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#1
11-11-2010, 03:12 PM
 sweeteviljesus Guest Join Date: Mar 2006 Posts: 2,405
Where does the mass of a kilogram come from?

Is it supposed to be the mass of one liter of water? (I know it is defined as the mass of a particular artifact, but is that what that artifact is supposed to represent?)

Thanks,
Rob
#2
11-11-2010, 03:20 PM
 Nunavut Boy Guest Join Date: Dec 2003 Location: Iqaluit, NU Posts: 945
I was taught that a cubic decimeter of water at its greatest density is equal to one kilogram.

Likewise, one cubic centimeter of water = 1 gram

One cubic meter of water = 1 tonne.

I love the metric system!

Last edited by Nunavut Boy; 11-11-2010 at 03:22 PM.
#3
11-11-2010, 03:25 PM
 sweeteviljesus Guest Join Date: Mar 2006 Posts: 2,405
Oops, this was supposed to be in GQ.

Rob
#4
11-11-2010, 03:28 PM
 Marley23 I Am the One Who Bans Join Date: Apr 2002 Location: Brooklyn Posts: 78,234
Moved from Great Debates.
#5
11-11-2010, 03:33 PM
 Simplicio Guest Join Date: Dec 2003 Location: Vermont Posts: 11,805
The chunk of metal used to formally define the Kilogram was machined so that it would be close to the weight of 1,000 cubic centimeters of water at 4 degrees. I'm not really sure why they haven't taken the obvious step of just using that as the definition. Perhaps water doesn't have as fixed a density as I'd think at a set temperature?

Last edited by Simplicio; 11-11-2010 at 03:34 PM.
#6
11-11-2010, 03:38 PM
 ChrisBooth12 Guest Join Date: Sep 2005 Posts: 1,768
Yeah I never got that either. Why not just say a kilo is this much pure water at this temp at this pressure..?

I mean we can't exactly count the molecules here
#7
11-11-2010, 03:48 PM
 Malacandra BANNED Join Date: Jan 2003 Location: England, Britain, UK Posts: 18,480
More likely it's hard to measure the volume of water to the necessary degree of accuracy, and ensure the purity of same. May as well use an imperishable standard lump (well, as imperishable as can be managed - nothing's 100.0000000000000%) and be done with it.
#8
11-11-2010, 04:01 PM
 beowulff Member Join Date: May 2001 Location: Scottsdale, more-or-less Posts: 15,097
Quote:
 Originally Posted by Simplicio The chunk of metal used to formally define the Kilogram was machined so that it would be close to the weight of 1,000 cubic centimeters of water at 4 degrees. I'm not really sure why they haven't taken the obvious step of just using that as the definition. Perhaps water doesn't have as fixed a density as I'd think at a set temperature?
The new standard will be a really cool Single-isotope Silicon sphere.
#9
11-11-2010, 04:06 PM
 Leaffan Guest Join Date: Aug 2005 Location: Ottawa Posts: 22,811
Quote:
 Originally Posted by beowulff The new standard will be a really cool Single-isotope Silicon sphere.
Oh shit. The Hortas are not going to like this.
#10
11-11-2010, 04:15 PM
 RealityChuck Charter Member Join Date: Apr 1999 Location: Schenectady, NY, USA Posts: 40,916
They have been working for years on trying to find a standard that did not depend on the kilogram in Paris. They replaced the meter stick years ago, but finding something that has the required accuracy is pretty damn hard. The sphere is a good start, but ultimately, it's just another object.

