Ohm's law: V–I characteristics of a resistor
Build the classic ammeter-voltmeter circuit, vary the current with a rheostat, and show that V/I is constant for a fixed resistor at constant temperature.
WHAT YOU'LL LEARN
- ✓Wiring an ammeter in series and voltmeter in parallel — and why they go there
- ✓Using a rheostat to sweep current smoothly
- ✓Plotting a V–I graph and reading resistance from the gradient
ON YOUR BENCH
- □2 × 1.5 V cells in holder
- □Fixed resistor (nominally 10 Ω)
- □Ammeter (0–1 A), voltmeter (0–5 V)
- □Rheostat, switch, connecting leads with crocodile clips
- □Graph paper
The protocol, step by step
The same guide is printed and waiting at your bench.
- 01
Wire the series loop
Connect cells → switch → ammeter → rheostat → resistor and back to the cells in one loop. Keep the switch open while wiring.
- 02
Add the voltmeter
Connect the voltmeter in parallel across the resistor only — not across the rheostat or the cells. It measures the potential difference across just the component you're studying.
- 03
Check zero errors
With the switch open, both meters should read zero. Adjust the zero screws if not, and record the correction.
- 04
Take readings
Close the switch, set the rheostat for the smallest current, and record I and V. Move the rheostat to raise the current in roughly equal steps — take 6–8 pairs of readings. Open the switch between readings so the resistor doesn't heat up.
- 05
Plot and conclude
Plot V (y-axis) against I (x-axis). Draw the best-fit straight line through the origin and calculate the gradient — that's your resistance.
What you should see
- →A straight line through the origin: V ∝ I, which is Ohm's law for a metallic conductor at constant temperature.
- →Gradient within ~5% of the resistor's marked value; the small difference is mostly meter resistance and contact resistance at the crocodile clips.
- →If your line curves upward at high current, the resistor is heating — that's not an error, that's physics. Note it in your conclusion.
| I /A | 0.10 | 0.15 | 0.20 | 0.25 | 0.30 |
|---|---|---|---|---|---|
| V /V | 1.02 | 1.51 | 2.03 | 2.49 | 3.05 |
⚠ BEFORE YOU START
- •Open the switch between readings — a continuously loaded resistor gets hot enough to burn.
- •Never connect the ammeter in parallel across the cells; its low resistance makes that a short circuit.
Try it, then run it for real.
Practise the whole thing on the virtual bench, then book real lab time and do it with your own hands.