Practical diagram for meter bridge is as shown in figure.
(a) G deflects zero, when `l_(1)=20cm` and `R=100Omega` then find X.
(b) If R and X are interchanged then find `l_(1)` for zero deflection.

Explanation

(a) `l_(1)=20cm, l_(2)=100-l_(1)=100-20=80cm,R=100Omega`
`(P)/(Q)=(R)/(S)rArr(l_(1))/(l_(2))=(R)/(X)rArr(20)/(80)=(100)/(X)rArrX=400Omega`
(b) `(l_(1))/(100-l_(1))=(X)/(R)=(400)/(100)rArrl_(1)=80cm` or R and X interchane then `l_(1),l_(2)` interchanged.