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## Experts Suggest Replacing Definition of Kilogram844

fenimor writes "The kilogram is the only one of the seven basic units of the international measurement system defined by a physical artifact rather than a natural phenomenon. International team of scientists suggest replacing the kilogram artifact -- a cylinder of platinum-iridium alloy about the size of a plum --with a definition based on one of two unchanging natural phenomena, either a quantity of light or the mass of a fixed number of atoms. They propose to adopt either one of two definitions for the kilogram by selecting a specific value for either the Planck constant or the Avogadro number."
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## Experts Suggest Replacing Definition of Kilogram

• #### The last time this was mentioned (Score:4, Informative)

<tepples AT gmail DOT com> on Saturday February 26, 2005 @07:58PM (#11790237) Homepage Journal

• #### Re:artifact (Score:3, Informative)

on Saturday February 26, 2005 @07:59PM (#11790244)
the 'meter' isn't a unit. perhaps you're thinking of 'metre'?
• #### Re:Anyone Else? (Score:5, Informative)

on Saturday February 26, 2005 @08:00PM (#11790255)
The SI unit of mass is the kilogram, not the gram.
• #### Re:artifact (Score:2, Informative)

on Saturday February 26, 2005 @08:00PM (#11790256)
Wrong: On October 20, the meter was redefined again. The definition states that the meter is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second. The speed of light is

c = 299,792,458 m/s

This [nist.gov]

It's true that it was once defined that way, however, it has been redefined.

• #### Re:artifact (Score:2, Informative)

on Saturday February 26, 2005 @08:02PM (#11790278)
More recently (1984), the Geneva Conference on Weights and Measures has defined the meter as the distance light travels, in a vacuum, in 1/299,792,458 seconds with time measured by a cesium-133 atomic clock which emits pulses of radiation at very rapid, regular intervals.

Thus, the meter is not defined by a physical artifact.

• #### Re:artifact (Score:5, Informative)

on Saturday February 26, 2005 @08:04PM (#11790308) Homepage Journal
Just in case people care, here are the 7 base units:

Metre for Length
Second for time
Ampere for current
Kelvin for temperature
Mole for amount
Candela for "Luminous intensity" ... or something.

All the others are built up and defined from these, so these must be well defined. Change what exactly a Kg is changed more than just mass - it changes everything dependant upon it. Hence, these things must be got right.

The definition of second changes every now and then though, and I think the metre has changed a few times, too. I wrote a bit about the second here [f2s.com], in my AS-Level Physics coursework, if anyone want s a simplifed read.

(Wiki [wikipedia.org])

I don't see how this topics is maths, by the way.
• #### Density (Score:2, Informative)

on Saturday February 26, 2005 @08:05PM (#11790315)
Originally, yes. But the density of water varies based on temperature and pressure, so that really doesn't work for any kind of precision.

The pressure part really kills using water as a definition, because it has a mass component. Circular definitions are a no-no.

• #### Re:How about (Score:5, Informative)

on Saturday February 26, 2005 @08:07PM (#11790330)
The second and the meter have long since been based off of more fundemental measures. The second is defined as how long it takes for 9,192,631,770 cycles of microwave light to be emitted by the hyperfne transition of cesium-133 atoms. The meter is defined as the distance traveled by light in a vacuum in 1/299,792,458 of a second.
• #### Re:Anyone Else? (Score:2, Informative)

on Saturday February 26, 2005 @08:15PM (#11790402)
The definition is a kilogram because the abovementioned lump of metal is one kilogram. A redefinition may ofcourse change this.
• #### Re:Redundant definition? (Score:3, Informative)

on Saturday February 26, 2005 @08:17PM (#11790412)
We generally use the verb "weigh" to express units of mass, because there is no commonly used verbal form for mass. There is nothing inherently wrong with this, it's just a vagueness in the meaning of the word "weigh".

See NIST Special Publication 811 (1995 ed.), _Guide for the Use of the International System of Units (SI)_ by Barry N. Taylor (NIST is the National Institute of Standards and Technology, the successor agency to the National Bureau of Standards):

In commercial and everyday use, and especially in common parlance, weight is usually used as a synonym for mass. Thus the SI unit of the quantity weight used in this sense is the kilogram (kg) and the verb "to weigh" means "to determine the mass of" or "to have a mass of".

