Primary Standard

The Primary Standard is a bar of platinum-iridium alloy, 1.000 metres in length at 4 degrees above the freezing point of the Tower's cistern water, held under double seal in a vault on Floor 0. It is the ultimate length standard of the Tower, against which every ruler, tape, plumb-line, and stair-gauge in the archive is notionally traceable. It has not been removed from its vault since the Ninth Recension of the Ordinance of Standard Measures, and it has been formally compared with another object only eleven times in recorded history.

Description and custody

The bar is 1.000 m long, 26 mm wide, and 26 mm deep, with two engraved lines ruled on its upper face at the neutral plane; the distance between them, and not the ends of the bar, constitutes the standard. The engraving is 0.2 mm deep and is re-inked once per decade with Gallstone Ink by a clerk of the Division of Calibration working under the observation of two witnesses who are not permitted to speak.

Custody is divided by design. The Office of Weights and Seals holds the key to the vault; the Division of Calibration holds the key to the inner case; and neither office is permitted to hold both. The arrangement was introduced after the Affair of the Borrowed Fathom (Floor 44, winter of the year of the fourth audit), in which a vault clerk named Merish Dolve removed the bar to settle a dispute over the width of a landing and returned it, by his own account, "substantially the same length." Dolve was reassigned to the Standard Pacing Tiles and the double-key rule was entered into the record without further comment.

The comparison problem

The difficulty that has shaped the Primary Standard's institutional life is that length, unlike duration, cannot be referred to a void. Tower Standard Chronon is derived from an event; the Primary Null Standard is a silence against which instruments are verified; the Primary Resonant Standard is a sealed absence maintained by the Office of Harmonic Measurement. The Primary Standard, by contrast, is a thing that must be touched by another thing in order to be consulted.

Since every contact risks the engraved lines, the Division of Calibration long ago abandoned direct comparison in favour of working standards of the second and third remove, which are compared with one another and only very rarely with the bar itself. The result is a chain of measurements in which the last link is inspected more often than the first. A note in the Registry of Working Standards, volume 12, records the position plainly: "The bar is the standard because we do not use it."

Drift and the nominative question

The Primary Standard is subject to Standard Drift in its nominative subtype. No physical change to the bar has ever been demonstrated; what drifts is the sense of what it is the standard of. Early recensions describe it as the standard of the Tower's stone; later recensions describe it as the standard of the Tower's measures; the Third Concordance, Annex 4, describes it only as "the bar, and what follows from the bar."

This has produced a durable scholarly quarrel. The Commissive School, following Librarian Ossava Mend, holds that the Primary Standard defines the metre and that all other lengths are derived from it by division. The Declarative School, following the marginalia of Librarian Hest, holds the reverse: that the metre is a name given to a length the Tower already possessed, and that the bar merely witnesses it. The dispute is not academic. Under the Commissive reading, a damaged bar would require the re-measurement of the Tower; under the Declarative reading, it would require only a new bar.

"The bar does not make the fathom. The fathom was here before the vault, and will be here after the key is lost."

— Librarian Hest, marginalia to the Seventh Recension of the Ordinance of Standard Measures

The Concordance of 1140 on the Priority of Instruments appears to settle the matter in favour of the Commissive School, but its operative clause is held by many readers to be an interpolation, and the same document is cited by the Declarative School on the strength of its preamble. Both citations are considered correct.

Relationship to other primaries

The Primary Standard is one of four objects styled Primary, and the only one that is a solid. It is distinguished from the Primary Artifact, which is the physical manifestation of the Standard Lumen Unit and is therefore luminous rather than dimensional, and from the Primary Pendulum, which verifies the progression of Tower Standard Time but is not itself the standard of anything. The Primary Standard Plaque, which carries the canonical text of the Chain-of-Custody Acknowledgment, is frequently confused with the bar on account of its name; the Plaque is a plaque, is mounted, and may be read by anyone, and the confusion has been the subject of two memoranda from the Office of Weights and Seals.

Access and prohibitions

The bar may be viewed through the vault grille on the first working day of each Standard Tower Month by parties holding a Standard Terminal Unit endorsement. Viewing is limited to four persons at a time. Nothing may be passed through the grille, including gloves, and the grille is not to be leaned upon.

Requests for direct comparison are heard by the Standing Committee on Primary Measures and are granted, on average, once every sixty years. The most recent grant was to the Guild of Surveyors and Stair-Wardens in connection with the resurvey of Floor 0 itself, which was found to be 4 mm shorter than its own record and was amended rather than rebuilt. The bar was returned to its case at the close of the second day, and the vault was sealed with the Guild Standard 115delta1a impression, as is customary.