Interactive practical

Measuring electrical resistance

Read a voltmeter and ammeter at six supply settings to find a resistance from V/I and a V–I graph, then switch off and compare with an ohmmeter reading corrected for its leads.

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A-Level Physics

Measuring electrical resistance

CCEA 1210 — matched to your course.

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Practical focuses1

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Available question marks4

Practical focus

Determine resistance using ammeter-voltmeter measurements and compare with a multimeter or ohmmeter.

Practical method

Aim

Measure resistance using the ammeter–voltmeter method and an ohmmeter, and compare the methods.

Variables

Independent variable: the resistor being tested (and the method)
Dependent variable: measured resistance
Control variables: temperature; the same meters.

Apparatus

Resistors of nominal value
Ammeter and voltmeter
Low-voltage supply and variable resistor
Digital multimeter used as an ohmmeter

Method

Ammeter–voltmeter method: connect the resistor with an ammeter in series and a voltmeter across it; record several pairs of V and I and plot V against I (gradient = R).
Ohmmeter method: disconnect the resistor from any circuit and measure it directly with the multimeter on a suitable range.
Repeat for several resistors and compare with the nominal values and tolerance bands.
Consider the effect of meter resistance: a voltmeter draws a small current.

Sample results

Nominal 47 Ω ± 5% resistor.

V / V	I / mA
1.00	21.0
2.00	42.1
3.00	63.0
4.00	84.2

Analysis

V against I is a straight line with gradient 47.5 Ω. The ohmmeter reads 47.3 Ω. Both agree within the 5% tolerance (44.7–49.4 Ω).

Uncertainty

Each meter reading is ±1 in the last digit; the graph averages these. The ohmmeter's stated accuracy is typically ±1% ± 2 digits.

Errors and improvements

Problem	Effect	Improvement
Voltmeter draws current	Ammeter reading slightly too high	Use a high-resistance digital voltmeter
Measuring with the ohmmeter in a powered circuit	False readings or meter damage	Isolate the resistor first
Resistor heating	R drifts	Keep currents small

Risk assessment

Hazard	Risk	Control measure
Hot resistor	Burns	Keep power well below the rating
Short circuits	Overheating	Check the circuit before switching on

Common mistakes

Using an ohmmeter on a component that is still connected to a supply.
Forgetting to convert mA to A.
Taking only one V–I reading.

Worked analysis

Gradient of V against I = \(\dfrac{4.00}{0.0842}\) = 47.5 Ω. The ohmmeter reads 47.3 Ω, a 0.4% difference.

Common misconception

An ohmmeter supplies its own small current; it must never be used on a component that is part of a powered circuit.

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