Using a Micrometer Screw-Gauge

Determination of the diameter 'd' of a wire

 

You can use a mircometer to measure small (>2.5 cm) diameters that can fit within the 'jaws' of the screw-gauge can be measured to within a hundredth of a millimeter.

  • Close the jaws of the micrometer and check for a zero error.
  • Place the wire between the anvil and spindle end as indicated in the diagram.
  • Rotate the thimble until the wire is firmly held between the anvil and the spindle.
  • The rachet is provided to avoid excessive pressure on the wire. It prevents the spindle from further movement - squashing the wire!.

 

To take a reading:

  • First look at the main scale. This has a linear scale reading on it. The long lines are every millimetre, the shorter ones denote half a millimetre in between.
  • On the diagram this reading is 2.5 mm
  • Now look at the rotating scale. That denotes 46 divisions - each division is 0.01mm so we have 0.46mm from this scale.

 

The diameter of the wire is the sum of these readings: 2.5 + 0.46 = 2.96 mm

Sensitivity of the reading

You can read to half a rotating scale division - to within 0.005mm - but no more than that. That assumes you have experience and confidence in using the instument +/- one division is more realistic!

Practical Notes

  • SWG (Standard Wire Gauge) 32 wire has a manufacturer diameter value of 0.274 mm. If you measure it with a micrometer in school you could only say it was 0.275mm - that last digit of '4' would be impossible to measure with a micrometer....
  • Always check carefully for a zero error before using the micrometer!