Alternating current bridge

Fig. 19. Alternating current bridge circuit for the measurement of electrical conductivity.

For a full discussion see B. Hague, Alternating Current Bridge Methods, 2nd. ed. Sir Isaac Pitman and Sons Ltd., London (1930). 

The measurements of the dielectric characteristics of composites were carried out at a frequency of lO Hz using an alternating current bridge P-5058. The specimens prepared had a disc-like shape with a diameter equal to the diameter of the electrode (15-18 mm). The thickness of the specimen was about 1 mm. The temperature dependencies of the dielectric permeability (e) and of the tangent of the dielectric losses (tan 8) were found at the heating rate 3-4 K/min. 

R. D. Armstrong, W. P. Race, and H. R. Thirsk [ 1968] Determination of Electrode Impedance over an Extended Frequency Range by Alternating-Current Bridge Methods, Electrochim. Acta 13,... 

B. Hague and T. R. Foord, Alternating Current Bridge Methods, 6th ed.. Pitman, Bath, U.K., 1971. 

For the experimental determination of the impedance of a measured system the alternating current bridge is the most accurate method. This method, utilizing a... 

In single-phase bridge circuits for ac connections and for very low ac output voltages below 5 V, single-phase center tap circuits are used as rectifier circuits for CP transformer-rectifiers. They have an efficiency of 60 to 15% and a residual ripple of 48% with a frequency of 100 Hz. A three-phase bridge circuit for three-phase alternating current is more economical for outputs of about 2 kW. It has an efficiency of about 80 to 90% and a residual ripple of 4% with a frequency of 300 Hz. The residual ripple is not significant in the electrochemical effect of the protection current so that both circuits are equally valid. 

To avoid the errors of polarisation and stray currents, special resistivity meters are employed. One form of these uses an alternating current produced from batteries by a vibrator. The effective resistance is measured by a modified Wheatstone bridge with balance indicated by a galvanometer. 

A Wheatstone bridge, modified for use with alternating current, forms the basis of the measuring system. Usually two arms of the bridge are matched resistors of approximately 1,000 ohms, the third arm is the conductance cell and the fourth... 

The checkers measured these resistances with a Serfass Conductivity Bridge, Model ROM 15, employing a 1000-Hz. alternating current. 

There are two types of conductometric procedures commonly used. Firstly, a Wheatstone Bridge circuit can be set up, whereby the ratio of the resistance of unknown seawater to standard seawater balances the ratio of a fixed resistor to a variable resistor. The system uses alternating current to minimise electrode fouling. Alternatively, the conductivity can be measured by magnetic induction, in which case the sensor consists of a plastic tube containing sample seawater that links two transformers. An oscillator establishes a current in one transformer that induces current flow within the tube, the magnitude of which depends upon the salinity of the sample. This in turn induces a current in the second transformer, which can then be measured. This design has been exploited for in situ conductivity measurements. 

There is much to think about here. If one wishes to measure the dielectric constant of a liquid, not a conducting ionic solution, one simply uses an alternating current (ac) bridge containing a capacitor in one of the arms. Then the capacitance is measured in the presence of the liquid, the dielectric constant of which is to be measured, and then without it, i.e., in the presence of air. Since the dielectric constant is near to unity in the latter case, this gives rise to knowledge of the dielectric constant of the liquid because the capacitance of the cell in the bridge arm increases as the dielectric constant increases. 

The Parker effect is absent from cells with dipping electrodes, such as in cells of the flask type there are other sources, of error, however, as was pointed out by Shedlovsky. In the cell represented diagram-matically in Fig. 15, I, the true resistance of the solution between the electrodes is 72o, and there is a capacity Ci between the contact tubes above the electrolyte, and a capacity C2 in series with a resistance r between those parts immersed in the liquid the equivalent electrical circuit is shown by Fig. 15, II. When the cell is placed in the arm of a Wheatstone bridge it is found necessary to insert a resistance R and a capacity C in parallel in the opposite arm in order to obtain a balance (cf. p. 33) it can be shown from the theory of alternating currents that... 

Design of the A.C. Bridge.— Strictly speaking the condition of balance of the Wheatstone bridge given by equation (7) is applicable for alternating current only if /2i, R, Rz and R are pure resistances. It is um... 

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