# Jiffy (time)

Jiffy (time)

Jiffy is an informal term for any unspecified short period of time, as in "I'll be back in a jiffy". From this it has acquired a number of more precise applications for short, very short, or extremely short periods of time. Known since the 18th century, the word's origin is unclear, though one suggestion is that it was thieves' cant for lightning.[1]

## Beginnings in measurement

The earliest technical usage for jiffy was defined by Gilbert Newton Lewis (1875–1946). He proposed a unit of time called the "jiffy" which was equal to the time it takes light to travel one centimeter (approximately 33.3564 picoseconds).[2] It has since been redefined for different measurements depending on the field of study.[3]

## Use in electronics

In electronics, a jiffy is the time between alternating current power cycles,[2] 1/60 or 1/50 of a second in most building power supplies — see alternating current.

## Use in computing

In computing, a jiffy is the duration of one tick of the system timer interrupt. It is not an absolute time interval unit, since its duration depends on the clock interrupt frequency of the particular hardware platform.

Early microcomputer systems such as the Commodore 64 and many game consoles (which use televisions as a display device) commonly synchronize the system interrupt timer with the vertical frequency of the local television standard, either 59.94 Hz with NTSC systems, or 50.0 Hz with most PAL systems. Within the Linux operating system kernel, since release 2.6.13, on the Intel i386 platform a jiffy is by default 4 ms, or 1/250 of a second.[4] The jiffy values for other Linux versions and platforms have typically varied between about 1 ms and 10 ms.[5]

## Use in physics

The speed of light in a vacuum provides a convenient universal relationship between distance and time, so in physics (particularly in quantum physics) and often in chemistry, a jiffy is defined as the time taken for light to travel some specified distance. In astrophysics and quantum physics a jiffy is, as defined by Edward R. Harrison,[6] the time it takes for light to travel one fermi, which is the size of a nucleon. One fermi is 10−15 m, so a jiffy is about 3 × 10−24 seconds.

One isolated author[7] has used the word jiffy to denote the Planck time of about 5.4 × 10−44 seconds, which is the time it would take light to travel a Planck length if ordinary geometry was still relevant at that scale. Scattered reports have presented that obscure proposal as an established fact. Ironically, some of those reports seem to be based on a poorly written previous version of this very Wikipedia-article.

## References

Notes
1. ^ Douglas Harper (November 2001). "jiffy". Online Etymology Dictionary. Retrieved 2008-03-18.
2. ^ a b Gerard P. Michon (November 2002). "What's a jiffy?". Units of Measurement. Numericana. Retrieved 2008-12-30.
3. ^ Russ Rowlett (September 2001). "jiffy". How Many? A Dictionary of Units of Measurement. University of North Carolina. Retrieved 2009-08-31.
4. ^ "/pub/scm / linux/kernel/git/torvalds/linux-2.6.git / blob". git.kernel.org. Retrieved 2008-03-18. [clarification needed]
5. ^ "/pub/scm / linux/kernel/git/torvalds/linux-2.6.git / tree". git.kernel.org. Retrieved 2008-03-18. [clarification needed]
6. ^ "The Cosmic Numbers" in Cosmology, The Science of the Universe, 1981 Cambridge Press
7. ^ Lieu, Richard; Hillman, Lloyd W. (2003-03-10). "The Phase Coherence of Light from Extragalactic Sources: Direct Evidence against First-Order Planck-Scale Fluctuations in Time and Space". The Astrophysical Journal 585 (2): L77–L80. Bibcode 2003ApJ...585L..77L. doi:10.1086/374350.

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