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Graphene and Quantum Hall Effect Could Help Redefine Metrics

timothy posted about 3 years ago | from the no-paywall-and-that-rocks dept.

Science 92

eldavojohn writes "The National Physical Laboratory has published research in Nature that could lead to redefining two of our most commonly used metrics. There's been a lot of trouble stemming from defining an exact Kilogram as some lump of platinum-iridium sitting in a glass case somewhere, so the proposal was put forth to study the quantum hall effect with different materials. Enter the Nobel prize winning, super strong, silicon usurping graphene. NPL now says you can add quantum resistance metrology to the list of graphene's many conquests as they say the quantum hall effect in graphene is 'very robust and easy to measure.' With this at their disposal, the Kilogram may be redefined in terms of h, the Planck constant, and the Ampere may be redefined in terms of e, the electron charge (alias Elementary charge or the charge of a proton). You can find the full paper here."

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I guess I always assumed... (1)

Beorytis (1014777) | about 3 years ago | (#37553462)

...that the ampere was already defined in terms of the charge of the electron.

Re:I guess I always assumed... (0)

Anonymous Coward | about 3 years ago | (#37553576)

No, hence why the electron charge is 1.602e-19 Amperes/s and not something "nicer".

Re:I guess I always assumed... (1)

ceoyoyo (59147) | about 3 years ago | (#37555430)

Irrelevant. The metre is defined as 1/ 299,792,458 of the distance light travels in a second. If the standard fro the Ampere changes, it will change to 1.602e19 electron charges per second.

(and you meant e = 1.602e-19 As, not A/s)

Re:I guess I always assumed... (3, Informative)

Goaway (82658) | about 3 years ago | (#37553598)

The definition of a unit must be physically instantiable. That is, you have to be able to use the definition to build a device or artifact that can be used to calibrate a meter for said unit. Otherwise, the unit is useless.

This means that some units still have cumbersome and strange definitions, as we do not have the technology to use the obvious definitions to calibrate measurement devices.

Re:I guess I always assumed... (1)

Pf0tzenpfritz (1402005) | about 3 years ago | (#37554766)

But the lump of metal is located "somewhere" i.e. outside the U.S. So it can not be used. Now, we have an american definition of "kilogram" and kilograms have become a real unit, ready to be used in the real world. As soon, as someone can claim intellectual property, that is.

Re:I guess I always assumed... (1)

Goaway (82658) | about 3 years ago | (#37557074)

But the lump of metal is located "somewhere" i.e. outside the U.S. So it can not be used.

Oh, you gravely underestimate the amount of work that goes into this system.

Every country has their own copy of the weight. Every now and then, they very, very carefully bring their own weight to Paris, and calibrate it against the weight that sits there. Then they equally carefully bring it back. Once it is back, more copies are manufactured locally, and sent out to institutions and industry who need it to calibrate their own equipment.

And so in the end, through many intermediate steps, your kitchen scales are calibrated against the single kilogram in Paris.

Re:I guess I always assumed... (1)

Chris Burke (6130) | about 3 years ago | (#37558034)

And so in the end, through many intermediate steps, your kitchen scales are calibrated against the single kilogram in Paris.

The compounding error, it burns... it burns!

Heh. Not that it's really a terrible way of doing things. Just glad to see there's enough confidence in an experimentally reproduceable metric to replace the 'lump of metal defined to be 1kg' model.

Re:I guess I always assumed... (1)

Medievalist (16032) | about 3 years ago | (#37558514)

And so in the end, through many intermediate steps, your kitchen scales are calibrated against the single kilogram in Paris.

The compounding error, it burns... it burns!

Oh, definitely. Any attempt to equate physicalities through intermediaries must be assumed to incorporate compounding error.

But a balance is very, very simple; it has fewer internal sources of error than a voltmeter, for instance. You validate a balance the same way you validate a level; it's absolutely dead simple and requires no tools other than human hands, eyes, and the existence of the planet Earth.

On of my cow-orkers was stunned when I told him how you initially set an atomic clock. (Strip away the jargon, and you're just referencing against Flamsteed's stick - when the stick has no shadow, it's noon.)

Re:I guess I always assumed... (1)

Chris Burke (6130) | about 3 years ago | (#37558870)

On of my cow-orkers was stunned when I told him how you initially set an atomic clock. (Strip away the jargon, and you're just referencing against Flamsteed's stick - when the stick has no shadow, it's noon.)

