## Conversion Perversion

“Be there in a jiffy.”

If someone says that to you, then you know that that person should arrive soon. But did you know that jiffy is a technical term? Similarly, the expression “two shakes of a lamb’s tail” used to indicate a short period of time, but the unit of time known as a shake now has a specific designation.

• 1 sec = 100 jiffies = 100,000,000 shakes

I’m big into conversions. I often tell folks, if you need to convert between televangelists and expatriate poets, the following picture may be helpful to you:

That is, 1 Ezra Pound ≈ 454 Billy Grahams.

The following are some other fun conversions.

• π sec ≈ 1 nanocentury

It’s interesting that this is so accurate. It is within 0.5%.

• 1 furlong per fortnight (FPF) ≈ 1 cm/min

This one is even better. The error is less than 0.000025%.

• 1 m/s = 1 Hz/dpt (Hertz/dioptre)

This is what can happen when common units are replaced with uncommon units. Hertz per dioptre is an inside joke among physicists and yet another reason not to hang out with them. (Dioptre is a unit of measure for the optical power of a lens.)

• 1 square = 100 square feet

The term square is used in the construction industry, typically to measure a roof. For example, if a roof has an area of 1,000 square feet, then the contractor would order 10 squares of shingles. But you wouldn’t want to use this unit in regular conversation, because it leads to awkward phrases like a “one-square square,” which would be a square that measures 10 feet on a side.

• 1 gal ≈ 3 + π/4 L

This is one of my favorite conversions. It’s accurate to 0.00000003%.

• 1 Hubble-barn ≈ 13.1 L

A Hubble length is the length of the observable universe (a very, very big length), and a barn is 100 square femtometers (a very, very small area), so it’s neat that their product gives a very tangible volumetric result.

• 1 stone = 14 pounds

When asked for my weight, I usually respond, “About 13 stones.” Such a reply leaves room for interpretation, and it could be assumed that I weigh as little as 175 pounds or as much as 189 pounds. And I’m fine with that. What kind of rude bugger asks your weight, anyway?

On a related note, the following formula can be used to approximate the U.S. population for a given year. Let x = the last two digits of the year, and let y = the projected U.S. population for that year (in millions). Then,

• y = πx + 276

This result is based on projections from the Pew Research Center. This formula provides an accurate estimate (within 1%) of the actual population for every year since 2000, and it should give a reasonable projection for the next several decades, assuming there are no major catastrophes.

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• 1. Joshua Zucker  |  June 23, 2012 at 4:20 pm

There shouldn’t be an approximately sign on your m/s in Hertz per diopter conversion. Also most of us Americans spell it diopter.

I was confused by your G abbreviation, too — gravitational constant? I would write “gal” instead. Parentheses around the numeric part might make it more clear too, perhaps?

• 2. venneblock  |  June 24, 2012 at 4:08 pm

D’oh! We have a lesson about Gallon Man on Illuminations, and it uses a very large G to represent Gallon Man. That must be what I was thinking. (Then again, I lost 5 points on a 2-point question in high school when I used “pds” instead of “lb” as the abbreviation for pounds, so screwing up unit abbreviations is something with which I have a long history.)

As for Hz/dpt… in deference to my friend Guy, who studied optics at Cambridge, I’m going to continue to leave the international spelling. That’s the one he used when he showed me this conversion. (Though on all other things optical, I’ll defer to you. You’re clearly more of an expert in this area than I am.)

• 3. Karen Craigs  |  July 17, 2012 at 9:42 am

Maybe “dioptre” is like “metre” in that it’s the correct spelling for countries that regularly use the metric system? 😉

• 4. venneblock  |  July 17, 2012 at 10:39 am

No doubt, Karen. But those countries also have words like “favour” and “colour,” so maybe we should spell it “dioptour”? 🙂

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