Bridge Measurement Circuits

A measurement bridge determines the value of a component by comparing it against known standards rather than measuring it directly. The classic null method arranges four impedances in a diamond, applies a source across one diagonal and connects a detector across the other. When the products of opposite arms are equal the bridge is balanced and the detector reads zero. Finding this null lets an unknown component be calculated from the known ones, and because the result depends on a balance condition rather than on an absolute reading it can be very accurate.

For direct current the Wheatstone bridge measures resistance, comparing an unknown resistor against three known resistors. A refinement, the Kelvin double bridge, is designed for very low resistances where the resistance of leads and contacts would otherwise distort the result. These DC bridges are the foundation on which the alternating-current versions build.

When the bridge is driven by an alternating source and uses capacitors and inductors as well as resistors, it can measure reactive components and their imperfections. The Wien bridge balances at a frequency set by its components and is used both to measure capacitance and frequency and as the frequency-determining network in oscillators. The Maxwell bridge measures inductance by balancing it against a capacitor and resistors, while the Schering bridge measures capacitance and the loss in dielectrics, making it valuable for insulation testing. In each case balance requires matching both magnitude and phase, so two adjustments are made together.

Frequently asked questions

Why is a bridge more accurate than a direct measurement?

A bridge compares an unknown against known standards and is balanced to a null. The result depends on that balance and on the known components, not on reading an absolute value.

What does the Maxwell bridge measure?

The Maxwell bridge measures inductance by balancing an unknown inductor against a known capacitor and resistors in an AC bridge, avoiding the need for a standard inductor.

What is the Kelvin double bridge for?

It is a variant of the Wheatstone bridge designed to measure very low resistances accurately, by arranging the connections so lead and contact resistance does not corrupt the result.