So this only works at the equator, eh? We need to find a better metric, or Ecuador and the other equatorial nations will hold this over us like the Sword of Damocles!

Re:I guess I always assumed... (1)

Medievalist (16032) | about 3 years ago | (#37559744)

Oh, it's worse than that. Flamsteed's observatory is near Greenwich, England... Universal Time Co-ordinate Zero.

Actually it's always been backwards like that (1)

alispguru (72689) | about 3 years ago | (#37553658)

The SI base unit is the Ampere. The Coulomb is a derived unit (Ampere-seconds).

Which is definitely silly - the base unit is charge flow, and the derived unit is charge?

Re:Actually it's always been backwards like that (1)

ZankerH (1401751) | about 3 years ago | (#37553720)

Agreed, the charge unit (preferably one defined via the elementary charge) should be the base.

Re:Actually it's always been backwards like that (1)

belg4mit (152620) | about 3 years ago | (#37562462)

Farady forever!

(a Faraday is the charge of a mol of electrons, or 96,485.30899 Coulombs)

Re:Actually it's always been backwards like that (1)

ZankerH (1401751) | about 3 years ago | (#37565210)

That kind of conflicts and would be easily mistaken for the F (farad), the unit for capacitance. Also, in physics you generally don't name two units after the same person.

Re:Actually it's always been backwards like that (1)

belg4mit (152620) | about 3 years ago | (#37566924)

Except that they already have, sort of, they call it a constant http://en.wikipedia.org/wiki/Faraday_constant>

Re:Actually it's always been backwards like that (2)

khallow (566160) | about 3 years ago | (#37553800)

Which is definitely silly - the base unit is charge flow, and the derived unit is charge?

Not really. Simple algebra can easily convert from one unit to another. And the second is far better defined than the Coulomb is. So there's no measurable error introduced by using the Ampere as the base unit rather than the Coulomb.

Re:Actually it's always been backwards like that (2)

fizzup (788545) | about 3 years ago | (#37554366)

If you spent the time needed for two laboratory exercises, one to prove that you had created a circuit with an Ampere of current and another to prove that you had amassed a Coulomb of charge, then you would understand why the base unit is Ampere, not Coulomb.

Re:Actually it's always been backwards like that (1)

Unequivocal (155957) | about 3 years ago | (#37556390)

I love it when someone who actually knows something post on slashdot!

Re:Actually it's always been backwards like that (1)

newcastlejon (1483695) | about 3 years ago | (#37557268)

I love it when someone who actually knows something post on slashdot!

Hear, hear!

It's like Christmas, but apparently somewhat less frequent.

Re:Actually it's always been backwards like that (1)

Goaway (82658) | about 3 years ago | (#37557098)

Which is definitely silly - the base unit is charge flow, and the derived unit is charge?

This is because we are better at measuring the effects of charge flow than of charge, and thus it is easier to find a unit definition based on it. Practicality.

What a terribly written summary! (-1)

Anonymous Coward | about 3 years ago | (#37553480)

It's inaccurate.

Hillbilly Mutt 20 is now an existentialist armageddon.

Graphene - the smartest material known to Man! (3, Funny)

lxs (131946) | about 3 years ago | (#37553588)

It even won a Nobel prize.

The gift (0)

intellitech (1912116) | about 3 years ago | (#37553892)

The gift that keeps on giving.

did nothting to earn it (5, Funny)

Thud457 (234763) | about 3 years ago | (#37554094)

It even won a Nobel prize.

They only awarded that because it wasn't George Bush.


"Yay, inanimate carbon, errr, sheet!

Not true.. (0)

Anonymous Coward | about 3 years ago | (#37554196)

That's not true..

Re:did nothting to earn it (1)

black soap (2201626) | about 3 years ago | (#37555656)

By that standard, the Catholic Church should make me a saint, because I'm not the Devil.

I like where this is going.

Re:Graphene - the smartest material known to Man! (1)

Jeng (926980) | about 3 years ago | (#37554410)

The inanimate carbon rod was robbed!!

Obligatory Simpsons Reference (0)

Anonymous Coward | about 3 years ago | (#37554604)

And the award goes to... this inanimate carbon sheet!

