The document discusses the Wheatstone bridge, which is a circuit used to measure an unknown electrical resistance. It consists of two known resistors, one unknown resistor, and one variable resistor connected in a bridge formation. The bridge is balanced when the ratio of resistances equals the ratio of opposing arm resistances, allowing the unknown value to be determined. Applications include precise low resistance measurement and converting other parameters like temperature, strain, impedance into resistance values for analysis. Limitations are insensitivity for high resistances and resistance changes due to heating effects in the circuit.