Explain AC Bridges
In this article we will explain AC Bridges. Resistances can be measured by direct-current Wheatstone bridge, shown in Figure (A) for which the condition of balance is that
Inductances and capacitances can also be measured by a similar four-arm bridge, as shown in Figure (B); instead of using a source of direct current, alternating current is employed and galvanometer is replaced by a vibration galvanometer (for commercial frequencies or by telephone detector if frequencies are higher (500 to 2000 Hz)).
The condition for balance is the same as before but instead of resistances, impedances are used i.e.
Z1/Z2 = Z4/Z3 or Z1 Z3 = Z2 Z4
But there is one important difference i.e. not only should there be balance for the magnitudes of the impedances but also a phase balance. Writing the impedances in their polar form, the above
condition becomes
Z1∠φ1 . Z3∠φ3 = Z2∠φ2 . Z4∠φ4 OR
Z1 . Z3∠φ1 + φ3 = Z2 . Z4∠φ2 + φ4
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Hence, we see that, in fact, there are two balance conditions which must be satisfied simultaneously in a four – arm a.c. impedance bridge.
(i) Z1 . Z3 = Z2 . Z4 ………………. for magnitude balance.
(ii) φ1 + φ3 = φ2 + φ4 …………. for phase angle balance.
Read article – Capacitances
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