Examples: the child's weight is 23 kg the briefcase weighs 6 kg Net wt. 227 g

• #### Re:Hmm... (Score:3, Informative)

on Saturday February 26, 2005 @08:18PM (#11790419)
Planck's constant would be a very elegant solution - it being the smallest possible quantity of energy

Huh? The units of Planck's constant are energy times time (eg., J s).

• #### Re:1L of water == 1kg? (Score:2, Informative)

on Saturday February 26, 2005 @08:25PM (#11790491) Homepage
From the Wikipedia article on "kilogram":
The kilogram was originally defined as the mass of one litre of pure water at a temperature of 4 degrees Celsius and standard atmospheric pressure. This definition was hard to realize accurately, partially because the density of water depends ever-so-slightly on the pressure, and pressure units include mass as a factor, introducing a circular dependency in the definition of the kilogram.
• #### Re:What? (Score:2, Informative)

on Saturday February 26, 2005 @08:31PM (#11790536)
F = m a
(N = kg m s^-2)
• #### Force (Score:2, Informative)

on Saturday February 26, 2005 @08:31PM (#11790537)
Force is nothing but mass * distance / time^2. 1 Newton = 1 kg*m/s^2
• #### Re:Just wait. (Score:3, Informative)

on Saturday February 26, 2005 @08:35PM (#11790566) Homepage
!!

Bhutan is a devout BUDDHIST country, you hoser. And they have TVs now, although when I first read about that (more than 5 years ago), the Bhutanese were quite reasonably shy about appearing on it. Hence they had to cajole some poor sod to read the national news in a pained monologue. If you see some of the recent Bhutanese movies, though, it appears things are changing fast. I have no idea how they weigh their TVs and Buddhas, though.

On topic, I think it's great that they're using Avocado's Number to define the kilo. So let's see, 6.022 x 10^23 avocados would weight ~1.8 x 10^23 kilos, or 28 Earths (this measurement based on the standard platinum-iridium avocado that was shipped to me from Paris, which is significantly larger than a plum, but was free to ship, since any attempt to weigh it would cause fatal recursion).
• #### Re:Hmm... (Score:3, Informative)

on Saturday February 26, 2005 @08:48PM (#11790671)
Plank's constant is a single Quanta of energy.

No. Planck's constant gives the amount of energy carried by (and hence gives a meaning to the momentum of) a photon of a certain frequency. Its units are Joule-seconds, which is not a unit of energy. Since the frequency of a photon can be arbitrarily low, so can its energy.

• #### Re:How about (Score:1, Informative)

by Anonymous Coward on Saturday February 26, 2005 @08:55PM (#11790737)
Because he's wrong. The modern definition is "1650763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton-86 atom".
• #### Re:And in other news... (Score:1, Informative)

by Anonymous Coward on Saturday February 26, 2005 @09:11PM (#11790890)
In 1897 the General Assembly of Indiana, USA enacted in Bill No. 246 stating that Pi was de jure 4. [4to40.com]
• #### Re:I suggest (Score:5, Informative)

on Saturday February 26, 2005 @09:13PM (#11790904) Homepage
Not only that, but it is also the only "basic measuring unit" that's not even a measuring unit, but a multiple of another one that's not considered "basic". At least in name. Kilo-gram. Get it ?
There are two common systems of units, mks (meter-kilogam-second) and cgs (centimeter-gram-second). The mks system is now more often referred to as the SI. In the cgs system, the gram is a base unit. In any case, what you're referring to is utterly trivial and/or irrelevant when it comes to the real work of defining the units. Any definition of the gram suffices to define the kilogram, and vice-versa.

Because Avogadro's number is JUST an artifact of the definition of the (kilo)gram, not a fundamental constant - it's (been originally) defined as the number of atoms in 12 grams (or, whatever, 0.012 kilogram) of Carbon-12.
It's happened before that they've changed things around so that something different was considered to be the more fundamental quantity: the speed of light used to be a measured quantity, but now it has a defined value. The whole issue is that as techniques change, you want to base your system of units on the things that can be most accurately measured (and reproduced) with the latest techniques.

Now, basing the definition of the kilogram (might I suggest they also change that basic to gram instead of kilogram... please) on Planck's constant somehow would be a MUCH better ideea. However, the value of that constant [i.e. 6.6260693111111 * 10^-34 and so on] makes it pretty wierd to work with unless you multiply it with 9 [to get exactly 5.96346238 * 10^-33 which makes more sense somehow].
I'm not sure where the <joke> tags belong here. Anyhow, giving h a defined value would be very much like the step they took when they gave c a defined value -- they did it because when techniques changed to the point where c was one of the most accurately measurable things in nature.

