Micrometer Screw Gauge
To measure the diameter of a wire:
Place the wire between the anvil and spindle end.
Rotate the thimble until the wire is tightly held.
The ratchet is provided to avoid excessive pressure on the wire. It prevents the spindle from further movement and squashing the wire.
To take a reading:
look at the main scale which has a linear scale reading. The upper lines label the millimetres and the lomer ones label the half millimetres.
The reading is 12+1/2=0.23=12.93 mm.