Prime Metre Bar

The Prime Metre Bar is a physical artifact of platinum-iridium alloy, one metre in length by its own definition, held under permanent vacuum within the Register of Primary Standards on Floor Π-7 of Scriptorium Prime. It is one of the seven foundational artifacts of the Register and the ultimate reference against which the Prototypical Metre and, by extension, all sanctioned measurement in the Tower is calibrated. Unlike the Ideal Metre, which is an abstract standard and cannot be handled, the Prime Metre Bar is a tangible object that has been handled on eleven recorded occasions, each of which required a separate instrument of permission.

Construction and custody

The Bar was cast in the alloy works of Corinthian Alloy in a year given in the Register as "the ninth of the third recension," a date that cannot presently be converted to any other system of reckoning. It consists of ninety per cent platinum and ten per cent iridium, with trace inclusions that the Office of Metric Assurance has ruled are not to be investigated further. Two ruled lines are incised on its upper face at a separation of exactly one metre; the distance between them is defined as one metre because it is the distance between them.

Custody rests with the Keeper of the Register, an office presently held by Librarian-Prime Anselm the Damp. Access requires the concurrence of three officers and the physical presence of a witness drawn by lot from the resident Librarians of Floor Π-7. The Bar is stored in a chamber evacuated to a pressure of less than one part in ten to the ninth; the pumps are serviced quarterly, and no one is permitted to speak while they are running.

The Dispute of the Expanding Bar

In the winter of the year recorded as 4,102, a routine comparison between the Prime Metre Bar and Reference Bar #9 in the Vestibule of Measure returned a discrepancy of eleven parts per million, which widened over four months before stabilizing. The episode became the Dispute of the Expanding Bar, the longest-running controversy in the history of the Register.

The Office of Metric Assurance maintained that the Prime Bar had not changed and that Reference Bar #9 had absorbed moisture through a faulty gasket. The Surveyor-Prime Althea maintained the opposite, arguing that a standard which has never varied is a standard which has never been tested. The matter was referred to Concordance Prime, whose Annex 12 (as amended) states:

"Where two standards disagree, the senior standard shall prevail. Where seniority is contested, both standards shall be retained and the disagreement recorded as a property of the interval between them."

— Concordance Prime, Annex 12, clause 4

This ruling has never been rescinded and has never been applied, since no one has been able to establish which Bar is senior. The gasket was replaced in 4,104. The discrepancy was not mentioned again.

Relation to the Prototypical Metre

The Bar is frequently confused with the Prototypical Metre, the polished rod kept for everyday calibration. The distinction is maintained in the Register's preamble: the Prototypical Metre is calibrated against the Prime Metre Bar, which is calibrated against itself. Surveyor-Prime Elara Vance observed in her survey notes that "a standard which certifies itself is either the most trustworthy object in the Tower or the least, and the Register declines to say which" (Survey Notes, vol. xiv, fol. 22). The observation was struck from the fair copy by order of the Silence Edicts but survives in the draft.

Calibration of the silvan unit proceeds from the Bar through the Master Standard Rule and the Master Orthographic Table, in that order. The order is not negotiable. In 3,988 a clerk reversed it and was reassigned to the inventory of stairwells for nine years; the reassignment is recorded in Dossier Alpha 707 without comment.

Incidents of record

Eleven handlings are recorded. The most discussed is the sixth, in which the Bar was removed from vacuum for cleaning and returned, by the unanimous assessment of the three concurring officers, to have grown by one ten-thousandth of a millimetre. The growth was attributed to the cleaning. The cleaning procedure was suspended and has not been reinstated.

The ninth handling occurred during the audit preceding the Presidium Accord 889-ω and produced no anomaly. The tenth produced two. The eleventh is recorded only as "completed," with the signature of the witness drawn by lot rendered illegible by damp; Librarian-Prime Anselm the Damp has declined to have the signature re-inked, on the grounds that a legible record of an illegible event would be a false record.

Visiting and handling

Visits to the chamber are conducted on the fourth day of each month, in groups of no more than four, without exception for rank. Visitors must surrender all measuring instruments at the door; the surrender is logged, and instruments are returned in the order surrendered, not the order requested. The chamber is cold. The visit lasts eleven minutes, which is longer than it sounds.

Disputes concerning the Bar itself — its length, its seniority, its custody, or the number of times it has been handled — are heard by the Register's own tribunal and may not be appealed to the Librarians, a restriction upheld in Concordance 1140 and quietly ignored in practice. The Office of Metric Assurance continues to receive such appeals and to file them.