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Experts Suggest Replacing Definition of Kilogram
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."
The SI unit of mass is the kilogram, not the gram.
Just in case people care, here are the 7 base units:
... or something.
Metre for Length
Kilogram (what this article is about) for Mass
Second for time
Ampere for current
Kelvin for temperature
Mole for amount
Candela for "Luminous intensity"
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, in my AS-Level Physics coursework, if anyone want s a simplifed read.
(Wiki)
I don't see how this topics is maths, by the way.
- Jax
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.
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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.
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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).
How many people can read hex if only you and dead people can read hex?
Given that the kg prototype has lost 50 micrograms over the last 100 years I'm guessing 0.999995kg?
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.
If you disagree, post your argument. (-1, Overrated) isn't your personal censorship tool for views you don't like.
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 interesting.
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
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.
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Cost free eBook I read (by iBook/Kobo/Amazon/ObookO/Gutenberg etc.): "The Green Odyssey" by Philip Jose Farmer.
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.