Resistance of a Wire: The Required Practical Explained

In this required practical you connect different lengths of a thin wire into a circuit, measure the potential difference across the wire and the current through it, and calculate resistance using R = V ÷ I. Plotting resistance against length gives a straight line through the origin: resistance is directly proportional to length.

What circuit do you need?

The circuit is a single series loop containing the power supply, an ammeter and the test wire, with a voltmeter connected across the test wire.

• Power supply or cell — a low voltage, typically 1.5 V to 6 V.
• Ammeter, connected in series with the test wire, so it measures the current flowing through it.
• Voltmeter, connected in parallel across the test wire only, so it measures the potential difference across that wire and nothing else.
• Test wire, usually constantan or nichrome, taped flat along a metre ruler.
• Two crocodile clips or flying leads to connect to the wire at whatever length you are testing.
• A switch, so the current only flows while you are actually taking a reading.

Getting the meters the right way round is worth a mark on its own: ammeter in series, voltmeter in parallel. A voltmeter placed in series would block almost all the current; an ammeter placed in parallel across the wire would short it out.

What is the method?

1. Tape the test wire alongside a metre ruler, so lengths can be read directly and the wire stays straight.
2. Set up the circuit as above, with the switch open.
3. Attach the crocodile clips so that the length of wire in the circuit is your first value — for example 100 cm.
4. Close the switch, take the ammeter and voltmeter readings quickly, then open the switch again. This is the single most important instruction in the practical, and the next section explains why.
5. Calculate the resistance for that length using R = V ÷ I.
6. Move one crocodile clip to shorten the wire — for example to 90 cm, 80 cm, and so on down to 10 cm — repeating steps 4 and 5 at each length.
7. Repeat the whole set of readings at least three times and take a mean resistance for each length.
8. Plot resistance (y-axis) against length (x-axis) and draw a line of best fit.

Why must the wire not be left switched on?

Because current heats the wire, and a hotter wire has a higher resistance. The metal ions vibrate more when heated, so electrons collide with them more often as they drift through the wire.

If the wire is left connected between readings, the later readings are taken at a higher temperature than the earlier ones. Temperature would then be a second variable changing alongside length, and the investigation would no longer be a fair test. Opening the switch between readings keeps the wire near room temperature throughout.

Many schools also place a fixed resistor in series to keep the current low, for the same reason and for safety — a thin wire carrying a large current can become hot enough to burn.

How do you calculate resistance from the readings?

Resistance is the ratio of potential difference to current:

$$R = \frac{V}{I}$$

where R is resistance in ohms (Ω), V is potential difference in volts (V) and I is current in amperes (A).

Worked example: at a wire length of 50 cm, the voltmeter reads 1.5 V and the ammeter reads 0.30 A.

R = 1.5 ÷ 0.30 = 5.0 Ω

Worked example: at 100 cm, the voltmeter reads 1.8 V and the ammeter reads 0.18 A.

R = 1.8 ÷ 0.18 = 10 Ω

Doubling the length has doubled the resistance, which is exactly the relationship the graph should show.

What should the results look like?

A typical results table looks like this:

Plotted, these give a straight line passing through the origin, which is the signature of direct proportion: resistance ∝ length. Physically, a longer wire means the electrons travel further and collide with more metal ions, so more energy is transferred and the current for a given potential difference is smaller.

The line passing through the origin matters. A line with a positive y-intercept suggests some resistance in the circuit that is not part of the test wire — usually the connecting leads or a poor contact at a crocodile clip.

What are the control variables and common errors?

Keep the same throughout: the material of the wire, its thickness (cross-sectional area), the supply voltage, and the temperature of the wire.

Common mistakes to avoid:

• Leaving the switch closed so the wire heats up, giving resistances that creep upwards.
• Measuring length inconsistently — always measure from the same edge of each crocodile clip.
• Using a kinked or coiled wire, so the length in the circuit is not the length read off the ruler.
• Loose crocodile clips, which add contact resistance and scatter the points.
• Swapping the meters over, or reading the ammeter on the wrong range.

Frequently asked questions

Why is constantan or nichrome used rather than copper?

Copper is designed to be a good conductor, so a metre of thin copper wire has a resistance so small that the meters could barely measure the difference between lengths. Constantan and nichrome have a much higher resistivity, giving readings in a comfortable range of a few ohms. Constantan has a further advantage: its resistance changes very little with temperature, so small amounts of unavoidable warming affect the results less than they would with most metals.

How does the thickness of the wire affect resistance?

A thicker wire has a lower resistance, because there is a greater cross-sectional area for the electrons to move through — rather like a wider pipe letting water through more easily. Doubling the cross-sectional area roughly halves the resistance. Thickness is a control variable in this particular practical, but some specifications set it as an alternative investigation, where you keep the length fixed and vary the wire's diameter instead.

Why does the graph go through the origin?

Because a wire of zero length would offer no resistance at all. Direct proportion means that when one quantity is zero the other must be zero too, and the straight line through (0, 0) is the evidence for it. If your line of best fit misses the origin by a noticeable amount, look for a systematic error — the leads, the clips, or measuring the length from a different point each time.

Do I need to remember the circuit diagram for the exam?

Yes. Questions frequently ask you to draw or complete the circuit, and the marks are for placing the ammeter in series with the test wire and the voltmeter in parallel across it, using the correct standard symbols. Practise sketching it from memory: a cell, a switch, an ammeter and the test wire in one loop, with the voltmeter branching across the wire alone.

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