Answer: This is the best commonest method of measuring medium resistance. It is popular for laboratory use. The circuit diagram of typical wheastone's bridge is shown below.

The source of emf E is connected between points A and B . Galvanometer G is connected between points C and D . The resistors $R_1 , \ R_2 \ , R_3 \ $ are known resistance and $R_x$ is unknown resistor to be measured. When switch is closed , current flows through the circuit and gets divided as $I_1 , \ I_2$  at point A . The current $I_3 \ and \ I_4 $ flows through resistor $R_3 \ and \ R_x$ respectively. The bridge is said to be balanced if no current flows through the galvanometer. 

When bridge is balanced, $I_1R_1=I_2 R_2$

$\therefore I_1=I_3= \dfrac{E}{R_1+R_3}$

Similarly, $I_2=I_4= \dfrac{E}{R_2+R_x}$

Substituting all the equations , we get

$\dfrac{E\times R_1}{R_1+R_3}=\dfrac{E\times R_2}{R_2+R_x}$

$\dfrac{ R_1}{R_1+R_3}=\dfrac{ R_2}{R_2+R_x}$

$\therefore R_1R_2+R_1R_x=R_1R_2+R_2R_3$

$\therefore R_1R_x=R_2R_3$

$\therefore R_x= \dfrac{R_2R_3}{R_1}$

For measurement of unknown resistor , $R_3$ is adjusted such that the bridge is balanced and galvanometer current is zero. Then by using the above equation , we can calculate the value of unknown resistance.