Meter-Bridge Balance and Data Consistency
For a uniform meter-bridge wire at a true null point l measured from the end adjacent to resistance R, the gap-resistance ratio is R/S = l/(100-l), subject to the actual circuit orientation and negligible end corrections.
Why this shows up in the exam
Unknown-resistance measurement · Detecting an inconsistent observation-table row · Comparing resistance changes
Learn the idea
At meter-bridge null, the resistance ratio equals the ratio of the two wire lengths. A uniform bridge wire acts as two resistors cut by the jockey. At zero galvanometer current, corresponding potential drops match, so geometry replaces a direct current measurement.
🧠 Memory hook: At null, resistance ratio follows adjacent length ratio.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- R/S = l/(100-l) — balance relation when l is measured from the R end
- R = S l/(100-l) — unknown resistance for the stated orientation
How to approach it
- 1Label both gaps and the measured end
- 2Write the ratio before substituting
- 3Check each table row for the same inferred unknown
Common slip-ups that cost marks
- •Using l/(100-l) without checking which resistor is adjacent
- •Using a non-null reading in the balance formula
- •Ignoring implausible end-point balance lengths
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
In a meter bridge, resistance 2 ohm is in the left gap and balance occurs at 40 cm from the left end. Find the right-gap resistance.
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