An a.c. double bridge with inductively coupled ratio arms for precision platinum-resistance thermometry

A simple double bridge with inductively coupled ratio arms for precision resistance thermometry is postulated, and the advantages to be gained from the accuracy, stability, low temperature coefficient, high input impedance and low output impedance of such ratio arms are considered. The equation of bridge balance is examined, and it is shown that the errors in measurement due to the resistance of the potential leads of the thermometer can be reduced to 0.0001°C for any temperature change between −100°C and +100°C and are unlikely to exceed 0.0003°C over the range −183°C to +630°C. A double bridge having eight decade dials and operating at 400c/s has been constructed, and the details are given. The errors of voltage division within the ratio arms are less than 2 parts in 107, and the input impedance is approximately 650kΩ.