A.C. bridges form the most convenient method of measuring effective resistance, self-inductance, mutual inductance, capacitance, and other associated quantities (such as the Q-factor of coils and loss angles of capacitors) at frequencies up to the lower radio-frequency range. A great many a.c. bridges are in use, dependent upon the measurements to be made and the equipment available for measuring, but they all have a common origin in the d.c. Wheatstone bridge. A.C. bridges, however, are more complicated to use and greater care is necessary to avoid errors, mainly because two quantities are usually required (for example, both the resistance and the inductance of a coil). Two variable standard components are therefore required in the bridge network, because two conditions must be satisfied simultaneously in order to achieve balance. It is an advantage to choose an a.c. bridge in which the two balance conditions are independent of each other, because then a true balance of the bridge can be rapidly obtained by alternate adjustments of the two variable standard components. The bridges dealt with in this chapter are of this nature.
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