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The Ohm's law practical looks like the simplest experiment in Class 12: connect a wire, read a voltmeter and an ammeter, plot a straight line. Getting a good value of resistivity from it is harder, because the result depends on a diameter of a fraction of a millimetre and on keeping the wire cool. This post covers the method, works a full set of sample readings, and shows which measurement decides your error.
Which syllabuses include it?
The CBSE Physics curriculum for 2026-27 lists, as experiment 1 in Section A of the Class 12 practicals: "To determine resistivity of two / three wires by plotting a graph for potential difference versus current." The JEE (Main) 2026 syllabus includes "The resistance of a given wire using Ohm's law" under Experimental Skills. The JEE (Advanced) 2026 Information Brochure lists "Verification of Ohm's law using voltmeter and ammeter" among experiments for error analysis. Check the documents for your year.
What is the principle?
Experiment 1 of the NCERT Class 12 physics lab manual combines two relations:
Ohm's law: V = IR, as long as the wire's physical state, including its temperature, does not change.
For a uniform wire, R = ρl/A, where ρ is the resistivity of the material.
So V against I is a straight line through the origin whose slope is R (or 1/R if you put V on the horizontal axis). Then ρ = RA/l, with A = πd²/4 from the wire's diameter.
The circuit: battery, key, rheostat, ammeter and the wire in series, with the voltmeter across the wire alone. The rheostat changes the current.
A worked set of readings
Sample numbers for an alloy wire. Length between the voltmeter terminals l = 1.000 m. Diameter from a screw gauge, mean of several readings: d = 0.25 mm.
V = 0.40 V, I = 40 mA
V = 0.80 V, I = 81 mA
V = 1.20 V, I = 119 mA
V = 1.60 V, I = 160 mA
V = 2.00 V, I = 201 mA
V = 2.40 V, I = 239 mA
The points lie close to a straight line through the origin. The best-fit slope is 10.0 V per ampere, so R = 10.0 Ω.
Area: A = π(0.25 × 10⁻³)²/4 = π × (1.25 × 10⁻⁴)² = 4.91 × 10⁻⁸ m².
Resistivity: ρ = RA/l = 10.0 × 4.91 × 10⁻⁸ / 1.000 = 4.9 × 10⁻⁷ Ω m.
The NCERT chapter on current electricity puts the resistivities of metals in the range 10⁻⁸ to 10⁻⁶ Ω m, so the result is in the right range for a metal alloy.
Which measurement decides the error?
The manual estimates the error in R as ΔR/R = ΔV/V + ΔI/I. With least counts of 0.05 V and 1 mA (sample values) at the top reading, that is 0.05/2.40 + 1/239 ≈ 2.5 percent.
Now the diameter. A screw gauge with least count 0.01 mm on d = 0.25 mm is a 4 percent error, and since A depends on d², it becomes 8 percent in ρ. The length, 1 mm in 1000 mm, adds only 0.1 percent.
So the honest result is ρ = (4.9 ± 0.5) × 10⁻⁷ Ω m, and most of that uncertainty comes from the diameter. Two habits help:
Measure d at several places along the wire, in two perpendicular directions, and average. Correct for the screw gauge's zero error.
Use a longer wire if you can. It does not reduce the d error, but it keeps R large enough to read V and I precisely.
Why does heating spoil the line?
The manual's procedure says to remove the key between readings and let the wire cool if it has heated. Resistance changes with temperature, so a warm wire gives a higher V/I at large currents and the line curves upward. NCERT notes that alloys like nichrome, manganin and constantan have resistivities that change very little with temperature, which is one reason wires like these are used in standard resistors. Even so, keep the current modest and the key closed only while reading.
Common mistakes and how to avoid them
Ammeter across the wire, or voltmeter in series. The manual's first precaution: voltmeter in parallel, ammeter in series, current entering at the + terminals.
Radius used as diameter. Using d where d/2 belongs makes A, and ρ, four times too big.
Measuring the whole wire. l is the length between the voltmeter's terminals, not including the ends wound round them, as the manual points out.
Forcing the line through the first point. Draw the best line through all points, then take the slope from two widely spaced points on it.
Unit slips. Convert mA to A and mm to m before calculating.
For what resistivity means at the level of electrons, see our post on drift velocity and resistivity. To compare with the bridge method, see metre bridge and potentiometer.
A practice routine
Recompute R and ρ from the sample readings, and the 8 percent error from the diameter.
Redo the calculation with d = 0.24 mm and d = 0.26 mm to see how far ρ moves.
Draw the circuit from memory, marking + terminals on both meters.
In the lab, take one reading again at the end to check the wire has not drifted with heating.
If a reading or a slope does not behave as you expect, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
Before the practical, measure the diameter of any thin wire at home with a screw gauge, or note what least count you would need.