What they want is some way to measure it without needing an artifact. For instance, the standard meter is defined now as "the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second." This allows people to create their own meter sticks without having to measure them against any standard.
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#11
11-11-2010, 04:36 PM
 beowulff Member Join Date: May 2001 Location: Scottsdale, more-or-less Posts: 15,097
Quote:
 Originally Posted by RealityChuck They have been working for years on trying to find a standard that did not depend on the kilogram in Paris. They replaced the meter stick years ago, but finding something that has the required accuracy is pretty damn hard. The sphere is a good start, but ultimately, it's just another object. What they want is some way to measure it without needing an artifact. For instance, the standard meter is defined now as "the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second." This allows people to create their own meter sticks without having to measure them against any standard.
Except a clock...
#12
11-11-2010, 04:45 PM
 Squink Guest Join Date: Oct 2000 Location: Yes Posts: 20,327
Quote:
 Originally Posted by Leaffan Oh shit. The Hortas are not going to like this.
How do you think Iridium feels about the change?
That old Kg standard made up 90% of its reason for being.
#13
11-11-2010, 04:51 PM
 Chronos Charter Member Moderator Join Date: Jan 2000 Location: The Land of Cleves Posts: 74,529
Quote:
 What they want is some way to measure it without needing an artifact. For instance, the standard meter is defined now as "the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second." This allows people to create their own meter sticks without having to measure them against any standard.
And the second, in turn, is defined in terms of a particular oscillation mode of a cesium atom of a particular isotope. This does, in a sense, rely on an artifact, but all atoms of the same isotope are identical, so it's an artifact that any lab can study independently.

There are groups working on techniques to precisely count out large number of atoms, with the hope of defining a kilogram as the mass of so-and-so many atoms of carbon-12, but so far, this is not yet as precise as comparisons to a standard lump of material in a vault somewhere.
#14
11-11-2010, 04:52 PM
 glowacks Guest Join Date: Aug 2008 Posts: 1,629
Quote:
 Originally Posted by beowulff Except a clock...
Yes, but you know your clock to be accurate because you've used the definition of a second, the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom, to build it.
#15
11-11-2010, 05:31 PM
 LouisB Member Join Date: Apr 2000 Location: Seminole, FL Posts: 8,515
Quote:
 Originally Posted by glowacks Yes, but you know your clock to be accurate because you've used the definition of a second, the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom, to build it.
I used to quote this number during presentations I put on concerning the frequency of quartz crystal units. When asked what the hell it meant, I could only shrug and say that I had no idea, nor did anyone I've ever talked to. For our purposes, I always said, one second was equal to one sixtieth part of a minute. No one ever quibbled.

How does one isolate a single atom, determine the two hyperfine levels of that atom, and make whatever measurements one must make in order to determine what the hell one second actually is? Perhaps even more importantly, how does one determine which atom one needs to isolate? There are so many things I wish I knew and so little time left to even begin.
#16
11-11-2010, 05:43 PM
 Spatial Rift 47 Guest Join Date: Dec 2000 Location: Boston, MA Posts: 3,222
Quote:
 Originally Posted by LouisB How does one isolate a single atom, determine the two hyperfine levels of that atom, and make whatever measurements one must make in order to determine what the hell one second actually is? Perhaps even more importantly, how does one determine which atom one needs to isolate? There are so many things I wish I knew and so little time left to even begin.
In order: you don't, you use the equations of atomic structure and energy from quantum mechanics, and you use a spectrometer to measure the radiation emitted by a large assembly of cesium-133 atoms as they make that particular transition when you induce it with a magnetic field.

The choice of that particular transition from that particular atom is largely arbitrary, since it serves only to identify photons of a particular frequency. Any quantum mechanically specified transition would serve the same purpose. We could just as solidly define the second as 2.4967x1015 periods of the radiation corresponding to the Lyman-alpha transition of neutral hydrogen atoms (in the absence of any fine or hyperfine splitting).
#17
11-11-2010, 05:59 PM
 Chronos Charter Member Moderator Join Date: Jan 2000 Location: The Land of Cleves Posts: 74,529
The choice of that particular transition of that particular atom was probably because, for some reason, that one was easier to measure precisely than, say, the Lyman alpha of hydrogen. What reasons those are exactly, I don't know, nor probably would anyone except experimentalists working in that particular field.
#18
11-11-2010, 06:44 PM
 Spatial Rift 47 Guest Join Date: Dec 2000 Location: Boston, MA Posts: 3,222
Chronos - yeah, probably.

