Measuring Resistance Against Length: Why a Better Ruler End Makes a Better Physics Experiment

The GCSE Physics Practical That Looks Simple — Until You Actually Try It

One of the standard GCSE Physics required practicals in the AQA syllabus is the investigation into how the resistance of a wire changes with its length.

On paper, it looks beautifully straightforward:

Take a length of resistance wire.
Connect it into a circuit.
Measure the potential difference and current.
Calculate resistance using:

R = \frac{V}{I​}

Then change the length of the wire and repeat.

The expected result is also wonderfully satisfying: as the length of the wire increases, the resistance increases. In fact, for a uniform wire at constant temperature, resistance should be directly proportional to length.

So far, so good.

But like many school physics experiments, the real learning begins when students discover that the practical is not quite as neat as the textbook diagram suggests.

The Standard School Method

In many schools, this experiment is carried out using:

a metre ruler
a length of resistance wire
crocodile clips
a power supply or cell
an ammeter
a voltmeter
connecting leads

The wire is usually stretched along the metre ruler and fixed in place at each end. One crocodile clip remains at the start of the wire, and another crocodile clip is moved along the wire to select different lengths.

Students might test lengths such as:

10 cm
20 cm
30 cm
40 cm
50 cm
60 cm
70 cm
80 cm
90 cm
100 cm

For each length they measure the current and potential difference, calculate the resistance, repeat results, and then plot a graph of resistance against length.

This is a good experiment because it links practical work directly to theory. It also gives students a chance to practise graph work, repeat readings, control variables and discuss sources of uncertainty.

But there is a problem.

The Problem With Crocodile Clips

Crocodile clips are useful. They are cheap, common, robust and easy for students to use.

They are not, however, very precise measuring instruments.

When a crocodile clip is used to make contact with the wire, several small errors can appear:

The contact point is not sharply defined
A crocodile clip touches the wire over a small area, not at a single point.
The wire may not start exactly at 0 cm
If the wire is tied, taped or clipped to the ruler, the electrical length may not match the ruler reading.
The wire can move
As students move clips around, the wire may slip or bend slightly.
The clip can damage or kink the wire
This changes the quality of contact and can affect results.
Students often measure from the wrong place
They may read from the edge of the clip, the middle of the clip, or wherever seems most convenient at the time.

A few millimetres may not sound like much, but in a GCSE practical it matters. If the first point is supposed to be 10 cm and the actual electrical length is 11 cm, that is already a 10% error.

That is before we even consider heating of the wire, parallax errors, loose connections, fluctuating current or poor graph scales.

Why Accuracy Matters

It is tempting to say, “Well, the experiment still works.”

And it does.

Students will usually still find that a longer wire has a larger resistance. The graph will usually show a clear positive correlation. The teacher can still explain the relationship.

But practical science is not just about getting the “right sort of answer”.

It is about learning how scientists improve measurements.

A key part of GCSE Physics is understanding that experiments should be:

repeatable
reproducible
accurate
carefully controlled
honestly evaluated

If the equipment introduces avoidable uncertainty, then students should be encouraged to think about how the apparatus could be improved.

That is where this small piece of design work comes in.

Designing a Better End for the Ruler

To improve the experiment, we designed a simple end piece for the ruler.

The idea is straightforward: the resistance wire is wound around a peg at the end of the ruler so that the electrical starting point of the wire is fixed exactly at the 0 cm mark.

This means the length being measured is much more clearly defined.

Instead of saying, “The wire starts somewhere near the end of the ruler,” we can now say:

The wire starts at zero. The length measured on the ruler is the actual length used in the circuit.

That may sound like a small improvement, but in practical physics small improvements are often exactly what matter.

From Crocodile Clip to Jockey

The next improvement is to use a proper contact point, often called a jockey, rather than relying on a crocodile clip as the moving contact.

A jockey allows the student to touch the wire at a specific point on the scale. It gives a much clearer position than a crocodile clip wrapped around the wire.

This helps students understand that the length of wire in the circuit is not just a rough guess. It is a measured variable.

That is the point of the experiment.

We are not just waving a clip somewhere along a wire and hoping for the best. We are deliberately changing one variable — the length — and measuring its effect on resistance.

The Physics Behind the Experiment

The resistance of a wire depends on several factors:

R = \frac{\rho L}{A}

Where:

$R$ is resistance
$\rho$ is resistivity of the material
$L$ is length of the wire
$A$ is cross-sectional area

For GCSE students, the most important part is this:

R \propto L

provided the material and thickness of the wire stay the same, and the temperature does not change significantly.

A longer wire has more resistance because the electrons have to travel through more material. There are more collisions with the metal ions in the wire, so it is harder for charge to flow.

A useful analogy is to imagine walking through a crowded corridor.

A short crowded corridor is annoying.
A long crowded corridor is worse.

The longer the route, the more collisions and delays you experience.

A Practical Example for Students

Suppose a student records these results:

Length of wire / cm	Potential difference / V	Current / A	Resistance / Ω
20	0.40	0.80	0.50
40	0.80	0.80	1.00
60	1.20	0.80	1.50
80	1.60	0.80	2.00
100	2.00	0.80	2.50

The resistance increases as the length increases. If we double the length from 20 cm to 40 cm, the resistance doubles from 0.50 Ω to 1.00 Ω.

