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The Changing Definition Of 'Kilogram'
DrLudicrous writes "The NYTimes is reporting that the platinum-iridium standard mass for the kilogram is shedding at an appreciable rate -- at least compared to other reference masses. The Pt-Ir cylinder is kept in France, and measured annually, and the slight discrepancy is important because the kg is an SI base unit- thus other quantities such as the Volt are based on it. A new standard is being sought- the two frontrunners are counting the number of atoms in a perfectly spherical single crystal of silicon, and another technique uses a device known as the Watt balance."
Here is a site that gives some reasons why the are thinking of replacing the standard: http://physics.nist.gov/News/TechBeat/9501beat.htm l.
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It's not a matter of dark ages, it's a matter of infrastructure... while not the largest country in the world (the US is probably third or fourth, I'm not sure), we have by far the most technological infrastructure. It is not feasible to change all that in a short period of time.
A friend is in construction, and guestimates that it will take over 100 years to replace all failing/obsolete tech with the versions in metric equivalents. It just does not make any economic sense to replace a set of, say, water pipes with the metric standard if the current ones will last 20 years. It'll have to be a gradual thing.
Just to be difficult, though, I'd mention that most construction is done in 'tenths of feet', even the surveying equipment is marked this way. Has nothing to do with the metric system, it just makes the math easier...
"Faith: Belief without evidence in what is told by one who speaks without knowledge, of things without parallel." - A.B.
Scientists Struggling to Make the Kilogram Right Again
By OTTO POHL
RAUNSCHWEIG, Germany -- In these girth-conscious times, even weight itself has weight issues. The kilogram is getting lighter, scientists say, sowing potential confusion over a range of scientific endeavor.
The kilogram is defined by a platinum-iridium cylinder, cast in England in 1889. No one knows why it is shedding weight, at least in comparison with other reference weights, but the change has spurred an international search for a more stable definition.
"It's certainly not helpful to have a standard that keeps changing," says Peter Becker, a scientist at the Federal Standards Laboratory here, an institution of 1,500 scientists dedicated entirely to improving the ability to measure things precisely.
Even the apparent change of 50 micrograms in the kilogram -- less than the weight of a grain of salt -- is enough to distort careful scientific calculations.
Dr. Becker is leading a team of international researchers seeking to redefine the kilogram as a number of atoms of a selected element. Other scientists, including researchers at the National Institute of Standards and Technology in Washington, are developing a competing technology to define the kilogram using a complex mechanism known as the watt balance.
The final recommendation will be made by the International Committee on Weights and Measures, a body created by international treaty in 1875. 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. The weight change has been noted on the occasions it has been removed for measurement.
"It's part ceremony and part obligation," Dr. Richard Davis, head of the mass section at the research arm of the international committee.
"You'd have to amend the treaty if you didn't do it this way."
That ceremony has become a little sorrowful as the guest of honor appears to be, on a microscopic level at least, wasting away.
The race is already well under way to determine a new standard, although at a measured pace, since creating reliable measurements is such painstaking work.
The kilogram is the only one of the seven base units of measurement that still retain its 19th-century definition. Over the years, scientists have redefined units like the meter (first based on the earth's circumference) and the second (conceived as a fraction of a day). The meter is now the distance light travels in one-299,792,458th of a second, and a second is the time it takes for a cesium atom to vibrate 9,192,631,770 times. Each can be measured with remarkable precision, and, equally important, can be reproduced anywhere.
The kilogram was conceived to be the mass of a liter of water, but accurately measuring a liter of water proved to be very difficult. Instead, an English goldsmith was hired to make a platinum-iridium cylinder that would be used to define the kilogram.
One reason the kilogram has lagged behind the other units is that there has been no immediate practical benefit to increasing its precision. Nonetheless, 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.
A total of 80 copies of the reference kilogram have been created and distributed to signatories of the metric treaty. The sometimes colorful history of these small metal cylinders underscores how long the world has used the same definition of the kilogram.