I should note a correction: the cesium-133 transition used to define the second is taken from a hyperfine correction to the ground state, with no external magnetic field present. The extremely small separation between the energy levels is produced by internal magnetic fields, i.e. those of the subatomic particles. So you would just get a bunch of cesium-133, make it as cold as possible, and wait. Once you record the radiation, you correct for the remaining temperature and for various other effects like time dilation (Earth's gravity will change the measurement depending on altitude).
#19
11-11-2010, 06:59 PM
 JoelUpchurch Guest Join Date: Dec 2009 Location: Florida Posts: 3,813
At this level, I think you also have to compensate by time compression. IIRC it you put an atomic clock on a plane flying east and another plane flying west you will get a slightly different answer when the return to their starting location.

I just goggled it and here is a description.

http://hyperphysics.phy-astr.gsu.edu...airtim.html#c2
#20
11-11-2010, 07:25 PM
 Dewey Finn Charter Member Join Date: Apr 2003 Posts: 25,136
Quote:
 Originally Posted by beowulff The new standard will be a really cool Single-isotope Silicon sphere.
This part ("A team of scientists based in Germany measured the [Avogadro] constant by counting the atoms in a painstakingly crafted one-kilogram sphere of silicon-28.") of that article is hilarious. So the standard kilogram is something for which the number of atoms have been counted?
#21
11-11-2010, 07:55 PM
 wheresmymind Guest Join Date: Mar 2010 Posts: 1,010
Quote:
 Originally Posted by beowulff The new standard will be a really cool Single-isotope Silicon sphere.
I remember reading (possibly in Wired, actually) that there are other teams trying to define a new standard using ultra-sensitive watt balances.
#22
11-14-2010, 06:43 PM
 LouisB Member Join Date: Apr 2000 Location: Seminole, FL Posts: 8,515
Quote:
 Originally Posted by JoelUpchurch At this level, I think you also have to compensate by time compression. IIRC it you put an atomic clock on a plane flying east and another plane flying west you will get a slightly different answer when the return to their starting location. I just goggled it and here is a description. http://hyperphysics.phy-astr.gsu.edu...airtim.html#c2
IIRC, the referenced paper was included in the Proceedings of the Frequency Control Symposium possibly one year after the original publication or it might even have been the original publication. Again, IIRC, one couldn't move an inch without running into another discussion of that paper; it generated more interest than any I could ever recall. My copy was lost somewhere over the years and I'm delighted to have access to it again. Thanks for the link.
#23
11-14-2010, 06:47 PM
 LouisB Member Join Date: Apr 2000 Location: Seminole, FL Posts: 8,515
Quote:
 Originally Posted by Spatial Rift 47 In order: you don't, you use the equations of atomic structure and energy from quantum mechanics, and you use a spectrometer to measure the radiation emitted by a large assembly of cesium-133 atoms as they make that particular transition when you induce it with a magnetic field. The choice of that particular transition from that particular atom is largely arbitrary, since it serves only to identify photons of a particular frequency. Any quantum mechanically specified transition would serve the same purpose. We could just as solidly define the second as 2.4967x1015 periods of the radiation corresponding to the Lyman-alpha transition of neutral hydrogen atoms (in the absence of any fine or hyperfine splitting).
Thank you; my ignorance has been fought but, regrettably, not vanquished. All I want to know is everything; that shouldn't be too much to ask.
#24
11-14-2010, 06:52 PM
 Rhythmdvl Charter Member Join Date: Oct 1999 Location: Shakedown Street Posts: 12,951
Quote:
 Originally Posted by glowacks Yes, but you know your clock to be accurate because you've used the definition of a second, the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom, to build it.
How does the counting work? A team of dedicated actuarial physicists with high-tech versions of those clicky-counter things umpires use? Some gadget that's capable of not only counting to 9 billion in a second, but hitting "start" on the Doomsday Device within the margin for error?
#25
11-14-2010, 07:13 PM
 beowulff Member Join Date: May 2001 Location: Scottsdale, more-or-less Posts: 15,097
Quote:
 Originally Posted by Rhythmdvl How does the counting work? A team of dedicated actuarial physicists with high-tech versions of those clicky-counter things umpires use? Some gadget that's capable of not only counting to 9 billion in a second, but hitting "start" on the Doomsday Device within the margin for error?
You just buy one of these.
#26
11-14-2010, 09:32 PM
 MikeS Charter Member Join Date: Oct 2001 Location: New London, CT Posts: 3,770
Quote:
 Originally Posted by Dewey Finn This part ("A team of scientists based in Germany measured the [Avogadro] constant by counting the atoms in a painstakingly crafted one-kilogram sphere of silicon-28.") of that article is hilarious. So the standard kilogram is something for which the number of atoms have been counted?
Well, not yet, but that's the general idea behind the new standard. One mole of carbon-12 is defined to be that number of atoms of carbon-12 such that their total mass is precisely 12 grams. We can use precision measurements to find out what the ratio of the mass of an atom of silicon to an atom of carbon is, so we know how many grams a mole of silicon should weigh. What we don't know is precisely how many atoms are in a mole. So if we can manufacture some physical object for which we know the number (and type) of atoms extremely precisely, and we weigh that object relative to our "old kilogram" standard, then that allows use to precisely measure how many atoms (of silicon, say) make up an "old kilogram". Then we define the "new kilogram" to be the weight of that number of atoms of silicon, and the iridium bar is no longer needed.
#27
11-14-2010, 10:02 PM
 ChrisBooth12 Guest Join Date: Sep 2005 Posts: 1,768
they should just say take 1 carbon-12 atom and multiply till you get 1 kilo. I don't get the standard block I mean are scales really calibrated using that? I doubt it so whats the point?
#28
11-15-2010, 12:25 AM
 Chronos Charter Member Moderator Join Date: Jan 2000 Location: The Land of Cleves Posts: 74,529
Quote:
 they should just say take 1 carbon-12 atom and multiply till you get 1 kilo. I don't get the standard block I mean are scales really calibrated using that? I doubt it so whats the point?
In practice, what's usually done is that you use the primary standard to make a bunch of secondary standards for major national labs around the world, and then you use those secondary standards to make tertiary standards that are used by all the other labs that need precise standards. And yes, the process of making those secondary and tertiary standards does involve putting the primary and secondary standards on scales of some sort.
#29
11-15-2010, 01:34 AM
 Cheshire Human BANNED Join Date: Jan 2010 Location: NY, USA Posts: 4,545
Quote:
 Originally Posted by LouisB Thank you; my ignorance has been fought but, regrettably, not vanquished. All I want to know is everything; that shouldn't be too much to ask.
Start here, and work your way back.
#30
11-15-2010, 07:59 AM
 MikeS Charter Member Join Date: Oct 2001 Location: New London, CT Posts: 3,770
Quote:
 Originally Posted by ChrisBooth12 they should just say take 1 carbon-12 atom and multiply till you get 1 kilo.
Well, yes, but multiply by what? That's what this precision silicon sphere project is supposed to do, in the end: tell us precisely how many atoms are in a mole (and, by extension, in a kilogram as currently defined.)
#31
11-15-2010, 09:17 AM
 Scuba_Ben Guest Join Date: Oct 2002 Location: Below the thermocline Posts: 5,702
Quote:
 Originally Posted by MikeS Well, yes, but multiply by what? That's what this precision silicon sphere project is supposed to do, in the end: tell us precisely how many atoms are in a mole (and, by extension, in a kilogram as currently defined.)
And that, right there, is the engineering problem. The SI meter is defined exactly, based on the SI second. SI mass would need to be defined exactly. No "6.02245 E23 +/- 1E18", but exactly.
#32
11-15-2010, 04:06 PM
 LouisB Member Join Date: Apr 2000 Location: Seminole, FL Posts: 8,515
Quote:
 Originally Posted by Cheshire Human Start here, and work your way back.
Oh, thank you, thank you; that greatly simplifies my task. I had hoped for a link to the first page, but one must start somewhere.
#33
11-15-2010, 04:08 PM
 LouisB Member Join Date: Apr 2000 Location: Seminole, FL Posts: 8,515
Quote:
 Originally Posted by beowulff You just buy one of these.
Just thought I'd mention------the first frequency counter I ever used was an HP with nixie tubes. I get headaches just remembering those things.
#34
11-15-2010, 11:16 PM
 RadicalPi Guest Join Date: May 2009 Location: Santa Barbara Posts: 1,365
Quote:
 Originally Posted by Simplicio The chunk of metal used to formally define the Kilogram was machined so that it would be close to the weight of 1,000 cubic centimeters of water at 4 degrees. I'm not really sure why they haven't taken the obvious step of just using that as the definition. Perhaps water doesn't have as fixed a density as I'd think at a set temperature?
I believe that part of the reason that they didn't do this was that the volume of water depends a bit on the pressure of the air above it. So, any definition that doesn't take pressure into account is going to be less accurate than the platinum-iridium cylinder. And any definition that does take it into account is going to be circular since pressure is force per unit of area, and force is mass times acceleration. So you'd need to have mass defined before you can define mass.

