Kilo != Kilo
May 29, 2003 10:32 AM   Subscribe

The kilogram has lost some weight (and/or mass, depending on your point of view) in the past 2 centuries. Scientists race so their spiffy idea will be the next benchmark. via Ars Technica
posted by Nauip (22 comments total)
oh, ya. It's a NY Times link - generic/generic works for a login/pass.
posted by Nauip at 10:33 AM on May 29, 2003


posted by goethean at 10:37 AM on May 29, 2003

Free link.

Well, perhaps if the grabby scientists would stop handling the damn thing every few decades or so. But no, they have to actually use it as a reference mass, or just show it off to their friends from other international standards bodies (a competitive sort, to be sure). Besides, don't they realize they're violating the law of conservation of mass?

Frankly, it's most surprising that it's taken this long to move toward a modern SI definition of mass, which has long been the case with units of length, time, and many others. This "reference mass" method is so nineteenth-century. The bit about the wayward sibling reference masses was interesting, though. Get a look at the US prototype kilogram and the UK's (and the real thing), all enclosed in a charming double bell-jar.
posted by dhartung at 11:10 AM on May 29, 2003

I realize the answer is probably just that it's easier to deal with an object of higher mass, but if not, why is the reference weight a kilogram and not a gram?
posted by jacquilynne at 11:32 AM on May 29, 2003

I love you nerds! ^_^
posted by thirteen at 11:50 AM on May 29, 2003

Whatever happened to the definition of a kilogram as the mass of a body of water at 4°C (max density of water) occupying a liter of volume, otherwise known as a cube 10 centimeters high, wide and deep or a cubic decimeter?
posted by Kip at 11:53 AM on May 29, 2003

In related news, President G.W. Bush asserted that the loss of mass was a result of the continued French conspiracy against the United States. He was quoted as stating "that instead of 600 tons of chemical agents, the French will demand that we find an additional 47 grams, and since we haven't found any, this was a darn significant increase".

He was later corrected by White House press secretary Ari Fleischer who stated "that the President actually meant to say 30 kilograms but made a slight but understandable error on his third page of calculations".
posted by Mr Stickfigure at 12:09 PM on May 29, 2003

If you're at all interested in the history of the metric system, I just finished an excelent book all about it. The measure of all things : the seven-year odyssey and hidden error that transformed the world /by Ken Alder, Ken
posted by dipolemoment at 12:33 PM on May 29, 2003

The agency guards the international reference kilogram and keeps it in a heavily guarded safe in a château outside Paris. It is visited once a year, under heavy security, by the only three people to have keys to the safe.

This just screams Hollywood:

When the international reference kilogram is stolen, there's only one man who can get it back as the world spirals toward unstandardized chaos...
posted by gottabefunky at 12:52 PM on May 29, 2003

Very interesting link. Anyone know why the Japanese had to turn their reference kilo over after WWII? Something to do with weapons research maybe?
posted by smcniven at 12:58 PM on May 29, 2003

Hang on! Hasn't anyone considered that maybe the thing they're weighing it against hasn't gotten bigger? This could be a whole lotta fuss over nothin'!
posted by vraxoin at 1:46 PM on May 29, 2003

dhartung, isn't the standard for length now based on the speed of light? what happens when/if we finally prove that the speed of light isn't constant?

special relativity is so 20th century.
posted by mrgrimm at 2:00 PM on May 29, 2003

I think the coolest thing about the whole deal is in this quote from the linked article:

An intriguing characteristic of this smooth ball is that there is no way to tell whether it is spinning or at rest. Only if a grain of dust lands on the surface is there something for the eye to track.

I want me one of them!
posted by MrMoonPie at 2:12 PM on May 29, 2003

There was an interview with the senior metrologist in Canada on As It Happens last night about this (it'll be on the website tomorrow). Apparently, there are thirty or so "national" standards that were created using the Paris standard as a reference. These are the masses that appear to be getting heavier relative to the primary reference mass. Apparently, just as your stove gets greasy from cooking oils, the platinum-iridium national standards are getting dirtied by hydrocarbons in the atmosphere. The secondary standards get used far more often than the primary, and come in contact with the atmosphere much more often. So, in the expert's view, the primary standard isn't getting lighter, the secondary national standards are getting heavier.

