Channels
Channels are the unit of subscription and the unit of stability. We have added two in eleven years — Objects in 2019 and Horology in 2022 — and removed none, because removing one would mean somebody's screen going blank.
9,140 items
Units, tolerances, and the documents that fix them. The most stable channel in the corpus and the one customers pin when they want something that will still be correct after they have left the company.
Photograph: Santeri Viinamäki · CC BY-SA 4.0, via Wikimedia Commons
Sample items
Since 20 May 2019 the kilogram has been fixed by setting the Planck constant to exactly 6.62607015 × 10⁻³⁴ joule seconds. The platinum-iridium cylinder kept at Sèvres since 1889 is now an artefact rather than a definition.
The ISO 216 A-series is built from a single sheet of one square metre with sides in the ratio √2. Halving it along the long side preserves the ratio, which is why A4 folded is A5 and why enlarging A4 to A3 needs no cropping.
One metre is the distance light travels in vacuum in 1/299,792,458 of a second. The definition has held since 1983 and fixes the speed of light exactly, which means it is no longer something that can be measured, only used.
7,822 items
Locks, lifts, junctions, towers and the arithmetic behind them. Consistently the highest-dwell channel: people will read about a boat lift for considerably longer than they will read about anything else.
Photograph: Sean Mack · CC BY 2.5, via Wikimedia Commons
Sample items
Because the two caissons carry equal masses of water whether or not a boat is in them, the wheel is very nearly balanced and a half-rotation costs roughly the energy of boiling eight kettles.
Roughly one bar for every ten metres of elevation. The tank's capacity determines how long the pressure lasts; the height determines what the pressure is. This is why towers on hills are short.
A conventional four-way intersection has thirty-two points at which two vehicle paths can cross. A single-lane roundabout has eight, all of them at shallow angles and lower speeds.
6,015 items
Classification, not forecasting. We describe what clouds are called and how the scales were built. We do not tell anybody whether it will rain, because that is a prediction and predictions are how a feed acquires stakes.
Photograph: LiCheng Shih · CC BY 2.0, via Wikimedia Commons
Sample items
Cirrus, cirrocumulus, cirrostratus, altocumulus, altostratus, nimbostratus, stratocumulus, stratus, cumulus and cumulonimbus. The scheme descends from Luke Howard's 1802 essay and has survived two centuries of revision largely intact.
Read since 1924 in a fixed order, clockwise from Viking off Norway. The late broadcast is held to about 370 words, which is why it is delivered at the pace it is.
A reference condition at mean sea level, not a measurement of anywhere in particular. Aircraft altimeters, engine ratings and wind-tunnel results are all quoted against it so that numbers taken in different weather can be compared.
8,431 items
Timetables, gauges, crossings and routes, held as published rather than as running. Nothing in this channel is live, which is precisely why it is safe to put on a departure board.
Photograph: Aldebaran · CC BY-SA 3.0, via Wikimedia Commons
Sample items
Westray to Papa Westray in Orkney is timetabled at two minutes over 2.8 kilometres. With a following wind it has been flown in under a minute, which is shorter than the safety briefing.
Moscow to Vladivostok is 9,289 kilometres. Until 2018 the entire network's timetables were published in Moscow time regardless of where the station was, which was confusing for everybody but consistent.
Bergen to Kirkenes and back takes twelve days. The service has carried mail, freight and passengers on the same schedule since 1893, and several of its calls last only fifteen minutes.
5,988 items
Manufactured things, their dimensions and their origins. Pallets, cones, fasteners, packaging. The channel most often pinned by industrial customers, who find it either useful or reassuring and decline to say which.
Photograph: Oliver Eltinger · CC BY-SA 4.0, via Wikimedia Commons
Sample items
The specification fixes the count and placement of the nails, not merely the outer dimensions, because the nailing pattern is what determines whether the pallet survives being handled by a machine that does not care about it.
Sealed between two shower curtains in 1957 and marketed as a textured wall covering. It did not sell. Its second commercial life was as greenhouse insulation. Packaging was the third attempt.
The Gem, the double-oval shape almost everyone pictures, appears in British manufacture from the 1890s with no patent attached. The designs that were patented are the ones nobody uses.
4,508 items
Time signals, scales and the machinery of keeping them. The smallest channel and the slowest-growing, which is appropriate. Popular with customers who want one item on screen for a long time.
Photograph: National Institute of Standards and Technology · Public domain, via Wikimedia Commons
Sample items
Specifically, that many periods of the radiation between the two hyperfine levels of the ground state of an unperturbed caesium-133 atom. The number is not round because it was chosen to match the second already in use.
The most recent was at the end of 2016. In 2022 the General Conference on Weights and Measures resolved to stop the practice by 2035, on the grounds that it causes more trouble in computers than it solves in astronomy.
Five short and one long, broadcast since 5 February 1924. The last pip marks the hour; the first five are lead-in. The convention exists because a listener needs warning that a moment is about to happen.
Ambient is free for one screen and one channel, and it does not expire. The key takes about a minute.
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