Wheatstone bridge technique

Fig. 21 The variation of the balancing tunneling current of the four branches four electrodes monomolecular Wheatstone bridge connected as presented in (a). In (b), the dashed line is for the current intensity 7W (in absolute value) measured by the ammeter A and deduced from the standard Kirchoff laws calculating each molecular wire tunneling junction resistance of the bridge one after the other from the EHMO-ESQC technique. In (b), Hie full line is the same tunnel current intensity but obtained with the new intramolecular circuit rules discussed in Sect. 2. (c) The resistance of the branch used to balance the bridge as a function of its rotation angle. The minimum accessible resistance by rotation is 78 MQ for the short tolane molecular wire used here...

Bridge methods are also a standard technique for conductivity measurements. The basic Wheatstone bridge circuit is illustrated in Fig. 18. When the bridge is balanced, i.e. the detector D indicates zero current, then the unknown resistance is given by... 

The specific conductance of a water sample provides a simple method to determine the total dissolved ionic solids present in the sample. It is also an inexpensive technique, which lends itself to continuous monitoring of a river or waste stream for the total ion content (Fig. 4.1), and can be easily used to check the accuracy of analyses conducted for specific ions. Specific conductance is measured via a pair of carefully spaced platinum electrodes, which are placed either directly in the stream to be measured or in a sample withdrawn from it [22]. The water temperature should be 25 °C, or the result corrected to this temperature. Voltages in the 12 to 14 range, and frequencies of 60 to 1000 Hz AC are used, plus a Wheatstone bridge circuit to obtain a conductivity reading in xmho/cm or xS/cm (microsiemen/cm). The response obtained is linear with the total ion content over a wide range of concentrations (Fig. 4.1). Examples of the conductance ranges and seasonal variation of some typical Canadian rivers are... 

The conventional TCD is configured with the filaments being connected to form a Wheatstone bridge. A property of the Wheatstone bridge is common mode rejection of the noise which is primarily due to the electronics (l.e. power supply stability and the amplifier circuit). The TCD noise spectrum resembles white (shot) noise rather than the 1/f (flicker) noise of ionization detectors. Modulation techniques for noise rejection of white noise is no better than a simple Wheatstone bridge. 

Weigh cells or load cells are typically used to measure the mass of the contents of a vessel. These are electromechanical devices which convert force or weight into an electrical signal. The technique is to construct a wheatstone bridge similar to that used in the RTD circu it with one resistor being a rheostat which changes resistance based on load. 

The method most widely employed to measure the resistance of electrolyte solutions is the a.c. Wheatstone bridge together with subsidiary equipment to amplify and detect the output signal of the bridge. Shedlovsky has reviewed the experimental techniques and the equipment employed for the measurements. 

Dielectric measurements have developed Ifom cumbersome Wheatstone-bridge measurements to an efficient, precise, and rapid spectroscopic technique. The new technique dielectric spectroscopy soon found interesting applications within the field of eolloid chemistry. The technique can be applied as a preeision method in a thorough mapping of the static and dynamic properties of colloidal systems. Industrially, dielectric measurements ean be utilized in the online eharaeterization of sueh eomplex systems. 

Ebulliometry. Ebulliometry (34,42-47) is another technique for determining the depression of the solvent activity by the solute. In this case the elevation of the boiling point is determined. The boiling-point elevation ATb is measured with sensitive thermocouples or matched thermistors in a Wheatstone bridge. The molecular weight Mn is calculated from... 

This detector is used with ion chromatography only. The conductivity in the flow cell is measured by a Wheatstone bridge coupling, with pulsed techniques to avoid formation of electrical double layers at the electrodes. 

The Wheatstone bridge shown in Figure 3.16 can be used to determine the electrolyte resistance (/I,). This is a classical technique reviewed by Braunstein -Robbins [42] who provided clear details on the current topic. Moreover, only one Wheatstone bridge case is briefly described hereafter. 

The AC impedance technique coupled to the complex plane method of analysis is a powerful tool to determine a variety of electrochemical parameters. To make the measurements, instrumentation is somewhat more complex than with other techniques. It requires a Wheatstone bridge arrangement with series capacitance and resistance in the comparison arm, a tuned amplifier/detector, and an oscillator with an isolation transformer. A Wagner ground is required to maintain bridge sensitivity, and a suitably large inductance should be incorporated in the electrode polarization circuit to prevent interference from the low impedance of this ancillary circuitry. Sophisticated measurement instruments or frequency response analyzers with frequency sweep and computer interface are currently available such as the Solartron frequency response analyzers. Data obtained can be analyzed or fitted into proper equivalent circuit using appropriate software. 

Nernst applied the electrical bridge invented by Wheatstone to the measurement of the dielectric constants for aqueous electrolytes and different organic fluids. Nemst s approach was soon employed by others for measurement of dielectric properties and the resistance of galvanic cells. Finkelstein applied the technique to the analysis of the dielectric response of oxides. Warburg developed expressions for the impedance response associated with the laws of diffusion, developed almost 50 years earlier by Fick, and introduced the electrical circuit analogue for electrolytic systems in which the capacitance and resistance were functions of frequency. The concept of diffusion impedance was applied by Kruger to the capacitive response of mercury electrodes. ... 

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