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Choosing instruments: range and resolution

Edexcel IAL Physics · WPH13 Practical Skills in Physics I

Choosing the right instrument is a planning skill examiners test on almost every paper. The key word is resolution.

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Resolution

The resolution of an instrument is the smallest measuring interval it can show. It is a source of uncertainty in a single reading. Typical school instruments:

Instrument	Typical resolution	Good for
Metre rule	1 mm	Lengths from about 10 cm to 1 m
Vernier calipers	0.1 mm (digital calipers 0.01 mm)	External and internal diameters, a few mm to about 15 cm
Micrometer screw gauge	0.01 mm	Wire diameters, sheet thickness, small spheres
Top-pan balance	0.01 g or 0.1 g	Masses
Digital stopwatch	0.01 s (but human reaction time is about 0.2 s)	Times of a few seconds or more
Protractor	1°	Angles
Thermometer	1 °C (digital 0.1 °C)	Temperatures
Digital multimeter	The last digit shown	Current, p.d., resistance

Range

The instrument must be able to measure the whole range of values. A 30 cm ruler cannot measure a 1.5 m drop; a micrometer cannot measure a 5 cm diameter. On a multimeter, choose the smallest range that still covers the reading, because it shows more digits.

Justifying a choice

Justify an instrument by its resolution and the resulting percentage uncertainty: 'a micrometer has a resolution of 0.01 mm, so the percentage uncertainty in a 0.50 mm diameter is only 1%'. Examiners do not accept 'it is more accurate' or 'it is more precise' as a reason. Compare your percentage uncertainty with a benchmark and state explicitly that it is small enough.

When a stopwatch is acceptable

A stopwatch reads to 0.01 s, but the real uncertainty comes from reaction time. Time longer intervals (for example 20 oscillations) so the reaction-time uncertainty is a small fraction of the total, or use light gates and a data logger for short events.

Worked example

Should a student use a metre rule or a micrometer to measure the diameter of a 0.40 mm wire?

Metre rule: resolution 1 mm, larger than the diameter itself, so it is useless.
Micrometer: resolution 0.01 mm, uncertainty 0.005 mm.
Percentage uncertainty = 0.005 / 0.40 x 100 = 1.3%

Answer: Use the micrometer: its percentage uncertainty is only about 1%

Key idea: Resolution is the smallest interval an instrument can show. Choose one whose range covers the measurement and whose resolution gives a small percentage uncertainty, and justify it with that resolution.

Check yourself

Which instrument is most appropriate for measuring the diameter of a wire of about 0.3 mm?
- Micrometer screw gauge
- Metre rule
- 30 cm ruler
- Tape measure
Show answer
Micrometer screw gauge. Only the micrometer has a fine enough resolution (0.01 mm).
Which instrument can measure the internal diameter of a pipe of about 2 cm?
- Micrometer screw gauge
- Top-pan balance
- Vernier calipers, using the internal jaws
- Protractor
Show answer
Vernier calipers, using the internal jaws. Vernier calipers have internal jaws for inside diameters; a micrometer only measures external sizes.
A micrometer (resolution 0.01 mm) gives a diameter of 0.50 mm. Find the percentage uncertainty, using half the resolution.
Show answer
1.0 %. Uncertainty = 0.005 mm; 0.005 / 0.50 x 100 = 1.0%.
Vernier calipers (resolution 0.1 mm) give a diameter of 18.6 mm. Find the percentage uncertainty, using half the resolution, to 2 s.f.
Show answer
0.27 %. Uncertainty = 0.05 mm; 0.05 / 18.6 x 100 = 0.27%.
Which justification for choosing a micrometer would examiners credit?
- It is more accurate
- Its resolution is 0.01 mm, giving a small percentage uncertainty
- It is more precise
- It is the most expensive instrument
Show answer
Its resolution is 0.01 mm, giving a small percentage uncertainty. Justify with resolution and percentage uncertainty; 'accurate' and 'precise' are not accepted as substitutes for resolution.
A p.d. of about 1.5 V is measured on a digital multimeter. Which range setting gives the best resolution?
- 0 to 20 V
- 0 to 200 V
- 0 to 1000 V
- 0 to 2 V
Show answer
0 to 2 V. The smallest range that still covers 1.5 V shows the most decimal places.
Why is the uncertainty in a hand-timed reading usually much larger than the stopwatch's 0.01 s resolution?
- Stopwatches are never calibrated
- Human reaction time is about 0.2 s
- Time cannot be measured precisely
- The display rounds to the nearest second
Show answer
Human reaction time is about 0.2 s. Starting and stopping by hand adds a reaction-time uncertainty much bigger than the display's resolution.

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Last updated 8 November 2026. Spotted a mistake? Tell us.