Sigmagpulse

Sigmagpulse is the regular cyclic discharge propagated along the Sigma Regulator and observed, at a distance, as a faint throb in the load-bearing walls of every floor between 300 and 302. It is catalogued as a phenomenon rather than a machine: nobody has established that anything generates it, and the Concordance on Naming of Recurring Events (Annex 2, clause 6) explicitly forbids describing it as equipment. The pulse recurs at intervals of 44 seconds, a period fixed by Protocol Sigma-7 and, according to the orthodox account, never since altered.

The name is a compression. The registers of the Sigma Registry Office record the full designation as sigma-gated pulse, regulated class, unassigned series; the abbreviation appears in a clerk's marginal note of 14 Frostmonth 612 and has been used in all subsequent holdings cataloguing. That the abbreviation survived and the full designation did not is a matter of some irritation to the Office, which re-enters the long form at the head of every new recension.

Period and measurement

The pulse is measured by three independent means, which agree to within two seconds and have never agreed more closely than that. The Observatory of the Lower Spiral counts the audible throb in the stair wall. The Office of Interval times the discharge at the Regulator itself using a Graded Column of mercury and salt. A third count, maintained since 588 by the Keepers of the Aperture, is not a measurement at all but a tally kept by hand on a slate, and has the best record of continuity.

The two-second spread is the subject of an old and unproductive dispute. The Eleventh Concordance holds that the pulse is exact and the observers are not; the Third Concordance, Annex 4, holds the reverse. A reconciliation attempted in 640 by Surveyor Adept Marn Vellick concluded that both documents were correct but referred to different pulses, a finding the Registry Office declined to enter.

Emission and the stairwell record

The pulse is most clearly felt in Stairwell Sigma-44, where the risers at landings 7 and 19 resonate with the discharge and produce the low tone described in the stairwell's own holding. Travellers using the stairwell are asked not to halt on the resonant landings during the discharge; the Standing Order on Stair Traffic, clause 11, states the rule without explanation, and it is enforced chiefly by the inconvenience of being moved.

Between 601 and 604 the pulse was recorded as absent. The Gap of 601 is the only interruption in the continuous record, and three explanations survive: that the Sigma Regulator was under repair (the Works Ledger of the Regulator shows no such entry); that the observers were reassigned to the Vertical Tram Line Sigma during its extension and no one was left to count; and that the pulse occurred but was not catalogued, which the Office of Interval regards as the least likely of the three. Librarian Overseer Halven Struce argued in a note of 607 that the Gap had been "not a silence but an absence of clerks", a phrase now fixed in the Office's correspondence.

Relation to Protocol Sigma-7 and 12-D

The pulse and Protocol Sigma-7 are frequently confused in popular holdings, and the Registry Office maintains a standing correction to the effect that the Protocol governs the interval, not the pulse. Protocol 12-D, by contrast, treats the pulse as a clock: several of its inspection schedules are timed by counting discharges rather than by hours, so that a delayed inspection is described as "three pulses late" (12-D, clause 9). This usage has spread beyond the Protocol and into ordinary speech on floors 300 to 302, where an appointment is more often given in pulses than in hours.

The Sigma Regulator is presumed to be the pulse's origin, though the presumption rests on proximity and the word regulated in the designation. No inspection of the Regulator's interior has been recorded since the Sealing of the Regulator Housing in 578, and the seal is widely held to be unbreakable without a warrant no one has applied for.

Dispute: the period

Whether the pulse has always run at 44 seconds is unresolved. Line Sigma's oldest survey sheet, dated 561, gives the period as 43 seconds and adds, in a different hand, "or thereabouts." The Office of Interval holds that the 561 figure is a copying error; the Observatory of the Lower Spiral holds that the pulse has lengthened by one second over four centuries and that the lengthening is accelerating; the Office has twice refused to hear the argument on the grounds that its own figures are the ones at issue. A fourth position, advanced by Archivist Pella Renn in 655 and largely ignored, proposes that both are right and that the pulse is not the same event from one decade to the next — "a succession of pulses wearing one name" (Renn, Notes on Recurrence, 655).

Regulation

By the Third Concordance, Annex 6, no work involving hammering, boring, or the driving of wedges may be undertaken within nine feet of the Sigma Regulator during a discharge. The Concordance on Interference with Recurring Phenomena extends this to floors 300 to 302 generally, with the single exception of the period between the thirty-eighth and fortieth pulse of each day, when the Maintenance of Load-Bearing Walls is permitted to make its inspections. The exception has never been explained in writing. The maintenance party uses it every day and has never asked.

"The pulse is not to be interrupted, discouraged, or answered."

— Concordance on Interference with Recurring Phenomena, clause 1

What is meant by answered has occupied the Committee on Unclear Clauses since 634. The Committee's working opinion, never adopted, is that the word was chosen to stop the singing.