This is the clean result we want students to see.

But real results are rarely this perfect.

A student might instead get:

Length of wire / cm	Resistance / Ω
20	0.54
40	0.96
60	1.49
80	2.08
100	2.45

This is still a good result. It shows the same general trend. The points would still be close to a straight line.

The important question becomes:

Why are the points not perfectly on the line?

That is where students begin to think like physicists.

Sources of Error Students Should Discuss

This experiment is excellent for teaching evaluation. Students can discuss:

1. Uncertainty in length

If the contact point is not clear, the length may be slightly wrong. This is why a proper ruler end and jockey can improve the method.

2. Heating of the wire

As current flows, the wire warms up. Higher temperature increases resistance in a metal wire. This can be reduced by switching off the circuit between readings or using a lower current.

3. Poor electrical contacts

Loose crocodile clips or oxidised wire can add extra resistance. This can make results less reliable.

4. Reading uncertainty

Voltmeters and ammeters have limited precision. Students must read them carefully, especially if using analogue meters.

5. Parallax error

When reading the metre ruler, the student’s eye should be directly above the scale.

6. Wire thickness and material

The same wire should be used throughout. Changing wire thickness or material would change resistance for a different reason.

Repeatability and Reproducibility

This is one of the most useful teaching points in the whole experiment.

A result is repeatable if the same student, using the same method and equipment, gets similar results when they repeat the experiment.

A result is reproducible if someone else, using the same method, can obtain similar results.

That is why apparatus design matters.

If the start point of the wire is vague, and the moving contact is vague, then another student may not be measuring exactly the same thing. The experiment becomes less reproducible.

By fixing the wire so that it begins exactly at zero, we remove one source of uncertainty.

That is good science.

What This Teaches Beyond the Syllabus

This small improvement to the apparatus teaches students something very important:

Science is not just about following instructions. It is about improving the method.

At GCSE, students often think practical work is simply a recipe:

Set up the apparatus.
Take readings.
Draw a graph.
Write “human error” in the evaluation.

But good practical science is much better than that.

Students should be asking:

What exactly am I measuring?
Is the measurement reliable?
Where could uncertainty enter the experiment?
How could I improve the apparatus?
Would another student get the same result?
Does my graph support the theory?

That is the difference between doing an experiment and understanding an experiment.

A Personal Reflection From the Lab

This is one of the reasons I enjoy designing and adapting practical equipment.

Many commercial school experiments work, but they are not always designed for the way students actually use them. A teacher may understand where the measurement should begin, but a GCSE student under exam conditions may not.

If the apparatus can make the correct method clearer, then the student has a better chance of understanding the physics.

The little ruler end in the photograph is not a glamorous piece of equipment. It is not expensive. It does not need a computer, a sensor or a complicated interface.

But it solves a real practical problem.

It makes the start of the wire definite.

And in a measurement experiment, definite is good.

How Students Can Improve Their Method

A strong GCSE answer might suggest improvements such as:

use a jockey instead of a crocodile clip for the moving contact
make sure the wire starts exactly at 0 cm
keep the wire straight and taut
switch off the circuit between readings to reduce heating
take repeat readings and calculate a mean
use a low current to reduce temperature changes
check all connections are secure
plot a graph of resistance against length
draw a line of best fit
identify anomalous results
calculate resistance using $R = V / I$

These are not just “extra details”. They are the difference between a basic practical and a high-quality investigation.

What the Graph Should Show

The graph of resistance against length should be a straight line through, or close to, the origin.

This means:

when length increases, resistance increases
the relationship is directly proportional
doubling the length should approximately double the resistance

If the graph does not pass exactly through the origin, students should consider why.

Possible reasons include:

contact resistance
zero error in the length measurement
heating of the wire
poor connections
measurement uncertainty

This is a lovely opportunity to teach students that imperfect graphs are not failures. They are evidence to be analysed.

Why This Matters for GCSE Students

Students often lose marks in required practical questions because they know the basic method but not the reasoning behind it.

They might remember:

“Use a metre ruler and crocodile clip.”

But higher-level answers need more:

why length is the independent variable
why resistance is the dependent variable
why temperature must be controlled
why repeat readings are needed
why a jockey improves accuracy
why a graph is useful
why the wire should start at zero

The experiment is really about measurement quality.

That is why improving the apparatus is not just a nice extra. It supports the whole purpose of the practical.

Conclusion: Better Apparatus, Better Thinking

The resistance wire experiment is a classic GCSE Physics practical because it is simple, visual and mathematically useful. It links circuits, resistance, current, potential difference, gradients, proportionality and experimental method.

But simple experiments still deserve careful design.

Using a metre ruler and crocodile clips may be enough to show the basic trend, but it is not ideal if we want students to think seriously about accuracy and reproducibility.

By designing a ruler end that fixes the wire so the starting point is exactly at zero, and by using a jockey for a clearer contact point, we make the experiment more precise.

More importantly, we show students what practical physics is really about.

Not just getting an answer.

Getting a better answer.

And knowing why it is better.

21 May 2026