Using a Micrometer (Edexcel International AS Physics): Revision Note
Exam code: XPH11
Using a Micrometer
A micrometer, or a micrometer screw gauge, is a tool used for measuring small widths, thicknesses or diameters
For example, the diameter of a copper wire
It has a resolution of 0.01 mm
The micrometer is made up of two scales:
The main scale - this is on the sleeve (sometimes called the barrel)
The thimble scale - this is a rotating scale on the thimble
Components of a micrometer
The spindle and anvil are clamped around the object being measured by rotating the ratchet
This should be tight enough so the object does not fall out but not so tight that it is deformed
Never tighten the spindle using the barrel, only using the ratchet. This will reduce the chances of overtightening and zero errors
The value measured from the micrometer is read where the thimble scale aligns with the main scale
This should always be recorded to 2 decimal places (e.g. 1.40 mm not just 1.4 mm)
The micrometer reading is read when the thimble scale aligns with the main scale
Examiner Tips and Tricks
The most common mistake in exam answers when reading from a micrometer is not giving the reading to the correct number of significant figures. Record the reading in millimetres to two decimal places — that is, to the nearest 0.01 mm, matching the resolution of the instrument. The number of significant figures then follows from the size of the reading: 17.90 mm is 4 significant figures, 2.30 mm is 3 and 0.27 mm is 2. Keeping the trailing zero matters: 2.30 mm shows the reading was taken to the nearest 0.01 mm, whereas 2.3 mm claims less precision than the micrometer gives.
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