A Jiffy Is a Real Unit of Time

“Back in a jiffy” sounds like the vaguest possible promise, the verbal equivalent of a shrug. It could mean a minute. It could mean twenty. But in several technical fields, a jiffy is nothing of the sort. It is a defined length of time, and if you ask specialists in different subjects how long one lasts, you will get several different answers, each of them precise and none of them the same. The friendly little word for “soon” has had a surprisingly scientific career.

It started out as slang. According to the science education site Science Notes, sometime in the 18th century “jiffy” was slang for lightning. From there it came to mean a very short moment, the sort of time it takes for a flash to come and go. And like many handy words, it was eventually borrowed by people who needed a name for very small intervals and liked the idea of one that sounded quick.

In electrical engineering, the jiffy is tied to the mains power supply. Science Notes explains that in the United States, where alternating current runs at 60 cycles per second, a jiffy is one sixtieth of a second, or about 17 milliseconds. In places where the supply runs at 50 cycles per second, such as much of Europe and Asia, a jiffy is 0.02 seconds. So even a jiffy of electricity lasts a slightly different time depending on which country you are in.

Computers have jiffies too. Science Notes describes a jiffy in computer science as a single clock tick between interrupts, on the order of 1 to 10 milliseconds. The Linux operating system makes this very concrete. The book Linux Device Drivers explains that the kernel’s jiffies counter starts at zero when the computer boots and counts the clock ticks since it was turned on. The rate of those ticks is set by a value called HZ. The Linux Kernel Internals site lists common settings of 100, 250 and 1,000 ticks a second, giving ticks of 10, 4 and 1 milliseconds.

Physics offers the shortest jiffy of all. According to Science Notes, the American chemist Gilbert Lewis proposed that a jiffy should be the time it takes light to travel one centimetre. That works out to about 33.3 picoseconds, or 33.3 trillionths of a second. It is so short that a single blink of an eye would contain many billions of them. If a physicist ever promises to be with you in a jiffy, you may need to be very quick indeed.

What makes the jiffy delightful is the direction its meaning has travelled. Most technical words start precise and slowly get looser as they drift into everyday speech. The jiffy went the other way, from a casual bit of slang to a term with a number attached, and then several different numbers at once. So the next time someone says they will be back in a jiffy, it might be worth asking, very politely, which kind they mean.