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Reading vernier calipers and micrometers; zero error

Edexcel IAL Physics · WPH13 Practical Skills in Physics I

The specification names vernier calipers and micrometer screw gauges. You must be able to read them, check them for zero error and use them well.

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Vernier calipers

Read the main scale at the zero of the sliding vernier scale: this gives whole millimetres.
Find the vernier line that lines up exactly with a main-scale line. Its number gives tenths of a millimetre.
Reading = main scale + (vernier line x 0.1 mm). For example, 23 mm on the main scale with the 7th vernier line aligned gives 23.7 mm.
Outer jaws measure external diameters; inner jaws measure internal diameters; the rod at the end measures depths.

Micrometer screw gauge

Read the sleeve scale (whole and half millimetres) at the edge of the thimble.
Read the thimble scale (50 divisions, each 0.01 mm) at the line on the sleeve.
Reading = sleeve + thimble x 0.01 mm. For example, 5.5 mm on the sleeve and 28 on the thimble gives 5.78 mm.
Close the spindle using the ratchet, so the jaws grip with the same gentle force every time and do not squash the object.

Zero error

Close the jaws with nothing between them and take a reading. If it is not zero, every reading will be wrong by the same amount: a systematic error. Correct each reading:

corrected reading = reading - zero reading

A zero reading of +0.02 mm means subtract 0.02 mm; a zero reading of -0.03 mm means add 0.03 mm. At A2, examiners expect 'check for and correct zero error' as a technique for calipers and micrometers.

Using them well

Measure at several places along the object, because it may not be uniform.
For wires and spheres, also measure in different orientations (rotate by 90°), because the cross-section may not be perfectly circular. Examiners only credit 'orientations' for objects with a circular cross-section.
Average the readings.

Worked example

A micrometer shows 4.5 mm on the sleeve and 37 on the thimble. With the jaws closed it reads +0.02 mm. Find the corrected diameter.

Reading = 4.5 + 37 x 0.01 = 4.87 mm
Corrected = 4.87 - 0.02 = 4.85 mm

Answer: Diameter = 4.85 mm

Key idea: Vernier: main scale + aligned line x 0.1 mm. Micrometer: sleeve + thimble x 0.01 mm, closed with the ratchet. Subtract the zero reading. Measure at several places (and orientations for wires).

Check yourself

Vernier calipers show 18 mm on the main scale and the 6th vernier line is aligned. What is the reading in mm?
Show answer
18.6 mm. 18 + 6 x 0.1 = 18.6 mm.
A micrometer shows 7.0 mm on the sleeve and 23 on the thimble. What is the reading in mm?
Show answer
7.23 mm. 7.0 + 23 x 0.01 = 7.23 mm.
A micrometer shows 3.5 mm on the sleeve (a half-millimetre mark is visible) and 41 on the thimble. What is the reading in mm?
Show answer
3.91 mm. 3.5 + 0.41 = 3.91 mm.
A micrometer reads -0.03 mm when closed. It then reads 2.46 mm on a wire. What is the corrected diameter in mm?
Show answer
2.49 mm. Corrected = 2.46 - (-0.03) = 2.49 mm.
What type of error is a zero error?
- Random
- Systematic
- Parallax
- Human reaction
Show answer
Systematic. It shifts every reading by the same amount, so it is systematic and cannot be reduced by repeating.
Why is the ratchet used to close a micrometer?
- To speed up the measurement
- To remove parallax
- To lock in the zero error
- So the jaws grip with the same small force and do not squash the object
Show answer
So the jaws grip with the same small force and do not squash the object. Over-tightening compresses soft objects and gives readings that are too small and inconsistent.
Why is a wire's diameter measured in different orientations as well as at different places?
- The micrometer may have a zero error
- The cross-section may not be perfectly circular
- The wire stretches when measured
- It removes the need for a mean
Show answer
The cross-section may not be perfectly circular. Rotating by 90° checks for an oval cross-section; different places check for variation along the wire.

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