Chandler Wobble, Chandler Wobble
Dormez-vous? Dormez-vous?
How was that measured, given that water gets more dense as the ATMs get higher? IE the liter at the bottom is going to hold more water (at a higher pressure, and possibly a higher temperature) than the liter at the top, if you have, say, 10 liters or so one on top of another.
As It Happens did a very good interview about this on May 28th , it starts about 11 minutes into the shows part 2,
the direct link in Real Audio.
They think the platinum is absorbing environmental hydrocarbons…
The one liter of water weighs one kilogram isn’t a mere “rule of thumb”, it’s the old definition of the kilogram. There were certain other provisions, for example the temperature of the water was supposed to be 4 degree centigrade. Unfortunately I don’t have my books here, so this is all from memory.
I agree with Robot Arm that for practical purposes the defined relation between a mol of C12 and its weight isn’t really helpful.
Does anybody know for which practical use that platinum-iridium cylinder is used? I mean, for day to day life, we don’t need that kind of precision anyway.
No argument-these sort of computations presume something like 68°F, Sea Level, Republican President, Yankees down in the bottom of the 9th with three on, etc. 
To be more specific, I could have sworn that I learned a kilogram was equal to the mass of one liter of 4 degrees celsius water at standard atmospheric pressure
Blalron, the thing is that water can have varying weights depending on the purity and the isotopes of Hydrogen and Oxygen used.
Well, yeah, I figured I probably dropped a couple decimal points (and I didn’t do too badly, considering how long ago I learned this), but that wasn’t really what I was getting at. Even if you knew the exact number in a mole, that doesn’t help you in this case unless you can manufacture a sample with the required number of atoms. I don’t know of any way to do that. (Thanks for backing me up, Optihut.)
But here’s something I just thought of, how do they know the platinum-irridium kilogram is shrinking? What are they comparing it against?
Whatever it is, make that the new standard.
Oh, no doubt that the mole of C-12 equals 12 grams is a useless practical standard, but do you really think that the meter being defined in terms of the speed of light is a useful practical standard either? Or, for example, the ampere being defined in terms of the force between two infinitely long parallel wires of negligible cross-section?
The nice thing about an atomic standard of weight is that it pretty much can’t change, even if in every day life you need to use something else. Just like in every day life, you can’t use infinitely long parallel wires of negligible cross-section to calibrate your current.
Seeing as how there’s 1024 britsh bit units in a kilogram and the kilogram is shrinking are the bbus perhaps no longer sharing hectares with the adjusted millinewton micromoles?
I’m sure I read that somewhere.
So, does this mean that the gym standard of “Multiply by 2.2 and add one if it’s, like, a big number” has to change?
As a member of Canada’s lost generation ( the changeover happened while I was in grade school, so I know some things in Imperial, I know some other things in metric, and I give driving directions by time rather than either of those) I’m approximate in all things at a macro level and just can’t get too upset about anything short of the loss of an ounce in a pint. Of course, a pint varies from 18 to 24 ounces, depending where you are drinking, so I guess I really don’t care.
I tend towards the Discworld view in the minutiae of weights and measures. It’s good that there’s a place where people who worry about that sort of thing can go and keep themselves occupied, because who knows what they’d do if there wasn’t.
Check out the As It Happens link hampster posted back there. They did a great job of explaining it. A few points for questions above:[ul]
[li]It sucks to lose count at 8.3^13 atoms. Getting a good definition is hard.[/li][li]The One True Cylinder is shrinking when measured against the few reference cylinders made of it. Most scientists believe these are absorbing matter through regular use, whereas the OTC is not.[/li][li]This is important to everyone, cause these define other weights, which define other weights, which define other weights… which define your average market/bathroom scale. Errors magnify, so (according to the previously mentioned link), this actually does make a difference.[/li][/ul]Everyone had better pay attention, 'cause until this is fixed, you’re gonna weigh more! :eek:
That’s how it was originally defined, but because of the variability inherent in that measurement (nailing down “standard” atmopheric pressure, for example, could be a bit dicey), it was redefined with something more stable and permanent, or so they thought! All of this begs the deeper question, of course, about whether anything is truly constant and immutable.
If that was the case, the kg would be getting heavier, rather than lighter.
I’ve got a feeling this isn’t significant to people getting on the scales and saying ‘Damn! More pounds! HOW??? Those scientists must have made a mistake…’ but is a real pain to scientists.
I’m sure they’ll manage to find a definition in terms of atomic weights or something soon, and we can forget the whole sorry buisness 