• #### Re:I wonder... (Score:5, Informative)

<nick@nickjackson.me> on Saturday February 26, 2005 @09:15PM (#11790920) Homepage
No, there is one definitive 'Kilogram' which is kept in Paris, and then copies are made and shipped worldwide to save countires having to go to Paris to check their official weights. The copies are then compared to the one true kilogram every 10 or so years (dependant upon whether it's being used for a quest to save mankind at that point).
• #### Re:My thoughts (Score:3, Informative)

on Saturday February 26, 2005 @09:15PM (#11790924)
But a point mass should not be neccicary, the mass just needs to be perfectly sperical. So long as the distance from the center of the mass is greater than the radius of the mass + a few wavelengths to eleminate defraction, there should be no difference how the gravity field is generated. It simply needs to be uniformly constant for all points in every concentric shell larger than the mass. Am I right?
• #### Re:Just wait. (Score:3, Informative)

on Saturday February 26, 2005 @09:31PM (#11791057)
Depends how you look at it. We call the things we use "pounds" and "feet," but we abandoned the flawed British standards in the 1890's (their yard shrank, their pound leaked), instead basing them on the SI standards. A pound is defined as 0.45359237 kg and a foot is 0.3048 m.*

Also, we were the only ones sane enough to base our unit of volume/capacity on the cube of our linear standard (1 gal US = 231 in^3, as it's been since the 1800's or so). Both the British gallon and the SI liter both had ugly/cumbersome definitions involving a sample of perfect water at a given temperature, pressure and local gravitational acceleration (the French got the litre right from the first, but then they broke it, and then it was fixed again, meaning the definition of "liter" has changed by tens of microleters over the past century).

* These are actually numbers agreed upon by the foot/pound using world in 1959. Before that, in the US, the number of kilograms in a pound had more digits and a foot was 1200/3937 m.
• #### Re:I wonder... (Score:5, Informative)

on Saturday February 26, 2005 @09:55PM (#11791239)
Given that the kg prototype has lost 50 micrograms over the last 100 years I'm guessing 0.999995kg?
• #### Re:May I be the first to ask... (Score:2, Informative)

To clarify, all particles have mass, and this mass turns into energy as they go faster, turning completely to energy at the speed of light. Mass is measured at 'rest'. (That is, in the same frame of reference as the particle.)

Since photons, by defination, cannot go slower than the speed of light, the amount of mass they have is the amount of mass they would have if stopped. Or, to put it another way, it's the amount of mass they would have if you could measure their mass while traveling at the speed of light.

Which is why it's called 'virtual mass', as you can't actually do either of those. All you can do is calculate the mass from the energy of the photon. (Which, as another post pointed out, is set by the frequency.)

• #### It works in a hierarchy (Score:5, Informative)

on Saturday February 26, 2005 @10:27PM (#11791463)
Is there actually a method of directly using these definitions?

Where you actually need to use them directly, sure.

To give a real world example of how the standards work in practice... I used to write software for a company in the metrology (high precision measurement) business. They made machines that are used, for example, in quality control at the end of production lines. The gauges on the most popular machines gave accurate readings with resolutions of say 1-10m.

Those machines were calibrated from reference artifacts. These were themselves checked for accuracy on still higher precision equipment. (How they actually manufacture something so close to physical perfection is an interesting area in itself...)

Ultimately, there were white room areas with very careful decontamination procedures in place that were used almost exclusively for calibrating the company's most precise equipment and checking their reference artifacts.

From there, you were one step removed from the national standards laboratories. At that level the formal scientific definitions are just fine.

In other words, you work from major standards labs that can use the precise definitions effectively, and propagate the information (with some less, but little enough to be acceptable for the application in question) to more widely distributed testing facilities. A more trendy application of the same basic idea is the use of Internet-based real time clock services.

• #### Grrr... Slashdot filtering... (Score:3, Informative)

on Saturday February 26, 2005 @10:30PM (#11791474)

Looks like we lost a mu in there somewhere: the resolutions for the popular machines were around 1-10 micrometres.

• #### Re:Picture of the Kilogram Prototype (Score:3, Informative)

on Saturday February 26, 2005 @10:33PM (#11791495)
It's to prevent it from collecting dust, which would change the weight.. err, mass. Corrosion prevention might be a goal, too (yes, the alloy is corrosion resistant, but not entirely corrosion proof.) I don't think they're worried about evaporation, though (all substances outgas to a certain degree, some more slowly than others)

It's nested in several jars for redundancy.

