Key relationship
At null P/Q=R/X; X=(Q/P)R. G current follows the bridge Thevenin voltage and resistance.
Physics · Practical apparatus
Post-office-box resistance balance
Choose ratio-arm plugs, adjust the standard box and watch galvanometer direction reverse through zero. A null means the bridge midpoint potentials agree. Use the chosen ratio to calculate the unknown.
Open the interactive experiment →Post-office-box resistance balance
விகிதக் கை மதிப்புகளைத் தேர்ந்து மின்தடை பெட்டியைச் சரிசெய்து கால்வனோமீட்டர் பூஜ்யத்தைத் தேடவும்.
Key notes, relationships and model limits
Practical tip
Choose ratio-arm plugs, adjust the standard box and watch galvanometer direction reverse through zero. A null means the bridge midpoint potentials agree. Use the chosen ratio to calculate the unknown.
Common trap
Do not report the candidate(Q/P)R while the galvanometer is still deflected.
Model limits
Ideal Wheatstone topology with100 Ω galvanometer resistance and fixed ratio arms. Unknown is a virtual specimen control; candidate value is valid only at null. Contact/lead resistance omitted.
What you will learn
Determine a virtual unknown using a post-office-box null.
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
P100,Q100,R500 gives X500 Ω; P100,Q1000,R50 gives the same X500 Ω.
Why this matters
The model connects post-office-box resistance balance with an observable reading. Check the equation and assumptions before applying it.
Before you start
SI units · algebra · graph reading
Check your understanding
At a true bridge null, galvanometer current is?
- Maximum
- Zero
- Always negative
Read the explanation
Its two endpoints are at equal potential.
Course connection: Virtual apparatus preparation. Check exact board/exam membership in the syllabus sources. Physical practical work remains necessary.