Permanent Reference Bars

The Permanent Reference Bars are twelve platinum-iridium rods, each one Standard Tower Unit (STU) in length, held in the Vestibule of Measure on Floor 17,304. They are the Tower's primary material standard of length and, by extension, the ultimate authority for every measuring bar, ruler, and graduated instrument in the archive, including the Coincidence Ruler and the calibration instruments held under the Guild of Theoretical Mechanics. The Bars have not been moved from the Vestibule since the Fourth Concordance, Annex 12, and that Annex has been repealed twice and reinstated once.

Composition and custody

The twelve rods are numbered I through XII and are not interchangeable. Bar I is the master; Bars II through VII are the working copies issued, under seal, to the offices of survey; Bars VIII through XI are retained in the Vestibule against the failure of the master; and Bar XII is the arbitration bar, used only when two measurements disagree by less than the separation of graduation on either instrument.

Custody is vested in the Keeper of the Vestibule, an office presently held in commission by three clerks of the Bureau of Axiomatic Purity who sign the daily attendance ledger in rotation. The rotation has not changed since 4,408, though the ledger records that one of the three signs it in the dark as a matter of long habit, the Vestibule being lit only during the hours of official measuring.

History of the Vestibule

The Vestibule was constructed in 3,912 to house the then-eight Bars, which until that year had travelled with the survey caravan and were reported to have been shortened by handling. The Concordance of Enduring Measures records that the eighth bar, in particular, "had come to disagree with its own certificate" (Annex 2, clause 6). The four additional bars were cast in 4,001 from the same melt as Bar I, and their serial marks were struck by the same punch, which has since been lost.

In the winter of 5,113 a dispute arose between the Vestibule and the Waxwell Fabrication Office over whether the Bars could be removed to the Fabrication Office for comparison with the original bars of the Drain of Unfinished Thought. The Waxwell Fabrication Office held that they could not; the Keeper of the Vestibule held, at first, that they could. The matter was settled by Ordinance 702.4, which did not resolve the question, and by a subsequent amendment which forbade the removal of any Bar for any purpose, including measurement.

Use and calibration

Measurement against the Permanent Reference Bars is by contact comparison in the Vestibule, under the supervision of the Keeper or a clerk of the Bureau of Axiomatic Purity. Visiting instruments are first compared with Bar II, then with Bar VII if the first comparison is inconclusive, and finally with the arbitration bar if the two differ. Instruments found to be in error are marked and returned; instruments found to be in agreement twice are marked and retained for a year.

The Bars are also the reference against which Absence 417-Λ9 and Absence P-7 are measured by the coincidence method, though this is a derived use and is not acknowledged in the Vestibule's own register. The Reference Cylinder and the Oculus of Absolute Reference take their calibration from Bar X by an arrangement of long standing that no current clerk of the Vestibule can explain.

Disputed questions

There is a persistent disagreement, recorded in the marginalia of the Epsilon Reference Shelves, as to whether the Bars measure one STU or merely are one STU. The Concordance of Enduring Measures holds that the Bars are the standard (Annex 1, clause 1); the Directive Cross-Reference holds that the standard is the Standard Tower Unit itself and the Bars are its "most faithful likeness" (section 3, unpaginated). The two documents have never been reconciled, and both are cited in the Vestibule's daily register, depending on which clerk is on duty.

A second dispute concerns the twelfth bar. Recommendations for Cross-Reference, compiled in 5,208 by a sub-committee of two, recommended that Bar XII be retired on the grounds of an undetected irregularity said to be visible only under magnification. The recommendation was accepted and filed in the Gamma Ledger and never acted upon. Bar XII remains in service.

"The arbitration bar shall be used only where the parties have first confessed themselves unable to agree; and where they have so confessed, its finding shall be final; and where it is final, it shall not be appealed; and where it is not appealed, it shall be entered; and where it is entered, it shall be entered against one of them."

— Concordance of Enduring Measures, Annex 7, clause 3

Condition of the Bars

The Bars are inspected on the first working day of each quarter and found, on every recorded occasion but one, to measure exactly one STU. The exception is recorded in the Vestibule ledger for the quarter beginning 31,077, when Bar V was found to measure one STU to within the certificate's margin and the margin was not available. The entry reads: "Bar V within margin, margin in the annex, annex missing." The annex was later recovered; the margin agreed with the Bars. Visitors should not touch the rods.