Bridge circuits are commonly used to measure many quantities in electronics such as resistance ,capacitance, inductance and admittance. A bridge circuit is like two port network with a source of excitation connected to one branch of a network and the network is adjusted until the detector indicates no output, At this point the bridge is said to be balanced, the unknown value can be determined from known values of the circuit using equations. Because the unknown values are determined on the basics on known standard values , Bridges are often considered as “comparison instruments” though all based on the simplest of all bridges is the Wheatstone bridge which measures DC resistance. Other forms of this simple circuit can be measure capacitance, inductance and complex impedances.
The basic Wheatstone bridge has been extensively used in null-type instruments. A null reading is obtained on a Wheatstone bridge by a comparison of the voltage drops is the passive resistance arms of the bridge. The primary use of the basic Wheatstone bridge is to measure the resistance value of the known resistors
R1Rx = R2R3
If we know the values R1,R2 and R3 then we can easily calculate the value of unknown resistance by using following equation
Rx = R2R3 / R1
Note: For this experiment input voltage is 10V and detector is galvanometer.