Potentiometer null detectors

The accuracy of a voltage measurement depends most of all on the design and the quality of the potentiometer unit used. For a precision potentiometer, the relative accuracy of measurement (ability to measure very small changes in voltage) wUl depend a great deal on the null detector sensitivity, while the absolute accuracy will depend more on the accuracy of the standard-cell voltage value and the linearity of the slide-wire. The most accurate potentiometers replace the slide-wire by discrete resistors and switches. 

Emf measurements were made with a Leeds and Northrup K-5 potentiometer equipped with a Leeds and Northrup DC null detector (Model 9829). The temperature of the bath was regulated to within 0.02 K. Details of the experimental procedure, including preparation of the electrodes (19), cell design, preparation of solutions, purification of the hydrogen gas, and other experimental aspects, have been reported elsewhere (13,14). 

The temperature distribution within the cryostat was measured with six 20-gauge copper-constantan thermocouples located throughout the cryostat, these thermocouples being referenced to the resistance thermometer. Thermocouple emf s were measured with an L N Type K-3 potentiometer in conjunction with a Model 9834 electronic dc null detector. Inasmuch as these differential thermocouples generate small emf s, their accuracy is limited by the potentiometer, this accuracy being 0.02°C in this investigation. During operation the temperature distribution in the bath liquid was within 0.1°C of the resistance thermometer. 

The resistance ratio for Chromel-P thermocouple wire was determined with the circuit shown in Fig. 1. A fixed current was supplied to the specimen with a stabilized power supply (Trygon Model H20-1.5) having remote sensing. Two laboratory-type rheostats were used to give additional fine current adjustments the specimen current was maintained at a selected value of 0.10000 amp by adjusting the potential across the standard 1-ohm resistor, Rs, to 0.10000 v. The potential across the standard resistor and the specimen coil was determined with a potentiometer (L N K-3), a null detector (L N Model 9834-1), and an Eppley standard cell. 

A relatively high contact resistance is a characteristic of precision potentiometers. For this reason, the user may notice an apparent "noise" when obtaining a deep null with an external detector. This is normal and does not affect the rated accuracy of the instrument. 

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