OK Sorry, now I R'd the FA - summary is wrong (0)

Beorytis (1014777) | about 3 years ago | (#37553600)

SECOND POST! Both kg and A may be redefined in terms of h and e.

Re:OK Sorry, now I R'd the FA - summary is wrong (0)

Anonymous Coward | about 3 years ago | (#37553780)

More importantly, doing things technically right (i.e., not basing the kg on a eventually flimsy block of platinum) will make the USA adopt it and get us rid of the plethora of references to variable units (e.g. the pound) in "scientific" works? Pardon me, but I don't think so.

Re:OK Sorry, now I R'd the FA - summary is wrong (1)

black soap (2201626) | about 3 years ago | (#37555686)

When the imprecision of the definition of the unit is exceeded by the imprecision of the experiment, we usually call that "not a problem."

Re:OK Sorry, now I R'd the FA - summary is wrong (1)

jbengt (874751) | about 3 years ago | (#37557064)

The USA has already officially adopted the metric system, even though those units are not in common use in the US: Feet, pounds, etc., were legally defined in terms of metric measures more than 100 years ago.

Re:OK Sorry, now I R'd the FA - summary is wrong (0)

Anonymous Coward | about 3 years ago | (#37557818)

> The USA has already officially...

The word officially should (as I understand, at least) mean people would use it in traffic signs, term papers, newspapers et coetera... what is missing for the entire population, including scientists, to start using the SI?

It's not funny when there's a standard and nobody wants to use it. Do we need a campaign? Imagine telephony without standards... how could we call one another?

Re:OK Sorry, now I R'd the FA - summary is wrong (1)

Goaway (82658) | about 3 years ago | (#37557130)

The US system of weights is already fixed to the metric system, and calibrated against the prototype kilogram in Paris, just like everyone else.

Re:OK Sorry, now I R'd the FA - summary is wrong (0)

Anonymous Coward | about 3 years ago | (#37562148)

> The US system of weights is already fixed to the metric system, and calibrated against the prototype kilogram in Paris, just like everyone else.

Everyone else must use the international system of units (SI); read the next newsstory exactly here at /. : it talks about a solar sail which is 100 sq ft (or 9.29 m2).

So "like everyone else" is not exactly happening, is it?

Now, you can say it's a matter of Freedom, anyone does what they want... ok, if so then why is everyone using dollars inside the US? Shouldn't different people use Euros, pesos or whatever?

What can we do, you say? I don't know, and I have my own country with its quota of problems, but here's a thought: take care of what is yours, because your neighbours have their own houses to maintain...

So Metric will change..again. (1)

geekoid (135745) | about 3 years ago | (#37553614)

When was the last time someone redefined a pound?

Re:So Metric will change..again. (1)

Anonymous Coward | about 3 years ago | (#37553698)

Just about every second. [exchangerateusd.com]

Re:So Metric will change..again. (1)

Coisiche (2000870) | about 3 years ago | (#37553872)

Well, to be fair, while there are multiple definitions of "pound" there are also multiple definitions of "meter".

Although in terms of what's in the article, I don't think "gram" can ever be used in other contexts but "amp" is a contraction of more than just "ampere".

Re:So Metric will change..again. (1)

rossdee (243626) | about 3 years ago | (#37554010)

'there are also multiple definitions of "meter"'

Fortunately the System International unit of length is the metre

Re:So Metric will change..again. (0)

Anonymous Coward | about 3 years ago | (#37559294)

Weellll, if you're going to be pedantic, that would be "Système Internationale" with some possible other funny diacriticals along the way.

Re:So Metric will change..again. (0)

Anonymous Coward | about 3 years ago | (#37557388)

Department of lab rats | Operation | Wake Up Call | Gram VS Amp | Experiment #00001.
The difference was easily discerned when 800 grams of Marijuana was sitting on your ass vs 800 amps of electricity

different energy - Nightmare Hell On Earth vs Euphoria
weight - no discernible weight vs a few pounds on the ass (you know what I meant!)
emotional effects - torture vs pain free
lifespan - Continued shock results in death vs a couple months of being stoned
speed - Consuming Marijuana resulted in about a 90 minute delay vs the shock being instant.

common sense is all you need, half the possibilities are not even described in this white paper.

