Impedance balance conditions

TABLE 6.7 Impedance Balance Conditions for the Two Bridge Circuits of figure 6.24... 

Traditionally, the instrument of choice for accurate conductance measurements that are relatively free of capacitance effects has been the ac Wheatstone bridge illustrated in Figure 8.14. The details of operation and the derivation of the balance condition of the ac bridge are presented in considerable detail elsewhere [16,17], The balance condition is exactly analogous to that of the dc bridge except that impedance vectors must be substituted for resistances in the arms of the bridge when reactive circuit elements are present. 

In general, two adjustable elements are sufficient to satisfy relations (16) and (17). In practice, it is desirable to have the adjustments in the resistances independent of those in the reactances. This can be achieved in a ratio bridged Let us put the unknown element in arm 1 of the bridge, allow arm 3 to have a variable impedance Z3, and choose the ratio arms 2 and 4 so that Z2/Z4 = K, where Kis some real positive number (i.e., RilR = Z2/Z4 = K). This is equivalent to having the same phase shift in legs 2 and 4, i.e., 2 = tpi It follows from Eq. (13) that the balance condition for this ratio bridge is... 

Balanced-to-g und The condition when the impedance to ground on one wire of a two-wire circuit is equal to the impedance to ground on the other wire. 

Elg. 4 The change in the capacitance of the cathode derived from AC impedance analyses during the operation represented in Fig. 2. Anode/cathode, pure H2-Nj(20%), Hj-CO2(20%), or SRG (20% COj, lOppm CO, balance)/air. The other experimental conditions are the same as for Fig. 2... 

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