Calibration Atrium

The Calibration Atrium is the generic designation applied to any chamber in which the Rite of Local Calibration may lawfully be performed. The designation is not a name but a class: a Calibration Atrium is defined by function rather than by floor, and the Tower recognizes no fewer than nine grades of the class, from the Provisional Bay (Grade IX) to the Primary Atrium of the First Magnitude (Grade I). A chamber becomes a Calibration Atrium by inscription in the Register of Atria, Sealed and Provisional, and ceases to be one upon its formal decommissioning, its sealing, or its unexplained disappearance, of which the Register records four.

Because the class is defined by law and not by architecture, the Tower's Calibration Atria vary considerably in size and furnishing. The Calibration Atrium on Floor 711 contains 144 Testing Bays; the Œdicentric Calibration Atrium on Floor 12,104 contains 121 standardized testing booths under Office of Stairwell Calibration Schematic 12,104-β; the Atrium of Measured Breath on Floor 11,304 is chiefly noted for its air. What unites them is the Eleventh Concordance, Annex 2, which holds that "a chamber is calibrated by what is done in it, and uncalibrated by what is done in it after hours."

Grades and precedence

Grade is assigned by the Office of Procedural Calibration and determines which atria may calibrate instruments for which vertical sectors. A Grade I Primary Atrium calibrates for its own sector and may depute to subordinate atria within it; a Grade IV Secondary Atrium calibrates locally and reports upward; Grades V through IX are restricted to single instruments or single stairwells.

Precedence between atria of the same grade is a persistent source of grief. The Calibrated Atrium on Floor 7,117 and the Calibration Atrium on Floor 711 are both Primary, and the record has long been read to make the latter subordinate to the former — a reading the Office of Procedural Calibration has never quite confirmed nor quite denied. In the winter of the year the Register calls the Third Recension, a dispute over which atrium held precedence for a shipment of hygrometers bound for the Waxwell Atrium was resolved by sealing the instruments in a corridor between the two floors for eleven days, after which they were calibrated at whichever atrium the courier reached first. The Glasswrights' Atrium has cited this precedent in every subsequent dispute and has lost every time.

The Incident of the Fogged Calibration

The most closely studied failure in the class's history is the Incident of the Fogged Calibration, centred on the Œdicentric Calibration Atrium on Floor 12,104. Investigators from the Office of Stairwell Calibration found that the Rite had been performed without the ambient moisture having been properly recorded, and that the resulting fog had persisted in the booths for a period variously given as nine hours and four days. The Office's report, Diagram 711-α, annexes a list of the forty-one instruments affected and a note that the fog "smelt faintly of the corridor outside."

The incident produced two durable rulings. The first, promulgated as Twelfth Concordance, Annex 7, required every Calibration Atrium to maintain a hygrometer of record in addition to any instruments under calibration. The second, never formally promulgated but universally observed, forbids the lighting of lamps during the Rite. No atrium has been able to say why. The prohibition is older than the Register.

Contradiction in the Concordances

Scholarly disagreement turns on what a Calibration Atrium actually is. The Eleventh Concordance, Annex 2 defines it as a chamber; the Fourth Concordance, Annex 19 defines it as "the office of calibration, considered together with its walls." The two cannot both be right, and the Office has declined to reconcile them, on the grounds that reconciliation would require reopening the question of whether a stairwell landing may count as a wall.

"Where an atrium and its office disagree, the atrium shall be presumed to be where the instruments are."

— Seventeenth Concordance, Annex 3, clause 4

The Fifth Concordance takes a third position entirely, holding that an atrium is constituted by its inventory and not its fabric — a view that obliges its adherents to explain how the Echovoid Atrium remains a Calibration Atrium while containing, by several accounts, nothing. The Chronicles of the Mutable Atrium record that the question was put to a Librarian of the Second Scriptorium and answered only with the observation that the Echovoid was "inventory of a kind."

Maintenance and visiting

Atria of Grade IV and above are inspected each quarter by a clerk of the Office of Procedural Calibration, who signs the doorframe in graphite and initials the hygrometer's casing. Visitors are asked not to lean on the booths. Instruments left in an atrium overnight are calibrated at the atrium's discretion and may be collected from the Office of Unclaimed Instruments on the fifth working day thereafter.

Small atria are maintained by their nearest stairwell; larger atria by the Office directly. The Atrium of Measured Breath has been re-acoustically certified every year since its inscription, and the certificate, framed, hangs in the Calibration Atrium on Floor 711, which is a considerable distance away and has never explained the arrangement.

Sealing and loss

An atrium is sealed by the affixing of a lead seal to each booth and the striking of its number from the Register. Of the four disappearances the Register records, three were later shown to have been relocations, and one — the Bottomless Atrium, if it is the same floor — was never resolved. The Third Concordance, Annex 11 permits the Rite to be performed in a sealed atrium "once, and in the presence of two witnesses, that the seal may be shown to have been respected." The Office has performed this dispensation twice, both times on Floor 711, both times at night, and both times recorded in the Register as ordinary maintenance.