Someone please correct me if I'm wrong.

Last edited by RadicalPi; 11-15-2010 at 11:18 PM.
#35
11-16-2010, 12:55 AM
 Chronos Charter Member Moderator Join Date: Jan 2000 Location: The Land of Cleves Posts: 74,529
Well, you could specify that it be vacuum over the water (or rather, nothing but water vapor at its equilibrium vapor pressure), but you'd still have to specify a temperature. And the shape of the container and its orientation, since the water will be pressurized slightly by its own weight, and that'll depend on how tall the water is (and on the local gravity, for that matter). And then there's the question of how precisely you could measure the mass of that water, and how it compares to the precision of a metal proof mass.
#36
11-16-2010, 09:40 AM
 Saint Cad Guest Join Date: Jul 2005 Location: N of Denver & S of Sanity Posts: 12,394
Quote:
 Originally Posted by Squink How do you think Iridium feels about the change? That old Kg standard made up 90% of its reason for being.
Iridium is still used to make flares IIRC
#37
11-16-2010, 01:23 PM
 Chronos Charter Member Moderator Join Date: Jan 2000 Location: The Land of Cleves Posts: 74,529
Quote:
 Iridium is still used to make flares IIRC
An Iridium flare has almost nothing to do with iridium, and very little to do with flares. It's a specular reflection of sunlight off of an Iridium communications satellite, visible from the ground. And the Iridium satellites were named that because originally, there were 77 of them, and the structure of 77 satellites orbiting the Earth reminded the folks who founded the company of 77 electrons "orbiting" a nucleus.
#38
11-16-2010, 01:46 PM
 Keeve Guest Join Date: Aug 2000 Location: NY/NJ, USA Posts: 5,074
Quote:
 Originally Posted by LouisB Oh, thank you, thank you; that greatly simplifies my task. I had hoped for a link to the first page, but one must start somewhere.
Here you go!
#39
12-04-2010, 10:58 PM
 Squink Guest Join Date: Oct 2000 Location: Yes Posts: 20,327
Where does the mass of a kilogram come from?

Quote:
 the International Bureau of Weights and Measures (BIPM) outside Paris ... in the basement of the BIPM's main lab building, which houses a vault secured with three locks. Three antique keys, held by citizens of different countries, open these locks. ... the three key holders produce the keys, open the vault, unlock a safe inside the vault and inspect its principal contents: the platinum–iridium cylinder that defines the kilogram.
Au revoir, kilogram (with a nice picture of the kilogram inside its three nested bell jars).

So In Paris:In a Basement:In a Vault:In a Safe:Inside 3 bell jars.

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