Chip H.
• #### Re:Just to state the obvious (Score:3, Informative)

on Saturday February 26, 2005 @11:02PM (#11791678) Homepage Journal
The definitions are done for scientific accuracy as the ability to measure improves.

Celcius is trickier than just temperatures of boiling and melting, because I think it must also declare the pressure too. The temperatures that water boils and freezes depends on air pressure. Kelvins are defined as divisions such that the range from absolute zero to the triple point of water is 273.16 kelvin. At least that doesn't depend on a the standard for pressure.

The problem with a meter standard depending on a physical object is that the length of that object changes with temperature. A temperature independent standard is needed. Standards that can be measured independently without having to refer to a specific single object are necessary for maximum accuracy.
• #### Re:I suggest (Score:1, Informative)

by Anonymous Coward on Saturday February 26, 2005 @11:49PM (#11791936)
There is a far more fundamental difference between MKS and CGS units than just a simple scaling. It is that the units of electrical charge are actually quite different in both systems. Maxwell's equations in MKS units contain expermental constants; the so-called permittivity and permeability of free space. In CGS units Maxwell's equations only constant is c, the velocity of light.
• #### Re:Picture of the Kilogram Prototype (Score:5, Informative)

<erlogan.gmail@com> on Saturday February 26, 2005 @11:51PM (#11791950)
To keep its mass from changing. You may notice the calipers for handling it in the picture, too. Stray moisture, direct sublimation into the atmosphere, anything that could possibly affect it has to be kept away,

This is the definition of the kilogram. A kilogram is not 1L of H2O at STP (as mentioned elsewhere, pressure depends on mass), it's this little lump of metal. Changes in the mass of it are extraordinarily bad. They make copies of it for reference purposes, and then check the copies agains the original every 10 years. If there's a disagreement, the copy gets adjusted, not the original. The reference lump has actually lost about 50 micrograms in the last 100 years (and no one knows why). That's a lot (well, speaking at the level that micrograms get used at... 1 microgram = 0.000000001 kg), and the really highlights the need for an immutable reference point.

Readers may find the pertinent Wikipedia article [wikipedia.org] interesting.

• #### Re:Circular definition (Score:1, Informative)

by Anonymous Coward on Sunday February 27, 2005 @12:06AM (#11792037)
No, what you do is that you set Avogadros constant, to a....ehmmm...... constant (ie not dependant on KG). Then you define KG off that constant.
• #### Re:I suggest (Score:5, Informative)

on Sunday February 27, 2005 @12:13AM (#11792069) Journal

One of the nice things about the British system of measurement (which pretty nearly only the Americans use officially, though with a few changes) is that the units are exactly the sort of thing you often want about one of. A pint of beer, a gallon of kerosene, a bale of hay, a pint of milk if you live alone or a quart or a gallon depending on the size of your family, half an acre of land, etc. (yes, yes, I don't think a bale is an Imperial measurement).

The metric equivalents never seem to be just right, but we'll just have to live with them

But thats true for the metric system as well :D You only dont learn the "special" units in school I asume.

In german we have "pound" as well, which is just slightly bigger than yours. And ppl in shops still buy "half a pound" of meat or something.

Same for land, we have an "ar" and a "hectar" which is obviously 100 ar, and we have a "morgen" wich is 25 ar and the typical size of a field in older times.

A ar is similar big as an acre (IIRC).

Same for drinks, who cares about your pint? Do you really think we order 350ml Beer?

We order a glass of beer, obviously. And depending on beer brand it is served in a typical size.

The sizes are: 0.2l for Kölsch and Alt. 0.3l for some kins of "Pils" which consider themslelf noble. 0.4 for a standard everywhere pils,a nd your pint is just between 0.3 and 0.4. The enxt size is 0.5l for Weiten.

The same applies for nearly any metric size, no one is buying xyz litres or something except he buys 40l gasoline for his car.

Bottom line we have as many "human" metrics as you but sine the metric system is in use they got rounded to the next best number.

angel'o'sphere
• #### Re:artifact (Score:5, Informative)

on Sunday February 27, 2005 @12:22AM (#11792105) Homepage
What you're talking about are "fundamental" units versus SI base units.

