Calipers, Micrometers & Vernier Scales (OCR A Level Physics)

Micrometer Screw Gauge

• 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 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 (eg. 1.40 mm not just 1.4 mm)

The micrometer reading is read when the thimble scale aligns with the main scale

Vernier Calipers

• Vernier calipers are another distance measuring tool that uses a sliding vernier scale
  • They can also be used to measure diameters and thicknesses, just like the micrometer
  • However, they can also measure the length of small objects such as a screw or the depth of a hole

• Vernier calipers generally have a resolution of 0.1 mm, however, some are as small as 0.02 mm - 0.05 mm
• The calipers are made up of two scales:
  • The main scale
  • The vernier scale

• The two upper or lower jaws are clamped around the object
  • The sliding vernier scale will follow this and can be held in place using the locking screw

Components of a vernier caliper

• The value read from the caliper when the vernier scale aligns with the main scale
  • This should always be recorded to at least 1 decimal place (eg. 12.1 mm not just 12 mm)

The vernier caliper reading is read when the vernier scale aligns with the main scale

• In general, the micrometer has a smaller measuring range than a vernier caliper
• However, the micrometer has a better accuracy (due to better resolution)
• The vernier caliper is quicker to use, whilst the micrometer involves rotating the thimble
  • Therefore, to take many measurements, a caliper would be easier to use