Ammeter in series, voltmeter in parallel. Switch off between readings.
A length of resistance wire on a metre rule, a crocodile clip to change the length, and a circuit to measure the p.d. across it and the current in it. This comes up again and again in Paper 6. Watch the circuit built symbol by symbol, with the variables and the graph of R against l beside it, then build it yourself and check it against the mark points.
Past-paper referenceOriginal experiment, wording and figures, written in the style of Cambridge IGCSE Physics 0625 Paper 6 (Alternative to Practical). It is not copied from a past paper. The “what the examiners said” notes are our paraphrase of the Principal Examiner Reports for Teachers.
Practise next: 0625/62 February/March 2023, (resistance of different lengths of wire, graph and gradient) · official past papers
Practise next: 0625/63 October/November 2025, (three resistance wires, a fixed resistor to limit the current) · official past papers
Practise next: 0625/63 October/November 2024, (a resistor and a resistance wire, heating of the wire) · official past papers
Examiner reports used: 0625 March 2023, March 2024, June 2024, June 2025, November 2023, November 2024 and November 2025.
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What a full-mark diagram shows
The circuit
Battery, switch, ammeter, variable resistor and fixed resistor in one series loop, drawn with correct symbols and no gaps.
The voltmeter
Connected in parallel across the length l of wire: one lead to the zero end, the other to the crocodile clip. Never in series.
Reading the meters
Work out what one small division is worth before you read the scale. Record V and I in a table with units in the headings.
R and the graph
R = V / I to a consistent number of significant figures. Plot R against l, draw one thin best-fit line and use a large triangle for the gradient.
Habits the examiner reports keep asking for
Switch off between readings, or the wire warms up and its resistance rises.
The fixed resistor limits the current, so it cannot go past the ammeter’s full-scale reading. “Fair test” is not the reason.
Variable resistor symbol: a rectangle with an arrow through it, not a thermistor.
Measure l from the zero end of the wire to the clip, with an arrowhead at each end if you mark it on a diagram.
Sources of inaccuracy must be built into the method: the clip’s contact point is uncertain, or the wire is not uniform. Misreading a meter is just poor practice.