Micrometer is a high precision measuring device employed for testing lengths with an accuracy up to one thousandth of a millimetre. In particular, this tool is designed to measure thickness and external diameters of bares, wires, spheres or other generic lengths "outside" of a work piece.
Micrometer consists essentially in a frame made of forged steel or cast iron, at whose ends are located two coaxial cylinders, one fixed (anvil) and one mobile (spindle). The anvil is solidly fixed to the frame. At the other end of the frame, is coaxially fixed an internally threaded sleeve, in which is put to the mouth the threaded spindle. At the external end of the spindle, a coaxial thimble is fixed, which is graduated on the inside edge. Using the thimble, the spindle can be screwed on sleeve, in order to approach its inner end to the anvil, to the limit, coming to the stop.
The operating principle is very simple and immediate. Place the work piece between the prongs (the free ends of anvil and spindle), taking care to clean any debris, which would distort the measurement; screw the thimble using the pawl (or the ring) of the clutch (which prevents to apply an excessive torque) up to the 'clamping'; read on fixed scale the major component, usually millimeter or half millimeter, and on the thimble scale hundredth fractions.