it operates effectively over the range from 50 to 50,000 cycles per second and in that range the limit of error is considered to be the greatest one of the three following terms: (1) ±0.01 ohm (resistance, inductive, or capacitive reactance) (2) ±0.01 per cent (3) ±2π × frequency × 10
^-7
micromhos (conductance, inductive, or capacitive admittance) Item (1) is the limiting factor in the measurement of low impedances, (2) in the measurement of intermediate impedances, and (3) applies for high values. Thus at 1 kc, for example, impedances between 100 ohms and 150,000 ohms can be compared to 0.01 per cent. Below this range, measurements can be made to 0.01 ohm and above this range to 6 × 10
^-4
micromhos. In practice there is no difficulty about sufficient power and amplification for attaining the precision needed to realize this accuracy. The above figures refer to the ratio box only and do not include errors in the standards used with the ratio box. The above limits of error apply to measurements made under the most adverse conditions, as when measuring the smaller of the two components of an impedance. In many measurements it is possible to do much better than is indicated above, particularly in respect to item (3).