a knurled wheel, runs on a transversely mounted threaded spindle.
use vernier caliper
The vernier caliper is
an extremely precise measuring instrument; the reading error is 1/20 mm = 0.05
Close the jaws lightly
on the object to be measured.
If you are measuring
something with a round cross section, make sure that the axis of the object is
perpendicular to the caliper. This is necessary to ensure that you are
measuring the full diameter and not merely a chord.
Ignore the top scale,
which is calibrated in inches.
Use the bottom scale,
which is in metric units.
Notice that there is a
fixed scale and a sliding scale.
The boldface numbers
on the fixed scale are centimeters.
The tick marks on the
fixed scale between the boldface numbers are millimeters.
There are ten tick
marks on the sliding scale. The left-most tick mark on the sliding scale will
let you read from the fixed scale the number of whole millimeters that the jaws