In a fundamental system of units, there are three base units: charge, mass, and angular momentum. (Gee, those sound suspiciously like the three properties that a black hole can possess - I wonder why). Everything else can be derived from those units (for the most part - we'll ignore stuff like baryon number, lepton number, etc. because those theories aren't complete yet. For instance, we now know that only global lepton number is conserved, not mu, e, and tau lepton number separately. I won't even touch color, as color is completely hidden anyway).

In fact, the existence of those units can be derived from the fact that space is invariant under the Poincare group, and has gauge symmetry.

However, those base units come because you've defined other constants to 1.

The problem is that several of those constants are imprecise and difficult to measure. It is easier to define a kilogram, for instance, then it is to somehow base it on the gravitational attraction of two objects, because G is horribly imprecise.

Similarly, it is easier to treat Kelvin as fundamental rather than derived from other units *if* Boltzmann's constant has poor precision.

So while it's *possible* to use fundamental-based units, it's often *impractical* and less precise. The base units in SI are those that can generate all other units with no loss in precision.

To give a very practical example, the mass of a proton is typically given in atomic mass units (amu) as ~1.007 amu. You might think that it should be given in grams, as "amu" isn't a fundamental unit of mass. But the conversion from "amu" to "grams" is less precise than the mass of the proton in atomic mass units. So in this case, "amu" would be appropriate as a base unit, as well as mass, even though the two can be directly converted.

The benefit is that you can compare the mass of a proton and the mass of a neutron in "amu", for instance, to better precision than you could in grams. It's similar (or was similar when SI was developed) with the other units.
• #### Re:How is the US pound measured? (Score:3, Informative)

on Sunday February 27, 2005 @12:26AM (#11792120)
Pounds measure weight, my friend. As in the effect of a gravitational field on a certain mass.

Those who use pounds as force use slugs [wolfram.com] as the unit of mass. Same relationship as mass in kilograms and weight in newtons (i.e. Newton's 2nd Law), except for the weird-ass numbers.

Just how many hogsheads are there in a fortnight, anyway?

...laura

• #### Just a few corrections (Score:3, Informative)

on Sunday February 27, 2005 @12:28AM (#11792125)
A hectare is 2.5 acres, so there are 40 ares in an acre. An american pint is .475 L, very close to .5 L. A German pound is 500 g, whereas american is 454 g.
• #### Re:I suggest (Score:2, Informative)

on Sunday February 27, 2005 @01:25AM (#11792350)
Uhm, all the other SI units are defined off physical things. A second is defined as so many oscillations of a cesium atom, a meter is defined as 1/299792458 light*second, a kelvin is 1/273 the triple point of water and an ampere is 6.25*10^25 electrons/second passing a given point on a circuit. All of these things are based off a particular physical "thing" - it'd be impossible to create a definition that wasn't!
• #### Re:Obligatory Simpsons Metric Quote (Score:3, Informative)

on Sunday February 27, 2005 @02:36AM (#11792613) Journal
"metric" != "base-10"
• #### Re:artifact (Score:3, Informative)

on Sunday February 27, 2005 @02:52AM (#11792708) Homepage

Candela essentially measures the same things as watts.
Mole is just an number. It might be used in the definition of the kilogram, but in itself, it just relates the mass of a gram with 1/12 the rest mass of a carbon-12 atom.
Kelvin is just a unit derived from mass, momentum, and kinetic energy. It is not a base unit.

Ampere might or might not be a base unit, I'm not sure about that one.

You are talking about base units of physics (and you're still very wrong there), not base units of measurement.

Take Kelvin, for instance. We'll ignore the fact that temperature really relates both energy and *fundamental statistics* (the temperature of a gas of fermions at a given temperature is different than a gas of bosons at a given temperature). But even if it didn't, and it was just "average kinetic energy over Boltzmann's constant", you could say that Kelvin is just inverse joules...

if you set Boltzmann's constant to 1, and have it be unitless. The problem is that you've now shifted any imprecision of measuring Boltzmann's constant into *all measurements of temperature*, rather than just keeping it in the connection between energy and temperature. So when you calibrate your new temperature scale in "inverse joules", you now face the same precision problems that you would face in measuring Boltzmann's constant. That is, you have to measure the average kinetic energy of an ideal gas, and label that on your "inverse joule" thermometer.

This is dumb. Of course, what you do is use Kelvin as a base unit, and *define* the scale using other processes (the triple point of water, if memory serves) and now you've got a perfectly calibrated scale to huge precision, and the only imprecision from measuring the Boltzmann constant comes when you want to convert to energy.

