I remember reading (possibly in Wired, actually) that there are other teams trying to define a new standard using ultra-sensitive watt balances.
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.
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.
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?
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.
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.
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.
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.
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.
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.
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.
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.
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.