A.C. bridge for platinum-resistance thermometry

An a.c. bridge has been constructed for the measurement of the resistance of platinum-resistance thermometers. It is based on a multidial 2-stage ratio transformer, using a single reference resistor of 25¿, and operates at a frequency of 320Hz. The measuring current of 1 or ¿2 mA is carried by the standard and unknown resistors in series, and the potential drops are compared by means of the ratio transformer, using a single gain-stabilised detector. The ratio transformer is separately excited to ensure that the current in the potential circuit of the bridge is not significant. The bridge has five switched decades for a range of 111 ¿, and the residual unbalance is indicated on a direct reading meter with a most sensitive range of 1 m¿ full scale. Feedback of the quadrature component of signal from the detector eleminates the need for manual reactance balance. The resolution is better than 10 ¿¿ and the overall linearity better than 20 ¿¿. A recorder output (± 1V) is provided.