Some of the metal plugs were issued to countries that later vanished, including Serbia and the Dutch East Indies. The Japanese had to surrender theirs after World War II. Germany has acquired several weights, including the one issued to Bavaria in 1889 and the one that belonge
It doesn't exactly have to be measured. They just do that to check it's still right. Go read about the history of the Systeme International the NIST site and the definition of a kilogram at the same place
But essentially, its part of a way of ensuring that the measuring units Scientists use around the world are the same, not slightly different.
For instance, anyone around the world can reproduce (in a well equipped lab anyway) the definition for time (The second is 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 cesium 133 atom).
There are only 7 base SI units (meter, kilogram, second, ampere, kelvin, and candela) from which many more units are derived. Hence, if kilo is out/changing many of these are changing too.
and why should I care if it detoritates?
Presuming you're American, you would use feet, pounds, find metric too complicated, etc, etc - so probably wont care if it does.
The unit of "volt" can also be expressed as m^2kgs^-3A^-1.
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The various prefixes -- kilo, Mega, Giga, and so on -- are very precisely defined SI prefixes that have been in common use in the sciences for quite some time now. In computing though, 1024 bytes was originally termed a "kilobyte" because it was very close to an actual "kilo" of bytes (1000 bytes), and so was a convenient term to use. In other computer-related disciplines though, in particular engineering, the correct SI usage prevailed, so your 128 kbps mp3s actually have 128000 bits per second, not 128 * 1024.
The big problem is that 2^(10x) and 10^(3x) diverge as x increases: 1024 is 2.4% more than 1000, 1048576 is 4.9% more than 1000000, 1073741824 is 7.4% more than 1000000000, and so on. So obviously the "close enough" thing is getting less and less true -- when there's a 10% difference between the two measurements they're not even close enough for everyday colloquial speech.
So the solution of both the SI and the IEEE is to reassert the original meanings of the SI prefixes (kilo = 1000, Mega = 1000000, etc.), but to add new base-2 prefixes in recognition of their usefulness in computing. These are kibi, Mebi, Gibi, etc. (basically the same as the SI prefixes but with the last two letters replaced by "bi"). Their standard abbreviations are the same as for the SI prefixes, but with a lowercase 'i' appended (so ki, Mi, Gi, etc.).
The conversion is obviously nowhere near complete, and irritates some computer people who don't want to change the terms we've been using for decades, but this seems to be the only really reasonable way of doing things. The only other two options are to either force the rest of the sciences to change to use the base-2 definitions (which is obviously not going to happen, and they got there first anyway), or to maintain the current ambiguity, which is also obviously undesirable.
10 PRINT CHR$(205.5+RND(1)); : GOTO 10
I believe bhutan is a buddhist/hindu country.
The posted article, while interesting, is wrong about the volt being based on the Kilogram. Since about 1990, the volt is defined to be the voltage applied to a Josephson junction that produces a frequency of 483,597.9 GHz. This new standard was implemented in order to get away from relying on 'artifact' standards (such as the Kg cylinder). One quick source page on Josephson junctions (which completely revolutionized the field of Metrology back when I was a calibration tech in the AF) is:/ squid.html
http://hyperphysics.phy-astr.gsu.edu/hbase/solids
If I recall correctly, the eventual goal of the international standards organization was to find ways to define everything in terms of frequency/time since we can measure time so accurately/precisely.
Vacancies are not necessarily a problem. As you say, vacancies are entropically favored, but there is also a formation energy associated with a vacancy. So thermodynamics tells us there will be a balance between the energy required to create a vacancy with the entropy gained by creating one.
Thus, there is an equilibrium number of vacancies in any crystal. As long as you know what the equilibrium value is for a given temperature and you maintain that temperature, then you will also know how many vacant sites you will have on the crystal lattice. I don't have any of my texts handy, but I'm sure someone can chime in with the numbers for silicon.
To sum up. All crystals will have vacancies because vacancies are thermodynamically favored. However, the number of vacancies will tend towards an equilibrium value which allows them to be accounted for.
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The internet is the greatest source of biased information in the history of mankind.
a kilogram is, however, a measure of mass, not weight. it is therefore unaffected (if properly measured) by the force of gravity.
