Meaning and mechanism
The Wheatstone bridge compares the intermediate voltages of two voltage-dividing paths. An ammeter measures the potential difference between two intermediate points and displays it as current.
If you adjust the resistance to equalize the intermediate voltage, no current flows in the ammeter branch. However, there is no current flowing from the main source to another branch.
Relationships and conditions
In the experimental setup, the equilibrium condition is R₁/R₂=R₃/Rx. If you know the resistance values of three resistors, you can calculate the unknown resistance Rx using this equation.
The balance equation is applied by verifying the connection positions and names of the resistors. For actual zero measurements, contact resistors, instrument sensitivity, and temperature variations all have an impact.
What to observe in the simulation
At balance R₁/R₂ = R₃/Rx and the middle nodes have equal potential.
The source and resistors are ideal, with no contact resistance or galvanometer zero error. Experiment on screen; do not reproduce this with real power supplies, outlets or high voltage.
Key relationships and symbols
R₁/R₂ = R₃/Rx · Ig = (Vl−Vr)/Rg
| Symbol | Meaning | Unit |
|---|---|---|
| Rx | Unknown resistance to determine | Ω |
| Ig | Galvanometer current between the middle nodes | A |
Worked example
With 4 V at the left node and 4.5 V at the right, what is the sign of left-to-right galvanometer current?
Ig < 0
Common misconceptions
- The current flowing through the entire circuit is not necessarily 0 when the galvanometer reads 0.