Input resistance, high

This electrical field potential is measured by means of portable millivoltmeter with high input resistance. The field registration is carried out in the mountain areas at the distance of 5 km and more, at the same time the minimal gradient value is about 10 mV/km. 

Valve voltmeters were widely used in the past, but have been replaced by transistor voltmeters. With instruments of this type it is possible to achieve an input resistance of 50 MQ or more, the current required to operate the instrument being of the order of 10" A. The early instruments had a tendency to zero drift on the lower ranges, but this has been overcome in the modern transistor types. Such instruments are most often used to make potential readings in extremely high-resistance electrolytes. The accuracy of such instruments is of the order of 2% full-scale deflection. It is necessary to ensure that both types are so designed that they do not respond to alternating currents. 

As deducible from Figure 8, to apply a precise potential value to the working electrode means to apply a precise difference of potential between the working and the reference electrodes. Since the electronic circuit to monitor such potential difference, V, is properly assembled to possess a high input resistance, only a small fraction of the current generated in the electrochemical cell as a consequence of the applied potential enters the reference electrode (thus not modifying its intrinsic potential) most current is channelled between the working and the auxiliary electrodes. 

The pH meter is a specialized voltmeter that measures the potential difference (in mV) between the sensing and reference electrode and converts it to a display of pH. To provide an accurate measurement of the voltage of an extremely high resistance electrode (108 Q) [5], this specialized voltmeter must be designed with high input resistance or impedance characteristics (100 times that of the electrode used). Since the measurement potential difference per pH change is very small (59.16 mV/pH unit at 25°C), a reliable amplifier in the pH meter is also essential. It should be sufficiently sensitive to detect changes of at least 0.05 pH unit (or 3 mV). 

Any direct current source may be used as a current source (CS), in connection with a voltmeter with high input impedance, to control the applied potential at the working electrode. The applied potential can be measured via the reference electrode (R) using a voltmeter with a high input resistance (V). If a potentiostat is available, it will automatically control the working potential. 

If we take into account the finite input resistance Rm of any real electrometer, we recognise that any reading of a probe potential will decay exponentially with a time constant RmCm. If is of the order of 10 pF (it cannot be made high otherwise Vm will be too small to measure) the input resistance must be kept above about 1013 fl to keep the time constant long compared with the time required to take a reading. 

The effective input resistance of a DC amplifier may be greatly increased by the use of negative feedback. This principle is illustrated for charge measurement in Fig. 7.4, where a feedback capacitor is connected across an amplifier with k high input resistance Rm and gain —N. To make a measurement, the amplifier is first unshorted whilst the input probe faces an earthed shield. The shield or cap is then removed and the probe taken near to the charged surface in... 

The quantity a (j) in Eq. 18B is the actual potential measured between, say, two copper wires attached to the terminals of a battery. We note that potential is measured as a rule with a device having a very high input resistance which, in effect, prevents equilibrium between the electrons in its two terminals. 

For experiments under controlled potential conditions the electrode voltage may be controlled manually following the reading of a dc voltmeter with high-input resistance or automatically by a potentiostat. This is a device that is able to keep the potential of the working electrode within a small range around a fixed value (versus the reference electrode), independent of the cell current. The apparatus thus reduces the cell current as the electroactive substance is consumed and also follows possible changes of the cell resistance. 

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