How To Read A Micrometer Screw Gauge - Mini Physics

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How To Use a Micrometer (Digital + Screw Gauge) Show/Hide Sub-topics (Measurement | O Level Physics)
  1. Base Quantities & SI Units
  2. Prefixes
  3. Scalar & Vector Quantities
  4. Vector Addition (Graphical Method)
  5. Measurement Of Length
  6. Measurement Of Time
  7. How To Read A Vernier Caliper
  8. How To Read A Micrometer Screw Gauge
  9. Accuracy, Precision & Measurement Errors
  10. Practice On Reading A Vernier Caliper
  11. Practice On Finding The Zero Error Of A Vernier Caliper
  12. Practice On Reading A Vernier Caliper With Zero Error
On this page
  • 1. Definition
  • A. Digital micrometer (recommended)
  • B. Micrometer screw gauge (analogue) (optional / legacy)
  • 2. Key Ideas
  • 3. Detailed Explanations
  • A. Digital micrometer workflow (quick)
  • B. Analogue micrometer parts and scales (optional / legacy)
  • C. Reading an analogue micrometer (no zero error)
  • D. Another analogue example
  • E. Correcting for zero error (works for positive and negative)
  • F. Technique tips (accuracy + repeatability)
  • 4. Common Mistakes
  • A. Scale-reading mistakes
  • B. Zeroing and technique mistakes
  • 5. Exam Tips
  • A. How to present your working
  • 6. Worked Examples
  • A. Thimble scale: smallest and largest readings
  • B. Classify the error type
  • C. Read an analogue micrometer (no zero error)
  • D. Correct for zero error
  • E. Convert a micrometer reading into metres
  • 7. Mind Stretchers
  • A. Why take repeated readings for a wire?
  • B. Precision vs accuracy
  • 8. Practice

1. Definition

A digital micrometer measures very small thicknesses/diameters (e.g. wire) and displays the reading directly (usually in mm).

B. Micrometer screw gauge (analogue) (optional / legacy)

A micrometer screw gauge uses a sleeve (main scale) and a thimble scale to read lengths to a fine resolution (commonly ).

2. Key Ideas

  • Zero first: the micrometer should read zero when fully closed.
  • Use the ratchet (if present) so you always apply the same force.
  • Analogue reading:
    • sleeve (main scale) reading (including the line if visible)
    • thimble reading
    • observed reading sleeve thimble
  • Zero-error correction (both signs): .

3. Detailed Explanations

A. Digital micrometer workflow (quick)

  1. Close gently and press ZERO (use the ratchet if provided).
  2. Place the object between the anvil and spindle.
  3. Tighten using the ratchet until it clicks (consistent force).
  4. Record the display value with units (e.g. 1.76 mm).

If it won’t zero, treat the offset as a zero error: see Accuracy, Precision & Measurement Errors.

B. Analogue micrometer parts and scales (optional / legacy)

Optional / legacy

Many practicals use digital micrometers, but the analogue screw gauge still appears in questions. The steps below are for analogue micrometers (sleeve + thimble scale).

Micrometer sleeve and thimble scales; read the sleeve then the thimble division at the datum line
Read the sleeve (including any visible half-millimetre mark), then read the thimble at the datum line.

C. Reading an analogue micrometer (no zero error)

  1. Sleeve reading: read the last visible mm marking (and add if the half-mm line is visible).
  2. Thimble reading: read the thimble division that lines up with the datum line, then multiply by .
  3. Add: observed reading sleeve thimble.

Mini-example:

Example micrometer reading: sleeve 5.5 mm with thimble at 28 divisions (observed 5.78 mm)
Example: sleeve = 5.5 mm, thimble = 28 × 0.01 mm = 0.28 mm, so observed = 5.78 mm.

D. Another analogue example

Example micrometer reading: sleeve 2.5 mm with thimble at 38 divisions (observed 2.88 mm)
Example: sleeve = 2.5 mm, thimble = 38 × 0.01 mm = 0.38 mm, so observed = 2.88 mm.

E. Correcting for zero error (works for positive and negative)

Use one rule for both signs:

Zero-error correction rule
  • Positive zero error: the thimble’s zero is below the datum line when fully closed.
  • Negative zero error: the thimble’s zero is above the datum line when fully closed.

Mini-example:

Micrometer showing an observed reading and a closed-jaws reading (zero error); correct reading = observed − zero error
Use the closed-jaws reading as the zero error, then subtract it from the observed reading.

Observed reading , zero error .

Correct reading .

F. Technique tips (accuracy + repeatability)

  • Clean the anvil/spindle faces and the object.
  • Use the ratchet (don’t overtighten).
  • For a wire: take readings at several points and average (reduces random error).

4. Common Mistakes

A. Scale-reading mistakes

  • Missing the sleeve line when it is visible.
  • Reading the thimble from the wrong line (use the datum line).
  • Mixing up resolutions (e.g. writing when the micrometer reads ).

B. Zeroing and technique mistakes

  • Not checking the zero before measuring.
  • Forgetting to correct for zero error when it is given.
  • Squeezing the ratchet too hard, giving inconsistent force.

5. Exam Tips

A. How to present your working

  • Write your method in a mark-scheme-friendly order:
    • sleeve reading
    • thimble reading
    • observed reading
    • corrected reading (if there is zero error)
  • Quote to the correct resolution (often ).
  • If asked about reliability: mention repeat readings and average (random error) and correct zero error (systematic error).

6. Worked Examples

A. Thimble scale: smallest and largest readings

What is the smallest possible change (resolution) on the thimble scale? What is the largest value you can read from the thimble scale before it returns to zero?

Show Answer
  • Smallest division (resolution): .
  • Largest thimble reading: .

B. Classify the error type

In an experiment to measure the diameter of a wire, a student forgets to notice that the micrometer does not read zero when fully closed.

  1. Does this omission cause a random error or a systematic error?
  2. Explain briefly.
Show Answer
  • It causes a systematic error (a zero error).
  • The micrometer adds the same offset to every reading, so the readings are shifted in one direction.

C. Read an analogue micrometer (no zero error)

Sleeve reading and thimble alignment is at divisions. Find the observed reading.

Show Answer
  • Thimble reading .
  • Observed reading .

D. Correct for zero error

Observed reading and positive zero error . Find the correct reading.

Show Answer

Correct reading .

E. Convert a micrometer reading into metres

A wire diameter is measured as . Write this in metres.

Show Answer

7. Mind Stretchers

A. Why take repeated readings for a wire?

A student measures a wire diameter at one point only and gets . Another student measures at 5 points and averages. Why is the second method better?

Show Answer
  • A wire may not be perfectly uniform; readings can vary along its length.
  • Repeating and averaging reduces the effect of random error and gives a more reliable estimate.

B. Precision vs accuracy

A micrometer gives very consistent readings (same value each time), but the instrument has a zero error that the student ignores. Describe the accuracy and precision of the results.

Show Answer
  • Precision: high (readings are consistent / close together).
  • Accuracy: poor (all readings are shifted by the zero error).

8. Practice

Practice Time! Go to Measurement Quiz

Related lessons:

  • Measurement of Length
  • Accuracy, Precision & Measurement Errors
Previous Lesson How To Read A Vernier CaliperNext LessonAccuracy, Precision & Measurement Errors

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