Key relationship
At null P/Q=R/X; uniform metre wire X/R=ℓ/(100−ℓ).
Physics · Class 12
Wheatstone and metre bridge
Find zero detector voltage in the bridge. A uniform metre wire turns a resistance ratio into a length ratio. Ratio arms in a post-office box use the same null principle.
Open the interactive experiment →Wheatstone and metre bridge
வீட்ஸ்டோன் சமநிலையில் கண்டறிவி மின்னோட்டம் பூஜ்யம். மீட்டர் கம்பியில் நீள விகிதம் மின்தடை விகிதம்.
Key notes, relationships and model limits
Remember
Find zero detector voltage in the bridge. A uniform metre wire turns a resistance ratio into a length ratio. Ratio arms in a post-office box use the same null principle.
Common trap
The bridge ratio is justified at zero galvanometer deflection, rather than at an arbitrary slider position.
Model limits
Ideal voltmeter detector, uniform metre wire and negligible contact resistance. Wheatstone and metre readouts represent two alternative apparatus arrangements.
What you will learn
Use the controls and plotted relation to explain wheatstone and metre bridge.
Try the experiment
Try both scenarios, change one control, and predict the result before checking the graph and readouts.
Think of it this way
Change one input while keeping the others fixed, as you would in a controlled experiment.
Worked example
If R=20 Ω and the metre balance length is 60 cm, X=20×60/40=30 Ω.
Why this matters
The model connects wheatstone and metre bridge with an observable reading. Check the equation and assumptions before applying it.
Before you start
SI units · algebra · graph reading
Check your understanding
At balance, equal ratio arms P=Q imply?
- X must be twice R.
- The compared resistances R and X are equal.
- X must be half R.
Read the explanation
The compared resistances R and X are equal.
Course connection: Class XII Chapter 3. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.