Re:So Metric will change..again. (0)

Anonymous Coward | about 3 years ago | (#37557572)

Department of lab rats | Operation | Wake Up Call | Gram VS Amp | Experiment #00002.
The difference was easily discerned when 800 grams of Marijuana was in a plastic garbage bag vs 800 amps of Marshalls and gibsons and fenders pinning your ears back

different energy - Speakers could sooth the soul and so could the weed, some speaker lovers tried to visit the pot smokers
weight - If the speakers played a low enough tone the weight of the subject could be lessened by vomiting vs the weed guys got the munchies
emotional effects - both Weed and Speakers could send a shiver down your spine
lifespan - when the speakers go off and the THL (Temporary hearing loss) humm kicks in, the buzz from the weed helps mitigate
speed - Speakers 20 - 20KHz vs Pot -- the party hasn't ended yet man

Re:So Metric will change..again. (1)

bornwaysouth (1138751) | about 3 years ago | (#37561154)

Actually, English is pretty much a compilation of lots of languages, and "gram" does have lots of meanings, most associated with writing. It does allow one to consider adding (to microgram, kilogram and other aggregate units) the following...
pangram - typical weight of a chimp.
lipogram - typical excess weight of a human. (Useful as you only get heavier if you eat more than other fatties.)
seismogram - typical weight of a tectonic plate
urogram - something that weighs piss all
and so on. It's all quite simple. (And simplicity can be weighed in idiograms.)

Re:So Metric will change..again. (3, Informative)

Alpha Whisky (1264174) | about 3 years ago | (#37553858)

1963 when the UK parliament adopted the international definition (from 1959) of the pound as 0.45359237 kilogrammes. Ironically for you, it will change again if the definition of the kilogramme changes as per the article.

Re:So Metric will change..again. (0)

Anonymous Coward | about 3 years ago | (#37556124)

Last time I checked pound was a unit of force, and kilogram was a unit of mass.

Or is it just the British pound that that is a unit of mass?
What have I missed?

Re:So Metric will change..again. (1)

Chris Burke (6130) | about 3 years ago | (#37558130)

What have I missed?

That the pound is both a unit of force [wikipedia.org] and a unit of mass [wikipedia.org] .

Wonderfully confusing, no?

Re:So Metric will change..again. (1)

stardaemon (834177) | about 3 years ago | (#37564298)

Thank you:)

I was the AC who asked the question.

Re:So Metric will change..again. (1)

artor3 (1344997) | about 3 years ago | (#37554750)

Why bother? Pounds are only used in day to day life, where the sorts of precision being discussed here would go to waste. You may as well ask why a schoolkid's calculator only holds a dozen digits of pi.

Re:So Metric will change..again. (1)

Anonymous Coward | about 3 years ago | (#37555452)

Stop showing your ignorance. "Pound" has multiple definitions already, and has a schizophrenia about if it is a measure of force or mass. In ye olden physics, pound is used for force and the slug is the unit of mass.

For the "mass" concept, the commonly adopted definition is in fact 0.45359237 kilograms. In other words, it's defined in terms of SI.

Similarly, your beloved "inch" is defined as exactly 25.4 mm these days.

You can try defending the imperial system in a number of ways, but picking consistency is a battle already lost.

Re:So Metric will change..again. (1)

Jonner (189691) | about 3 years ago | (#37555942)

When was the last time someone redefined a pound?

Which pound? There are pound force and pound mass. There are at least US and Imperial varieties. At least modern pound mass [wikipedia.org] is generally defined in terms of the kilogram, so if it changes, so does the pound.

Re:So Metric will change..again. (0)

Anonymous Coward | about 3 years ago | (#37559154)

Probably about the last time anyone cared ...

Don't forget they aren't *changing* the kg. They're restating it based on more a stable and reproducible definition. No calibrated devices will need to change.

Comming soon ! (0)

Anonymous Coward | about 3 years ago | (#37553624)

Graphola! The all new graphene -strengthened energy drink along with many other wondrous effect can:

- Cure cancer and aids!
- Improve your memory and intelligence!
- Give you a minty fresh breath!

Coming soon to your local store! Be wary of imitations, ask for Graphola!

Re:Comming soon ! (1)

jamiesan (715069) | about 3 years ago | (#37553830)

PIYAN (Plus if you act now) We will throw in a serving of Graphola flavored Quantum Dippin' Dots!

Re:Comming soon ! (1)

game kid (805301) | about 3 years ago | (#37554130)

...in a container signed by Charli Dvoracek [slashdot.org] !