The hydrocarbons come mostly from vehicle emissions. Obviously, this is all the fault of you reduced-emission-controls SUV drivers.
posted by bonehead at 2:37 PM on May 29, 2003

Mrgrim: Mass is that last mediaeval unit. Everything else is now based on either an amount of something or a physical constant. There's a good page on this here.
posted by bonehead at 2:45 PM on May 29, 2003

Whatever happened to the definition of a kilogram as the mass of a body of water at 4°C (max density of water) occupying a liter of volume, otherwise known as a cube 10 centimeters high, wide and deep or a cubic decimeter?

Another article I read said that this was still the textbook definition of the kilogram, but 1 liter of water was hard to measure precisely, so they commissioned the cylinder, which was based on that.
posted by scarabic at 2:54 PM on May 29, 2003

Next they'll tell us that the poles are going to shift!
posted by riffola at 3:34 PM on May 29, 2003

These two quotes are from the article. Is it just me, or do they seem to be contradicting one another?

...the drift in the kilogram's weight carries over to other measurements. The volt, for example, is defined in terms of the kilogram, so a stable kilogram definition will allow the volt to be tied more closely to the base units of measure.

...the challenge of calculating the precise number of atoms in a silicon crystal is too imprecise with today's technology so they are refining a technique to calculate the kilogram using voltage.

So we're using volts, which are based on our current, inaccurate kilogram, to measure the mass of the next kilogram... which in turn will redefine the volt, which in turn will redefine the kilo... ad nauseum?
posted by five fresh fish at 5:03 PM on May 29, 2003

fff: exactly. In effect, we don't know what volts or kilograms are, so newer measurements will define them more precisely. New measurements will bootstrap off the old standards, but at its core the definitions are arbitrary. Ultimately, a kilogram is what the CGPM/CIPM says it is.
posted by bonehead at 6:19 PM on May 29, 2003

The volt is defined as the voltage necessary to make a particular type of Josephson junction (it's a superconductor thing) oscillate at a particular frequency. The units of voltage are fundamentally (m^2 * kg) / (s^3 * A), which includes mass, but you don't have to measure mass to measure voltage.

The "watt scale" approach mentioned in the article relies on those two things. The scale is basically measuring the voltage necessary to raise a mass some particular distance in some particular time with some particular current. That, of course, depends on how much you mass weighs, and for the error tolerances we're talking about, you need to know the local value of gravity really, really well.
posted by justin at 10:48 PM on May 29, 2003

I realize the answer is probably just that it's easier to deal with an object of higher mass, but if not, why is the reference weight a kilogram and not a gram?
Ease of use, really. The meter is a human-scaled measurement. So is the kilogram. If we weighed ourselves in grams, god would that be a pain. There is actually a system of measurement known as the CGS, or centimeter-gram-second system, but there's some conversion to be done to use the values in the standard MKS (meter-kilogram-second) system. Both of these units are based on SI (system internationale).
Further, both of these systems were developed as a foil to the standard English (foot - poundal(yes, you read that right...poundal is the mass of something with a weight of a pound) - second) system, which requires that you throw in the acceleration due to gravity (32.17 feet/sec) at various points. I grew up in the English system, but whenever I have to do engineering calculations, I convert to MKS - no conversion factors to forget, or accidentally throw in incorrectly.
posted by notsnot at 11:59 PM on May 29, 2003

But it would have been better if the designers of the SI had had used smaller meters -- say what we now call decimeters. Then a liter would be a cubic meter instead of a cubic decimeter, and a gram would be the weight of a cubic meter of water instead of a cubic centimeter. None of this converting between meters and decimeters and centimeters, not to mention having to remember which units are MKS and which are CGS.

And you'd have the benefit of being 15 or so meters tall.
posted by ROU_Xenophobe at 12:29 AM on May 30, 2003

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