Micrometer (Calibrated)

Definition and scope

Micrometer (Calibrated) is the designation given to any measuring instrument of the micrometer class that has been entered into the register of the Clerks of the Calibrated Measure and found, on three separate occasions, to agree with the standards held at the Calibrated Atrium. The parenthetical qualifier is not decorative: an uncalibrated micrometer of identical manufacture is catalogued elsewhere, and the two are not interchangeable in any procedure governed by the Ordinance of Calibrated Absence.

The instrument measures exterior thicknesses in the range 0–25 millimetres by the rotation of a threaded spindle against a fixed anvil, converting rotation into linear displacement at a pitch of 0.5 millimetres per revolution. Its accuracy in ordinary use is quoted as ±0.004 millimetres. Its accuracy within the Tower is quoted as ±0.004 millimetres outside the Chamber of Calibrated Volumes and is not quoted inside it.

Calibration

Calibration of a micrometer follows the procedure set out in Annex 9 of the Concordance most recently amended for that purpose, and involves comparison against the Master Standard Rule held among the five master standards of the Calibrated Atrium. Three concordant readings are required. A fourth is customary. The result is entered in the Register of Calibrated Measures with a seal, a date, and the name of the clerk who read the third reading.

The three-reading requirement dates from the Schism of the Calibrated Span, during which two offices each insisted that their own single reading constituted agreement, and a micrometer was certified twice on the same afternoon with results differing by 0.011 millimetres. The incident is preserved in the Index of Superseded Clauses as the case that made a second opinion mandatory and a fourth reading merely polite.

Standing disputes

Whether a micrometer remains calibrated if it is transported between floors is the subject of an unresolved disagreement between the Empiricists of the Calibrated Stair and the clerks of the Measure. The Empiricists hold that vertical displacement of the instrument introduces a residual stress in the frame and that readings taken above Floor 12,000 should be corrected by a factor published (and repeatedly withdrawn) in the Concordances. The clerks hold that the instrument's calibration is a property of its seal and not of its position, and that the seal does not know where it is.

The two positions are reconciled in practice by re-reading the micrometer on return, which satisfies neither party and has been the standard practice since the winter of the Incident of the Un-Calibrated Chime.

A second dispute concerns instruments calibrated before the current recension of the register. Their entries are valid, their seals are intact, and no clause has yet been found that says what to do with them. Until such a clause is written, they are cited as precedent and not used for measurement.

Notable holdings

• A micrometer (Calibrated), serial marking S-4, entered in the register in the forty-second year of the present recension and withdrawn the following spring for reasons given only as "unsound at the anvil".
• Two micrometers (Calibrated) held in reserve against the failure of the Calibrated Atrium standard, sealed and unopened since the last such report.
• The Bench Micrometer, a fixed-frame instrument of unusually long travel, installed in the Annex of Calibrated Entities and read exclusively by the Guild of Quantitative Idealists, who refuse to record its value in millimetres.

Visitors may observe a micrometer (Calibrated) in use during the second week of any month. Handling is restricted to the Clerks of the Calibrated Measure and to one named apprentice per season; the apprenticeship rotates, and the rotation has been known to lapse for years at a time without comment from any office competent to correct it.

Notes on notation

Recorded readings use the notation of the Register: a figure, the unit "mm", and a small superscript letter indicating which of the three readings was retained. Superseded readings are struck through rather than erased, on the grounds that erasure and calibration are both, in the words of the Ordinance of Calibrated Absence as amended, "statements about a thing that was measured", and a struck-through figure is the more honest of the two.