The smoot, the barn and the beard-second: units of measurement that are real

The Harvard Bridge between Boston and Cambridge carries two sets of distance marks. One is in feet. The other is repainted by MIT students and measured in the height of a man who stood five foot seven in 1958. It is not the only unit still in working use that began as a joke.

The smoot, and the ear that came with it

In October 1958 members of the Lambda Chi Alpha fraternity at MIT measured the Harvard Bridge by laying down their shortest pledge, Oliver R. Smoot, and painting a line each time he stood up. The span came out at 364.4 smoots, with the uncertainty recorded on the bridge itself as "plus or minus one ear." Cambridge police used the marks to log accident locations, and when the bridge was rebuilt in the 1980s the new sidewalk slabs were cut one smoot long instead of the usual six feet. Smoot later chaired ANSI and served as president of ISO. Google's calculator still converts smoots, at exactly 67 inches. (Wikipedia; MIT Alumni Association)

The barn, borrowed from a bad shot

Nuclear physicists needed a unit for the effective target area of an atomic nucleus, which works out at roughly 10⁻²⁸ square metres. In December 1942, over dinner at the Purdue Memorial Union, Marshall Holloway and Charles Baker settled on "barn," taken from the American expression about a shooter who could not hit the broad side of one. The name also did useful work: while the research was classified, it hid what they were measuring. Purdue's school of nuclear engineering still publishes a newsletter called The "barn" Chronicles. (Purdue University; OSTI, Los Alamos manuscript history)

The shed, which turned out to be unnecessary

Once a barn exists, somebody will build a smaller one. The shed is 10⁻²⁴ barn, or 10⁻⁵² square metres, coined for neutrino cross-sections so small that even the barn felt clumsy. It never caught on. Wikipedia's entry, which cites the second edition of the Oxford English Dictionary, calls it the smallest unit ever used in subatomic physics and notes that its use was minimal: a unit invented for a problem that never really arrived.

The shake, or ten nanoseconds

One shake is 10 nanoseconds, the informal unit Manhattan Project physicists attached to the gap between successive generations in a fission chain reaction. A single step takes about one shake, the whole reaction something like 50 to 100. The word came from "two shakes of a lamb's tail," a phrase in print since at least the 1830s. Thomas Cochran of the Natural Resources Defense Council wrote in 1994 that the interval was defined as one shake during the Manhattan Project. (Wikipedia)

The jiffy, which changes size by discipline

Jiffy began as slang for a moment and has been pinned down at least four times. The American physical chemist Gilbert N. Lewis defined it as the time light takes to travel one centimetre, about 33.3564 picoseconds. Electrical engineers used it for one alternating current cycle. Linux kernels use jiffies for timer ticks, typically between 1 and 10 milliseconds depending on the build. In quantum physics it shrinks again: Edward R. Harrison defined it as the time light needs to cross one fermi, roughly 3 × 10⁻²⁴ seconds. (Wikipedia; Russ Rowlett, University of North Carolina)

The beard-second

Modelled on the light-year and aimed, half seriously, at integrated circuit dimensions, the beard-second is how far an average beard grows in one second. Kemp Bennett Kolb fixed it at 100 ångströms, or 10 nanometres, and Nordling and Österman's Physics Handbook uses the same figure. Google's calculator uses 5 nanometres. Nobody has ever needed it. (Wikipedia)

Furlongs per fortnight

The furlong-firkin-fortnight system exists mainly to give students something painful to convert. One furlong per fortnight is 0.1663 mm/s, close enough to a centimetre a minute for casual work, and about the speed of the tip of a 3¾-inch minute hand. It also scales upward without complaint: light travels at roughly 1.8026 trillion furlongs per fortnight. The system has one genuine computer science credit, too. The VMS operating system sets its boot-time prompt wait in microfortnights, which works out at 1.2096 seconds each. (Wikipedia)

The banana equivalent dose

Bananas are rich in potassium, and a small fraction of all potassium is the radioactive isotope potassium-40. The US Environmental Protection Agency puts the dose from eating one banana at 0.01 millirem, about 0.1 microsieverts, and notes that you would need roughly 100 bananas to match the natural background radiation a person absorbs in an average day in the United States. Teachers and radiologists use it to make very small numbers legible. It is not a real exposure route: the body holds potassium levels steady and excretes the surplus, so the dose does not accumulate. (US EPA)

The micromort

Ronald A. Howard, a decision-analysis researcher at Stanford, introduced the micromort around 1979 to put very different dangers on one scale. One micromort is a one-in-a-million chance of death. Wikipedia's entry, drawing on Howard's 1980 chapter "On Making Life and Death Decisions," lists about one micromort for a few hundred miles of driving, around eight for a skydive, and roughly 24 as the baseline daily risk of death from all causes for an average adult.

The Garn

In April 1985 Senator Jake Garn of Utah flew on shuttle mission STS-51-D as a congressional observer and as a test subject for space motion sickness experiments. He had logged more flying hours than any NASA astronaut at the time, and he was still the sickest anyone on record has been in orbit. The astronaut corps started rating space adaptation syndrome on the Garn scale, with one Garn meaning the worst case a human can have. As Robert E. Stevenson put it, most people manage a tenth of a Garn at most. NASA named a training facility after him in 1992. (Wikipedia; NASA STS-51-D press kit)