So, again - base units of measurement are not the same as base units of physics. The base units of physics are the fundamental quantum numbers of a particle, mass, charge, spin (and color). The base units of measurement are the SI units.
• #### Re:I suggest (Score:2, Informative)

<vidarlo@bitsex.net> on Sunday February 27, 2005 @05:46AM (#11793092) Homepage
Avagadro's # sounds fine to me, as long as they define Avagadro's number independantly.

Why? The meter started as a platinium bar, and later we redefined it to something else, that matched this platinium bar. Why can't we do the same with kg? We're only trying to base it on something natural phenomen, not to redefine it. The point is that if it is a common criteria, and nnot a single object, anyone can measure a kg _exactly_

• #### Re:Mod Parent Up (Score:2, Informative)

<vidarlo@bitsex.net> on Sunday February 27, 2005 @05:49AM (#11793103) Homepage
Someone smart famous once said "Any technology, sufficiently advanced, is indistinguishable from magic."

Said by Arthur C. Clarke [wikipedia.org]

• #### Re:artifact (Score:3, Informative)

on Sunday February 27, 2005 @08:12AM (#11793395) Homepage
Candela essentially measures the same things as watts.

The candela is a weird unit, but it is not equivalent to watts. There are three units related to light:

• lumen -- comparable to watts, but weighted with a defined sensitivity curve that is supposed to represent the response of an average human eye;
• lux -- lumens per square meter
• candela -- lumens per steradian (unit of solid angle). It represents brightness, i.e. how bright the light source looks if you look into it from a specified distance.
For some reason the candela was chosen to be the base unit, rather than the lumen; probably because it is easier to calibrate for. The sensitivity curve is rather arbitrary. It is fundamentally impossible to measure this curve with high precision since individual humans are different and it requires test persons to judge subjectively whether, say, a red and a green light source are equally bright.

Since these units are defined to some hard-to-measure property of the human body, I think they shouldn't have a status as an SI base unit. Inches and feet don't have that status either, after all.

• #### Re:I suggest (Score:2, Informative)

on Sunday February 27, 2005 @01:12PM (#11794600)
2700 only has four significant digits if it's written as "2700." or the final zero has a bar over the top of it. Otherwise, it's two.
• #### Salami in decagrams (Score:1, Informative)

by Anonymous Coward on Sunday February 27, 2005 @01:56PM (#11794949)
When I was living in Europe I had to buy my salami (cheese, ham, etc) in decagrams. So I can tell you, they wouldn't sell you 80 grams of salami -- doesn't make sense. Now, 100 grams -- that, they would sell you (but only if you called it "10 deca"). Barely is enough for one sandwich, though -- I suggest you go with 20 deca as a minimum purchase for items like ham and salami.

And buying butter in chunks of 500 g is quite normal.

I cannot speak for all Euro nations, as I was living in only the one, but it's a good guess that it wasn't the only one that sold its deli items in this way.

• #### Re:I suggest (Score:3, Informative)

on Sunday February 27, 2005 @06:13PM (#11796890) Homepage
No. No one orders Kölsch as "0.2l of Kölsch". It's rather "Bringen Sie einen Meter!" (Serve a meter!), which refers to a wooden bar of roughly three feets (1 meter) length with holes, where a glass of Kölsch is put in every hole.
• #### Technicalities of mass measurement (Score:3, Informative)

on Monday February 28, 2005 @10:38AM (#11802152) Homepage Journal
Lucky you, you've got a bona-fide metrologist [wikipedia.org] replying. Admittedly one who specializes in automated calibration of electronic instruments, but we get the basic lectures on dimensional analysis too.

The main reason for platinum-iridium is that it's got a very low thermal expansion coefficient. Basically, it doesn't expand or contract much with change of temperature. However, densisty is also important. Don't ever ask a metrologist that old chestnut about which is heavier, a kilogram of lead or a kilogram of feathers, unless you're willing to sit through a few hours of lecture on buoyancy. Yup, it's not just for water and hot air balloons. A denser object of the same mass will weigh slightly less (assuming uniform shape and all that), as it will be slightly less bouyant in the air.

As for your comment regarding a smaller object being less accurate due to relative scale of dust, a smaller mass is also slightly less prone to the influence of the variability of the gravity constant across the Earth's surface. *wry grin* There are a lot of factors you have to deal with when you start working on the scales we do here. And that's not even getting into the gage blocks (length measurement) which have surfaces so smooth that they form a vacuum when touched together, and will spot weld to each other if left overnight...

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