Resistance of a wire: R depends on length (slide-wire method)
Tape ~105 cm of bare constantan wire to a metre rule, slide a contact along it to vary the length between a fixed terminal and the contact, and show that resistance is directly proportional to length.
WHAT YOU'LL LEARN
- ✓Wiring a slide-wire (potentiometer-style) circuit: supply, switch and ammeter in series with the whole wire, voltmeter tapped between one fixed terminal and a movable sliding contact
- ✓Why the circuit current stays constant as the sliding contact moves, while the p.d. it measures changes
- ✓Calculating R = V / I for each length and relating it to R = ρL/A
- ✓Plotting R against length, drawing a best-fit line through the origin, and reading the wire's resistance-per-unit-length off the gradient
ON YOUR BENCH
- □Power supply, 1.5–3 V, fixed by the supervisor
- □Switch (may be integral to the power supply)
- □Ammeter (0–1 A)
- □Voltmeter (resolution at least 0.1 V)
- □~105 cm constantan (Eureka) resistance wire, 0.28 mm diameter, ~8 Ω/m, bare, taped to a metre rule only at the 3–7 cm and 93–97 cm marks
- □Two terminals (crocodile clips), B at the zero end and C at the 100 cm end
- □Sliding contact S — a jockey, or a small screwdriver on a lead with a crocodile clip
- □Connecting leads
The protocol, step by step
The same guide is printed and waiting at your bench.
- 01
Wire the main loop
Connect the supply, switch and ammeter in series with the whole length of wire, from terminal B to terminal C and back to the supply. Keep the switch open while wiring.
- 02
Tap the voltmeter in at B and the sliding contact
Connect the voltmeter between terminal B and the sliding contact S, not between B and C. It reads the p.d. across only the length of wire from B up to wherever S touches down.
- 03
Close the switch and record the current
With the circuit closed, read and record the steady current I. Because the ammeter sits in the main loop, this current does not depend on where S is — it stays essentially constant for the whole experiment.
- 04
Take five length readings
Place S at d = 20.0, 40.0, 60.0, 80.0 and 100.0 cm from B in turn. At each position, read the p.d. V across B–S and calculate R = V / I. Record d, V and R in a table.
- 05
Plot and conclude
Plot R (y-axis) against d (x-axis). Draw the best-fit straight line through the origin; its gradient is the wire's resistance per unit length.
What you should see
- →A straight line through the origin: R ∝ d, because resistance is directly proportional to length for a uniform wire (R = ρL/A, with ρ and A both constant).
- →V increases steadily and monotonically as d increases — never dips — because you're measuring the p.d. across an ever-longer piece of the same wire.
- →The gradient of R against d should sit close to the wire's resistivity per unit length; for ~8 Ω/m constantan that's about 0.08 Ω/cm.
| d /cm | 20.0 | 40.0 | 60.0 | 80.0 | 100.0 |
|---|---|---|---|---|---|
| V /V | 0.26 | 0.51 | 0.77 | 1.02 | 1.28 |
| R /Ω | 1.60 | 3.20 | 4.80 | 6.40 | 8.00 |
⚠ BEFORE YOU START
- •Open the switch between readings if the circuit is left connected for any length of time — a low-voltage supply won't burn you, but a continuously loaded thin wire can still get warm enough to change its resistance slightly.
- •Handle the bare wire gently; it's thin (0.28 mm) and kinks or nicks it easily, which would locally change its cross-sectional area and throw off the R ∝ L relationship.
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.