In a bridge circuit, why are two test coils often employed?

In a bridge circuit, employing two test coils serves a specific purpose related to obtaining accurate measurements by comparing an unknown sample to a known standard. The setup allows for differential measurements, meaning that the bridge can detect subtle variations in electrical properties between the tested sample and the standard. The principle of null detection is applied, where ideally no current flows through the detector when the two coils are balanced. This balance indicates that the test sample has the same characteristics as the standard.

Using two coils increases the sensitivity and accuracy of the measurement. By systematically comparing against a known standard, any changes or discrepancies can be identified with greater clarity. This is particularly important in non-destructive testing methods like eddy current testing, where precise measurements are key to identifying defects or material properties related to thickness, conductivity, or permeability.

The other options do not align with the primary function of employing two test coils in a bridge circuit. They either relate to effects that are not typically mitigated by such means or do not directly address the comparative measurement process central to the use of a bridge circuit in testing scenarios.