Unanswered questions: (1)

cornface (900179) | about 3 years ago | (#37553974)

1) Can bitcoins be minted from graphene?

2) How does this affect the Packt constant?

Re:Unanswered questions: (0)

g4b (956118) | about 3 years ago | (#37554064)

in soviet russia, plancking constantly affects YOU.

Re:Unanswered questions: (1)

X0563511 (793323) | about 3 years ago | (#37554320)

Try not to waterboard yourself with your own tears.

3 orders of magnitude better than the lump (4, Informative)

Covalent (1001277) | about 3 years ago | (#37554022)

Right now the accuracy with which the kilogram can be measured is about 1 part per 1E8. The paper mentions a noise of around 1 part per 1.6E11. That's over 1,000 times better. That certainly suggests that this method will be sufficiently "better" to be used as the new standard.

I, for one, welcome our incredibly accurate overlords.

Re:3 orders of magnitude better than the lump (2)

Jamu (852752) | about 3 years ago | (#37555180)

It also doesn't depend on an artifact ("the lump"): It will be possible to determine one kilogram from the definition only.

Re:3 orders of magnitude better than the lump (0)

Anonymous Coward | about 3 years ago | (#37555956)

A kilogram can already be derived from definition only: it's 1000 grams.

Re:3 orders of magnitude better than the lump (0)

Anonymous Coward | about 3 years ago | (#37556526)

And what's the definition of a gram? 1/1000 kilogram?

dom

Re:3 orders of magnitude better than the lump (1)

Chris Burke (6130) | about 3 years ago | (#37558164)

And what's the definition of a gram? 1/1000 kilogram?

1,000,000 micrograms. The microgram is defined as 1/1000 milligrams. The milligram is defined as 1,000,000,000 picograms. The picogram is defined as shut up.

Re:3 orders of magnitude better than the lump (0)

Anonymous Coward | about 3 years ago | (#37559988)

Turtles all the way down.

thanks for that (1)

Medievalist (16032) | about 3 years ago | (#37560042)

I can't remember the last time I laughed out loud while reading a forum post.

Re:3 orders of magnitude better than the lump (1)

ImprovOmega (744717) | about 3 years ago | (#37559882)

Originally 1 gram was the mass of 1 cubic centimeter of pure water at precisely 0 degrees Celsius. Originally the meter was defined at 1/10,000,000 * the distance from the equator to the North Pole at sea level.

We've since redefined meter to be a fraction of the speed of light in a vacuum (good change) and a kilogram to be based on the mass of an object kept in a standards bureau (bad change).

Re:3 orders of magnitude better than the lump (1)

ImprovOmega (744717) | about 3 years ago | (#37559890)

Err, fraction of how far light in a vacuum will travel in a second. Should've proofread a little closer.

Re:3 orders of magnitude better than the lump (1)

treeves (963993) | about 3 years ago | (#37560272)

Ha ha. Kg is the base unit, not grams. I know it's crazy, since it has a prefix, but that's just how it is.

Re:3 orders of magnitude better than the lump (1)

devent (1627873) | about 3 years ago | (#37559020)

I don't know why AC was modded down. A kg is whatever you define as a kg. It could be the weight of my refrigerator, the weight of 1000 gold atoms, or as current "The kilogram is defined as being equal to the mass of the International Prototype Kilogram[1] (IPK),[Note 3] which is almost exactly equal to the mass of _one liter of water_.".

So now we define some other property as the kg. The important thing is, that you can measure the "lump" accurate.

Re:3 orders of magnitude better than the lump (2)

Chris Burke (6130) | about 3 years ago | (#37559836)

I don't know why AC was modded down. A kg is whatever you define as a kg. It could be the weight of my refrigerator

Yeah, but if the kg is defined as the mass of your refrigerator, then I can't arrive at the kg using just the definition. I also need your refrigerator. I can't build a refrigerator of my own and use that to calibrate my scale, because without access to your refrigerator mine is going to mass differently than yours, and the unit is defined in terms of yours and yours alone.

Whereas when the definition is based on a physical property of the universe, anyone anywhere can recreate the unit and calibrate their instruments without having access to a particular artifact. Using just the definition.

So it is quite different.

The AC was making a joke and didn't deserve to be modded down.

