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Better Equipped Practical Teaching Guides
A complete A Level Physics practical guide for measuring the resistivity of a metal wire, with apparatus, circuit setup, micrometer technique, graph skills, uncertainty, troubleshooting, risk assessment, technician notes and exam support.
Teacher note: This resource supports practical teaching and equipment planning. Teachers should adapt the procedure and risk assessment to match their exam board specification, local laboratory rules and departmental policies.
This practical determines the resistivity of a metal wire by measuring resistance, length and cross-sectional area. Students build a simple circuit, measure potential difference and current, calculate resistance using Ohm's Law, then use a graph of resistance against length to determine the resistivity of the material.
Current flowing through a conductor experiences resistance. For a metal wire, resistance depends on the material, length and cross-sectional area. Resistivity describes how strongly the material itself opposes the flow of charge.
Key equations: V = IR | R = ρL ÷ A | ρ = RA ÷ L | A = πd² ÷ 4 for a circular wire.
A longer wire has greater resistance because electrons experience more collisions as they travel through the metal. A thicker wire has lower resistance because it has a larger cross-sectional area.
Metals contain delocalised electrons moving through a lattice of positive ions. Resistance arises from collisions between drifting electrons and vibrating ions in the lattice.
Connect the power supply, ammeter, test wire, variable resistor and switch in series. Connect the voltmeter in parallel across the test wire only.
Students should keep the current low and close the switch only briefly while taking readings. This reduces heating of the wire, which can increase resistance and affect the measured resistivity.
Use R = V ÷ I.
Example: V = 1.20 V and I = 0.40 A, so R = 3.0 ohms.
For a circular wire, A = πd² ÷ 4.
If d = 0.46 mm, convert to 4.6 × 10-4 m before calculating area.
From R = ρL ÷ A, rearrange to ρ = RA ÷ L.
Using a graph, ρ = gradient × A.
Worked example: mean diameter = 0.46 mm, area = 1.66 × 10-7 m², graph gradient = 15 ohm m-1, so resistivity = 15 × 1.66 × 10-7 = 2.49 × 10-6 ohm m.
| Length / m | Voltage / V | Current / A | Resistance / ohms |
|---|---|---|---|
| 0.20 | |||
| 0.40 | |||
| 0.60 | |||
| 0.80 | |||
| 1.00 |
Plot Resistance on the y-axis against Length on the x-axis. The expected relationship is a straight line through the origin. The gradient is R ÷ L, so resistivity can be found using ρ = gradient × A.
| Material | Typical resistivity / ohm m | Notes |
|---|---|---|
| Copper | 1.7 × 10-8 | Very low resistivity; useful for wiring. |
| Aluminium | 2.8 × 10-8 | Low resistivity and low density. |
| Constantan | 4.9 × 10-7 | Common school practical wire. |
| Nichrome | 1.1 × 10-6 | Higher resistance; used in heating elements. |
Practical conclusion: Resistance increases directly with wire length. The gradient of the Resistance against Length graph, multiplied by the wire's cross-sectional area, gives the resistivity of the material.
| Problem | Likely cause | Quick fix |
|---|---|---|
| No readings | Faulty or incomplete circuit | Check power supply, leads, switch and meter connections. |
| Unstable readings | Loose contacts | Secure crocodile clips and replace faulty leads. |
| Resistance too high | Wire heating or contact resistance | Lower the current and switch off between readings. |
| Poor graph trend | Measurement errors | Repeat readings and check lengths carefully. |
| Incorrect resistivity | Unit conversion or area calculation error | Recheck mm to m conversion and cross-sectional area calculation. |
A thicker wire has higher resistance.
Correction: a thicker wire has lower resistance because it has a larger cross-sectional area.
Resistance and resistivity are the same.
Correction: resistance depends on dimensions; resistivity is a property of the material.
One diameter reading is enough.
Correction: real wires are not perfectly uniform, so several readings are needed.
Theory introduction: 15 mins | Apparatus setup: 10 mins | Diameter measurements: 10 mins | Resistance measurements: 20 mins | Graph and calculations: 20 mins | Evaluation and discussion: 15 mins
Demonstrate the micrometer reading method, then show the correct circuit layout. Highlight that the voltmeter must be connected in parallel across the test wire and that the switch should be opened between readings.
Overall risk: low. This practical presents a low level of risk when standard laboratory procedures are followed. Schools should complete their own risk assessment according to local procedures.
| Hazard | Risk | Control measure |
|---|---|---|
| Wire heating | Minor burns | Use low currents and switch off between readings. |
| Electrical equipment | Minor shock risk | Use low-voltage power supplies and check equipment condition. |
| Sharp wire ends | Cuts or scratches | Secure wire ends and handle carefully. |
| Trip hazards from leads | Falls | Keep leads tidy and away from walkways. |
| Micrometer use | Pinching fingers | Use carefully and under supervision. |
Diameter is the largest source of uncertainty, and real wire is not perfectly uniform. Repeated readings allow a more reliable mean value.
A high current heats the wire. For metals, heating increases lattice vibrations and increases resistance.
A graph uses all the data, reduces the effect of random error and allows resistivity to be determined from the gradient.
Resistance changes with length and area. Resistivity is a property of the material itself at a given temperature.
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This guide was written and reviewed by Better Equipped's technical team, drawing on experience supplying practical science equipment to schools, colleges, laboratories and science departments throughout the UK. Our technical team include ex-school laboratory technicians and are here to support schools, colleges and laboratories across the UK. If you have feedback on this guide, please contact us.
Last reviewed and updated: July 2026
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