← Back to Lab
Physics · 4.2 Electrical quantities · Paper 6 practical

Resistance of a wire. Measure it.

Investigate how the length of a resistance wire affects its electrical resistance. Close the switch briefly, read the voltmeter and ammeter, calculate R = V/I, then plot R against l.

0625 Topic 4.2.5 — Resistance 0625 Topic 4.3.1 — Circuit components Paper 6 — ATP
Setup — slide the jockey along the wire to choose a length, then close the switch to read V and I.

Drag the jockey along the wire. Shortcuts Space close/open switch · Enter record · R reset.

Circuit & wire

50
0.20
3.0

Live readouts

Voltmeter V
0.00 V
Ammeter I
0.00 A
Resistance R = V / I
— Ω
Length l
50.0 cm
Switch off the circuit between readings so the wire does not heat up.

Trial data — vary l, measure R

Close the switch and press Record reading at five lengths (20, 40, 60, 80, 100 cm).

R vs l — gradient = R per cm

📋 Method (Cambridge ATP procedure)
  1. Connect the power supply, ammeter, test wire, and switch in a single series loop.
  2. Connect the voltmeter in parallel across the wire, one lead at the 0 cm mark and the other on a sliding jockey.
  3. Move the jockey to l = 20.0 cm.
  4. Close the switch briefly. Read the ammeter (I) and voltmeter (V); open the switch as soon as you have the readings to prevent the wire heating up.
  5. Calculate the resistance at this length: R = V / I.
  6. Repeat for l = 40, 60, 80 and 100 cm.

Analytical control: plot R (y) against l (x). A straight line through the origin shows that R ∝ l; the gradient is the resistance per unit length of the wire.

⚠ Sources of error & precautions
  • Wire heating — current heats the wire, raising its resistance. Open the switch between readings and keep the current small.
  • Contact resistance at the jockey — press the jockey down firmly; clean the wire with emery cloth before starting.
  • Zero error on meters — check that ammeter and voltmeter read zero with the switch open before recording.
  • Parallax on the metre rule — view perpendicular to the scale when reading the jockey position.
  • Uniform wire — assume the wire has a constant cross-sectional area along its length.
🧪 Apparatus list
  • Low-voltage DC power supply (or cell with switch)
  • Ammeter (in series, ± 0.01 A resolution)
  • Voltmeter (in parallel, ± 0.01 V resolution)
  • Length of bare resistance wire (Constantan or Nichrome), ≥ 100 cm
  • Metre rule taped under the wire
  • Sliding jockey or crocodile clip
  • Connecting leads and switch
🎯 Syllabus reference (0625)
  • 4.2.5 Resistance — recall and use the equation R = V / I; describe an experiment to determine the resistance of a metallic conductor using a voltmeter and an ammeter; state that for a metallic conductor at constant temperature, current is directly proportional to potential difference.
  • 4.2.6 Electrical working — explain why a wire warms when a current flows through it.
  • Paper 6 — set up a circuit from a diagram; record results in a table; plot R against l; calculate the gradient using a large triangle on the line of best fit.

Ask the lab assistant