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btilly
Check it out if for no other reason than to read the rant about how poorly defined the candela is.
DrJokepu
Also, it's really hard to resist the temptation to "rant back" on the kilogram question. Unfortunately / fortunately, HN comments are not the right place for that.

(Basically, kg is the perfect unit of mass and if you disagree you are a heathen and should be burned)

btilly
His complaint is that if kg is the perfect unit of mass then make what we call the kg be the base unit and call that a gram instead. Makes perfect sense to me.

Of course this is an imperfect world. In a perfect world I'd like a meter chosen so that gravity is, to a good approximation, 1 m/s at sea level, and I'd like all of the units to be in base 12. (Easy divisibility by 3 is often very useful.) Including time, so that one doesn't have the awkward 3.6 factor converting from m/s to km/h. And our number system should also be base 12, and...I think you can see that I'm willing to complain about lots of things which will never change.

eru
I want my number system to be in base 11 or some other prime number. You see: The vast majority of numbers are never going to evenly divide our base, no matter what base we choose. So we might as well go for a solution that minimizes the special cases.
btilly
The vast majority of the time that people wish to divide something evenly, they want to divide it by a small number.
eru
Yes. I just wanted to give the standard mathematician's tongue-in-cheek reply to proposals like base 12.

To be earnest, octal is quite a nice base. Or -2 (minus two!). Or 10, but with digits from -5 to +5, though that representation should be redundant, I guess.

To speak of redundant representations: Fibonacci numbering is also interesting (http://en.wikipedia.org/wiki/Fibonacci_coding). Or skew binary numbers.

redcap
energy := c^2 // convert mass to energy

Looks like it's missing some mass.

btilly
You're right. I just emailed a bug report.
monkey
Thanks for the report, but this shouldn't be considered a bug. That unit is defined so that you can write "1 gram energy" or the like to find the energy content in that particular amount of mass. It's actually shorter (on a reasonable keyboard) to type "c^2" though, so this may not really be necessary. I may remove it from the data file.
btilly
That makes sense. I actually like the way that reads, and would suggest keeping it and adding sample usage to make it absolutely clear to people.
sesqu
There's also a comment in Kelvin regarding how the triple point of water is defined to be at 0.01 °C, not 0 °C, that reads a little confused. I wonder why it's defined like that, anyway.

The bit is used for information because information is almost defined as the expected length of an optimally (context-free) coded word, using a base 2 code. Some physicists prefer base e, in which case it's measured in nats.

bluesmoon
the triple point of water is defined as 0.01 °C in order for a difference of 1 °C to be equal to a difference of 1 K.

To put it another way, the triple point of water is known to be 273.16 K, and the boiling point 373.15 K. Now if this mapped on to 0 °C and 100 °C, then we'd be left with a situation where 100 kelvins would be equivalent to 100.01 Celsius degrees. Instead, by defining the triple point of water as 0.01 °C (still 273.16 K), we now get an interval of 100 kelvins being equal to an interval of 100 Celsius degrees.

This is also why the term centigrade is no longer used... it's no longer 100 even.

eru
By the way, they should re-rig the definition of the Kelvin, to make Boltzmann 1. (The Planck units already do this.)

We would probably need either very tiny or very large multiplies of this new Kelvin for anything practical, though. (Depending on your choice for the other units. The The Planck unit of temperature is way too big for practical uses for example.)

jerf
Relativity unified some of the elementary units in surprising ways that make this sort of file hard to write if you really, really want to be rigidly correct. Technically, in some sense, "time" and "length" shouldn't both be atomic units, we should have only one or the other. But that would make this less useful, even if more accurate.
jarin
It's not a complete list of units without the hella- prefix.
tome
hellacandela?
bluesmoon
what? no Helens

    beauty =!= H
    Helen := H      // The beauty required to launch a 1000 ships
                    // A mH is the beauty required to launch 1 ship
philwelch
Wouldn't that be a mH (milli-Helen)? A micro-Helen would only launch 1/1,000th of a ship, so perhaps a kayak or life raft.
[deleted]
(comment deleted)
RiderOfGiraffes
It's missing the hundred, an old English measure of area.
gjm11
It defines "hundred" = 100, and likewise for other spelled-out numbers. I don't think it has any way of making it mean 100 in one context and an area (of, AIUI, indeterminate size) in another.
kristiandupont
Does anybody know why the standard unit for mass is kilograms?
dfox
THis comes from practical and historical considerations. Mass is probably most commonly used quantity by non-technical people, so it is desirable to maintain continuity with previous systems which had gram as base unit. On the other hand kilogram as base unit makes many useful relations between various quantities simpler not to mention that it is significantly more practical to make and maintain prototype of kilogram, than prototype of gram.
[dead] torpor
> Ctrl-F "quaver" not found

Meh.

DougWebb
It's missing the megalithic yard and related units. http://www.ancient-wisdom.co.uk/megalithicyard.htm

This turns out to be a really cool system of measurement involving distance, volume, mass, frequency, etc, all based ultimately on the rotation of the earth.

Edit: the book Urial's Machine and its successors expand a great deal on this system of measurement, and how there are echos of it in Old English and Babylonian measurement systems. I can't find an online description, though.