# 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!.

• 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