Resistance measurement null detector

Apparatus. All electrical resistances were measured with an electrolytic conductivity bridge (Leeds and Northrup model 4666) which was constructed according to specifications set forth by Jones (28) and described by Dike (29). The audio-frequency source was a General Radio Co. type 1311-A audio oscillator used with the frequency regulated at 1000 Hz and the output at about 5 V. The detector circuit consisted of a high-gain low-noise tuned amplifier and null detector (General Radio Co. type 1232-A) and an oscilloscope (Heathkit model O-ll) ... 

The Wheatstone bridge shown in Figure 8.5 provides the most direct and best known circuit for comparison of unknown resistances against standard resistances. Resistances RA, RB, and R are standard resistance values that are used in the measurement of the unknown resistance R Resistance R is made variable and is adjusted until the null detector indicates that the bridge is balanced. 

Conductivity Measurements. Cell resistance measurements were made with a General Radio type 1650-A impedance bridge. It is equipped with an internal, 1000-cycle signal source and tuned null detector. For more sensitive balance at high resistances, a Hewlett Packard 400L vacuum tube voltmeter is used as an external null detector. 

Solution resistances were measured at 1000 Hz with an AC Wheatstone bridge an HP 200 AB oscillator provided the input and a tuned amplifier with a null detector was used to find the bridge balance (with simultaneous resistance and capacitance balancing). A homemade cell with Pt electrodes was used, similar to the one described in Ref. 27a. The cell was calibrated with 0.10N NaCl solutions. 

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. 

Measurement of conductance, which is a measurement of the reciprocal of resistance, is done with a Wheatstone bridge such as that shown in Figure 3. In its traditional form, the bridge consists of four resistances, the unknown cell resistance Rq and three precision variable resistances, Ri, Rx, and R3, which are manipulated to show zero voltage drop across BD as observed by the null detector ND. Calculation of the unknown resistance is then derived from Ohm s law, where V is the voltage and / the electrical current across a resistance R ... 

The resistances B, D, and E can be measured, and from these measuranents, the resistance and hence the conductance of the cell can be calculated. A small superimposed AC voltage ( 20 mV peak to peak) at 1000 Hz is best as a signal, because then faradaic polarization at the electrodes is minimized. The null detector may be a sensitive oscilloscope or a tuned amplifier and meter. Stirring is often used to minimize polarization. 

Surface resistivity is the leakage of current along the surface, expressed in ohms. It is also measured with a Wheatstone bridge and null detector, but with both electrodes on the same side of the sample. It is used to evaluate the effect of antistatic agents and suitability for service in charge dissipation. 

The temperature of the calorimeter was obtained by using the detector as a null instrument or the galvanometer in a more conventional way. During the fore and after periods, resistance was measured each minute. During the heating period, the time was noted at which several predetermined values of resistance were attained. Resistance could be measured to within 3 X 1(H ohms or about 0.0015° C. 

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