SM083     Micrometer     circa 1960     Mechanics

To measure small lengths with precision a useful instrument is the micrometer screw gauge.

The screw is accurately machined to have a definite pitch e.g. one turn per millimetre.   This means that one complete turn of the thimble around the screw will advance the blunt edge by 1 mm.   When this edge reaches the anvil or fixed part, the instrument is marked at zero on the longitudinal scale and zero on the thimble.   The thimble is marked in say, one hundred equal divisions so that the micrometer can be read to one hundredth of a millimetre.   The total reading of length is the number of whole turns plus the number of subdivisions on the thimble.

A friction device is usually incorporated on the screw so that a constant pressure can be applied on the object being measured.   This avoids errors of measurement.