Exactly One Kilogram Of Silicon

Ed Pegg Jr writes "You may know of the importance of 299792458 for length, and 9192631770 for time. However, the official standard for weight is still a block of platinum/iridium made a hundred years ago. A group of scientists from the Avogadro Project are hoping to change that, though, by producing a perfect sphere of ultrapure silicon."

Comments

why kilogram?

[Anonymous Coward]

the SI unit for length is the metre - not the kilometre

why is the unit for mass the kilogram when it should more logically be the gram?

using the gram might be easier to accurately measure too.

Re:why kilogram?

[Idarubicin]

But the English system goes by powers of two; e.g, 16 cups = 8 pints = 4 quarts = 2 half gallon = 1 gallon. Much nicer for computers; no roundoff error there in the floating point processor, and you can change units by a simple shift operation rather than the hugely more time-consuming multiplication by ten. Same logic, different base.

Yes, 3 feet to the yard (for surveyors, 66 feet to the chain), 1760 yards to the mile...

Please, tell me how to use a shift operation to divide by 1760. :P The other nice thing about metric is the consistent prefixes. There is one MKS symbol for length (m), not many (in, ft, yd, mi...) and it can be associated with a set of prefixes (micro, milli, kilo, mega, etc.) that have consistent meaning across all metric units. For us humans, it is easy to find a unit that lets you express values in "comfortable" form--living cells are on the order of 10 micrometers across, not 0.00001 meters; it's 100 kilometers to Grandma's house, not 100000 meters. And since it's in easy powers of ten, I can tell you immediately that you can line up 10^10 cells along the road to Grandma's, if you want to know.

Re:why kilogram?

[dublin]

There are good reasons to hate the metric system, other than because it's French.

Case in point - I was working with pressure instrumentation this week, and have a new appreciation of what absolutely *insane* units have to be used by the poor folks that prefer metric.

I'm talking, of course about that riduculous unit the Pascal, that defames the name of one of history's great scientists and thinkers.

The idiot who decided that a pressure as ridiculously low as one Newton per square meter was a useful unit of pressure should have been stood up against the wall and summarily shot. That such a thing exists as an ISO standard seems to fit the inherent silliness of every ISO-developed standard I've ever encountered.

I'm only half joking. This is such a ridiculously low pressure that any sort of real-world engineering use requires kilo-, or more likely megaPascals in order to express it. To put this silliness in perspective, realize that the very low pressure of 1 Atmosphere is equivalent to 101,325 Pascals. No wonder the civilized world calls it 14.7 psi instead... :-)

Why does it have to be a SPHERE?

[DancingSword]

Is there some inherent problem with other geometries?

Like, say, the cylinder ( as the original kilogram were )

Isn't machining a sphere, perfectly, more .. error-prone?

Please Correct me

[smurf975]

I always thought one kilogram is one liter of pure water?

If I'm correct what is the point? They should worry about what makes a liter and what is pure water.

Re:Honest Question

[scrawny]

To calibrate scales.

how will they measure the mass of their new sphere? that's right...against the cube (actually, the cube's specs). when the new kilogram is accepted, will they lend it out to calibrate scales? nope. it will sit in a room as an oddity, just like the cube has for most of its life.

that won't help much

[u19925]

the problem of mass standard is not that it was made 100 years ago. the problem is that there is no way to describe it so that it can be reproduced independently. as one of the famous scientist said, "we can communicate our definition of length and time to aliens 1000 light years away (if they are listening us), but we can't tell them what we mean by 1 kilo".

Secondly, it doesn't matter either what exactly is 1 kilogram. what matters is some reference atomic mass and then pick up Avogadro number (based on existing 1 kilo mass) and then get rid of the existing standard. this would allow independent reproduction (e.g. 1 kg is equivalent of 6.02...... x 10^23 atoms of Oxygen 16 in certain energy state. this scheme too has problem. there is no practical way of verifying that you have met the standard definition. so, two scientists can argue that each is possesing exact 1 kg and this cannot be arbitrated.

Re:To Mr. Pegg

[Captain Nitpick]

What do you think a newton is? The force exerted by one gram begin accelerated at one G (or its weight at one G).

A newton is defined as 1 kg-m/s^2. As the AC already stated, you're off by roughly a factor of 100, and even that isn't exact.

Re:Okay, you asked for it

[dublin]

It turns out that liquid water at 1 atmosphere pressure is most dense at about 4 degrees Centigrade, where its density is 0.9999750 g/cm^3. at closer to room temperature- at 22 degrees C- its density is only .9977735 g/cm^3. It never actually gets up to 1 g/cm^3 the unit system was originally designed to use, I think because of the limits of accuracy of measurements when the current definitions of individual units were set.

This extremely unusual quirk of water (along with its inverse density as a solid) is one of the chief reasons that stable oceans are pretty much impossible with other substances, despite waht science fiction authors like to imagine: Because water is densest at 4 C, the entire volume of a body of water has to first reach that temperature (since it sinks to the bottom at that temp) before freezing at the top. Any other scenario results in bad things happening, like, say, all bodies of water freezing solid, making the continuation of life through winter rather difficult for aqautic species... One more reason to question whether naturalistic notions of origins really hold up under close scrutiny. (Don't even get me started on eclipses...)

Re:Honest Question

[norton_I]

Actually, the pound is derived from the kilogram, as all English measument units are defined (by NIST) in terms of metric quantities, and have been for some time, now. So, the official defenition of an inch is 2.54 cm, and so forth.