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A “jiffy” is more precise in its definition than you may have thought. The earliest technical usage was defined by Gilbert Newton Lewis (1875–1946). He defined the “jiffy” as the time it takes light to travel one centimeter in a vacuum (approximately 33.3564 picoseconds). It has since been redefined for different measurements depending on the field in which it is used:
- In electronics, a jiffy is the length of an alternating current power cycle, 1/60 or 1/50 of a second in most main power supplies.
- In computing, a jiffy was originally the time between two ticks 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. Jiffy values for various Linux versions and platforms have typically varied between about 1 ms and 10 ms, with 10 ms reported as an increasingly more common standard in the Jargon File.
- The term “jiffy” is sometimes used in computer animation as a method of defining playback rate, with the delay interval between individual frames specified in 1/100th-of-a-second (10 ms) jiffies, particularly in Autodesk Animator .FLI sequences (one global frame frequency setting) and animated Compuserve .GIF images (each frame having an individually defined display time measured in 1/100 s).
- 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, the time it takes for light to travel one fermi, which is approximately the size of a nucleon. One fermi is 10−15 m, so a jiffy is about 3 × 10−24 seconds. It has also more informally been defined as “one light-foot”, which works out as approximately one nanosecond.
One author 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 were still relevant at that scale.
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