Uniform metre bridge wire
Provide a 100.0 cm wire whose resistance is proportional to length.
Physics / Metre bridge
FREE TO EXPLOREEXPERIMENT 08 · PHYSICS LABORATORY
Unknown resistance using a metre bridge
Determine an unknown resistance using balance lengths on a metre bridge and a straight-line graph. Explore a virtual apparatus, collect readings and analyse a graph.
CHANGE · OBSERVE · MEASURE
Your virtual workbench.
NULL BALANCE LENGTH L—
MEASUREMENT STATUS
Ready for a virtual observation.
Move the jockey to a null. Arrow keys change length by 0.1 cm.
Adjust the apparatus, then take a reading.
Readings are generated by an ideal model. The diagram is schematic and is not to scale.
Simulation assumptions and limits
The simulated wire is exactly uniform and 100.0 cm long. End resistances, jockey resistance, heating, thermoelectric offsets and galvanometer sensitivity are omitted. The unknown resistor is fixed at 5 Ω and known resistance settings are exact. Balance lengths are rounded to 0.1 cm; resulting fit scatter reflects quantization rather than a complete measurement uncertainty. The model places unknown R = 5 Ω on the left and known S on the right. It computes l = 100R/(R + S), rounded to 0.1 cm. The fitted graph uses x = S and y = (100 − l)/l, so resistance is 1/gradient. Range endpoints are available for sensitivity exploration; real measurements should favor central balances.
Know your apparatus.
Unknown resistor in the left gap
Connect the resistor R to the same side as the zero end used to measure balance length.
Resistance box in the right gap
Set the known resistance S in ohms.
DC cell, key, galvanometer and jockey
Detect a null using brief light contact; a physical galvanometer may require protective resistance.
FOLLOW THE METHOD
A reason behind every step.
Determine an unknown resistance using balance lengths on a metre bridge and a straight-line graph. Suggested independent settings: 2.5, 4, 5, 7.5, 10 Ω.
Fix the orientation
Connect unknown R in the left gap and known S in the right gap. Mark the wire zero at the unknown side and use L = 100.0 cm.
Why this step matters
The resistance-length equation depends on the side assignment.
Set a known resistance
Select S, starting near 5 Ω in this model to place the expected null near the midpoint.
Why this step matters
Central balance points are less sensitive to length reading error.
Find the null
Close the cell key and touch the jockey lightly at trial positions until galvanometer deflection passes through zero. In this simulation request the model balance reading.
Why this step matters
A null compares potentials without drawing detector current at balance.
Record and repeat
Record S and l to the shown resolution. Collect at least five distinct S values, preferably keeping physical balances around the central portion.
Why this step matters
Several balances support a graph and allow comparison of separate R estimates.
Calculate graph variables
For each reading calculate L − l and y = (L − l)/l. Fit y against S with a free intercept.
Why this step matters
This transformation makes the reciprocal unknown resistance the gradient.
Interpret the result
Calculate R = 1/m in Ω and compare individual values S l/(L − l). In real apparatus interchange the gap resistors and use the reversed formula, then discuss end effects and uncertainties.
Why this step matters
Consistency across settings and orientations provides a stronger check than one balance.
YOUR NOTEBOOK · 0 OBSERVATIONS
Turn measurements into a result.
Points are your observations. The fit uses SI x and y values before display scaling.
LINEAR FIT · y = mx + b
R/S = l/(100 − l); (100 − l)/l = mS + b; R = 1/m
| # | Known resistance S (Ω) | Null balance length l (cm) | Known resistance S (Ω) | Length ratio (100 − l)/l | Remove |
|---|
No readings yet. Take a virtual measurement and record it.
Calculate it yourself.
CHECK YOUR UNDERSTANDING