Simulation · Physics · Class 12
Metre bridge: find the unknown resistance
From the lesson Wheatstone bridge in Current Electricity. Change the values and watch what happens.
Metre bridge: find the unknown resistancePhysics · Class 12
The idea behind it
NCERT § "Wheatstone Bridge"
- Four resistors form a closed loop; a battery is connected across one pair of opposite corners and a galvanometer across the other pair.
- The bridge is balanced when no current flows through the galvanometer; the two ends of the galvanometer are then at the same potential.
- Balance condition, using our own labels: the battery is across corners A and C; path A→B→C has P (from A to B) then Q (B to C), and path A→D→C has R (A to D) then S (D to C); the galvanometer joins B and D. The bridge is balanced when P/Q = R/S, which is the same as PS = QR (products of opposite arms equal).
- Whatever letters a figure uses, check the condition this way: take the two resistors meeting at one battery corner; each one's ratio to the resistor that follows it on its own path must be the same for both paths.
- At balance, the galvanometer branch (or any resistor put in its place) can be removed, and the network reduces to two parallel series-pairs.
- The balance condition does not depend on the emf of the battery or the resistance of the galvanometer.
- If the ratios are not equal, the bridge is unbalanced and the middle resistor carries current; then Kirchhoff's rules must be used.
- The bridge lets an unknown resistance be found from three known ones by adjusting one of them until the galvanometer shows no deflection.