Micrometer with a negative zero error

An example figure from the Length Reading Generator.

Two micrometers: above, closed, its thimble’s 0 three marks above the datum line, reading −0.03 mm; below, holding a wire, reading 3.27 mm, each with a magnified view — The same micrometer twice. Above, closed, the thimble’s 0 has turned three marks past the datum line, so it reads −0.03 mm: a negative zero error. Below, holding a wire, it reads 3.27 mm. Taking off the zero error gives 3.27 − (−0.03) = 3.30 mm.

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Answer key

Reading: sleeve 3.0 mm + thimble mark 27 × 0.01 mm = 3.27 mm
Zero error (closed): the thimble’s 0 is 3 marks above the datum line, so −3 × 0.01 mm = −0.03 mm
Corrected reading: 3.27 mm − (−0.03 mm) = 3.30 mm

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