Hmm, so instead of a year being 365 days long you would want it to be 456 days long? (365 days * 1.25 = 465 days)
A leap year has nothing to do with anyone screwing up. The problem is that a year does not have an integral number of days. A year is 365 days, 5 hours, 49 minutes (365.2424 Universal days)*. That means that it takes about 526,297 minutes for the Earth to make a full trip around the sun. After the Earth has rotated about its axis 365 times it will still take about 350 minutes until it reaches the same spot it started from.
That means that if you tried to have the year be an even number of days, say 365, you would fall behind almost 1 full day every 4 years. It's not much but if you let it go for a while you will start having winter during the hottest times of the year. There are a few other rules that adjust the calendar besides the "extra day every 4 years" rule and because of these rules we are able to keep the seasons approximately where they should be.
To learn more about how the calandars were changed visit this web site.
*source: Timeline of interesting calendar facts
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Fundamentalist Islamic country without any telephones?
Can I have some of whatever your smoking please?
More than mere navel gazing.
Diamond spontaneously decays into graphite... no mass change, I suppose, but it has different absorption of gasses from the air, and different density (matters if they measure mass through weight in an atmosphere). Gold is much more long-lasting.
I've had this sig for three days.
The same deal with Plack's constat. It's value is not up to us, but up to nature. "Defining" it would be like defining pi as 3.
The same problems are still there, regardless of material. Changing the composition doesn't change the fact that there is only one standard in only one laboratory, that stray particles and cleaning will affect its mass upon measurement, and that the standard may be damaged in some way.
The other solutions presented as candidates to replace the standard rely on invariant physical constants, i.e. Avogadro's number. Distance and time standards are already defined in this way, from the speed of light and the frequency of a two-state cesium transition in the microwave region.
This shifts the accuracy of the standard from it's care and maintenance to the measurement of constants, with the added benefit of any appropriately equipped laboratory being able to measure the standard.
Except that no one has actually counted/created a structure composed of a mol of any particular element.
The minute you can do that, then you can reliably and predictably create a fixed metric by which any one in any place can measure mass.
GPL Deconstructed
If you really would try to build such a crystal, vacanies could very well be the problem. As you said, there is an equilibrium value of defects in any crystal. This equilibrium value is temperature dependant with a exp(-Eform/kT) law, where Eform is the formation enthalpy. High temperature means high rate of defects.
Si single crystals are usually prepared at very high temperatures out of molten Silicon (1414C, Czochralsky method). Essentially, this will lead to a freezing of the defect structure at temperatures close to the melting point, because the lattice reorientation kinetics (point diffusion) also are thermally activated.
You would have to temper the crystal for _very_ long times at temperatures of i.e. 300C to get a thermal equilibrium of defects at this temperature. These times could be >>years !
A kilogram is the measurement of a certain amount of matter, not it's weight. Sure the two are related, but not the same.
A kilogram is the same on the surface of the earth, in outer space, or one the moon. Weight however, varies with gravitational pull or acdeleration.
In other words, weight is basically the mass of on object multiplied by whatever gravitational field you happen to be in.
And the metre is defined properly these days (as is the second) in terms of wavelengths of radiation.
No. In SI units, c is not measured but defined. Physically, c is just a man-made constant of proportionality deriving from the fact that, for historical reasons, we measure time differently from space. In reality, both time and space are physical dimensions and so it makes perfect sense to express both in terms of the same units, be they seconds or metres.
That's why most theoretical physicists like to do their calculations in "natural units" -- i.e. you set c=1 and h/2pi=1 -- since in reality the values of the fundamental constants are artefacts of your measurement system. Scientifically speaking, it makes sense to set all independent constants to 1 since it brings out the fact that the "equivalence" of eg mass and energy, or distance and time, is really an identicality.
The author of this post asserts his moral rights.
Just as a minor correction ... the volt is no longer defined in terms of the kg. The international definition of 1 Volt is now defined in terms of the "Josephson Effect" and is an effect observed in superconducting materials that are interupted by a normal metal.
It turns out, that even without an applied voltage, there is still a current in the system, and after a voltage is applied, the current oscillates at a very predicable rate. Thus, the volt is now defined as the potential required to give a specific number of current osciallations in a Josephson Junction.
Nit-pickey I know, but maybe of interest.