Not good enough! (1)

vuo (156163) | about 3 years ago | (#37572076)

Redefining the kilogram with the Planck constant doesn't help with accuracy, since the Planck constant itself is known to the precision of 5e-8 only. So, in effect, the determination to the accuracy of 1e-8 isn't a major improvement. Just think of 1e-8 in terms of the prototype kilogram. 1e-8 means that the mass of the prototype (1000 g) can be fixed to within 0.01 mg, which is really a lot. This is an amount that can be measured by hand, without using any fancy and expensive machine. Our university department has a regular, everyone-and-their-dog-has-one, off-the-shelf, scientific garden variety, scale, which measures to this precision, five decimals to the gram. To put this in context, I use a four-decimal scale for everyday weighing in small-scale work; I usually avoid the five-decimal scale because it's prone to annoying hunting behavior (it "dances around" the correct value) more than the more robust four-decimal scale. In effect, it's just passing the buck if you define the kilogram based on the Planck constant.

No. 1 Unit Needing Urgent Definition (1)

srussia (884021) | about 3 years ago | (#37554030)

The US Dollar.

Re:No. 1 Unit Needing Urgent Definition (1)

drinkypoo (153816) | about 3 years ago | (#37554270)

It means precisely what it is intended to mean.

Re:No. 1 Unit Needing Urgent Definition (2)

peragrin (659227) | about 3 years ago | (#37554368)

As a financail advisor recently pointed out.

There is no trade imbalance with China. They give us tons of wortgless goods and we give them tons of eorthless dollars.

Seems fair wen you think about it.

Re:No. 1 Unit Needing Urgent Definition (0)

Anonymous Coward | about 3 years ago | (#37554974)

hihiuh

Re:No. 1 Unit Needing Urgent Definition (1)

black soap (2201626) | about 3 years ago | (#37555732)

But they are loaning us the dollars we are paying them with, and charging interest. As long as the interest rate they charge exceeds inflation, they'll still get paid.

Re:No. 1 Unit Needing Urgent Definition (0)

Anonymous Coward | about 3 years ago | (#37556168)

And since we are debasing our dollar, all capital goods in this country are now worth less, leading to an outflow of capital and hence a job shortage. But wait the unemployment rate is actually due to consumer retail sales down this quarter, right?

Re:No. 1 Unit Needing Urgent Definition (1)

inasity_rules (1110095) | about 3 years ago | (#37556980)

Its a guess, but I think I know where your keyboard was imported from...

Re:No. 1 Unit Needing Urgent Definition (0)

Anonymous Coward | about 3 years ago | (#37558354)

100 US Cents

Whew! (1)

argStyopa (232550) | about 3 years ago | (#37555072)

I'm glad to see that we can finally dump that silly imperial system and get to a set of eminently sensible standards and measures that aren't obscure and/or arbitrary.

Because when I want to buy meat, I certainly first think "how will this pile of hamburger relate to the Planck constant?"

Will this affect (1)

SirTicksAlot (576078) | about 3 years ago | (#37555770)

How much I pay for my baggage at the airport?

That's all well and good, but... (1)

ALeader71 (687693) | about 3 years ago | (#37555888)

Eventually Scotty will come up with a way around it at Kirk's behest.

Isotropically pure graphene (1)

Required Snark (1702878) | about 3 years ago | (#37556438)

Isotropically pure diamonds, either all C-12 or all C-13, have 50% higher thermal conductivity then isotropically mixed diamonds. http://en.wikipedia.org/wiki/Isotopically_pure_diamond [wikipedia.org] When using graphene for this kind of measurement, do they also use a single isotope of carbon? Does it make any difference if the carbon used in not isotropically pure?

Re:Isotropically pure graphene (0)

Anonymous Coward | about 3 years ago | (#37558888)

From the article, they are measuring the Quantized Hall effect. What they have found is that the measurements of the effect in Graphene are the same as the measurements already made in a GaAs or GaAlAs semiconductor, meaning that the Quantized Hall effect does not depend on the material producing the effect. Thus, since the effect is independent of the material producing it, it is not necessary for the sample to be isotropically pure.

The eventual re-definition of the kilogram may assume an isotropically pure amount of graphene carbon. There may also be a static definition of the Mole at that point as well.

Water? Kilograms are based on water! (0)

Anonymous Coward | about 3 years ago | (#37556458)

I had always assumed that a meter is based on the speed of light (so relativistic effects can change the size of the meter, have fun with that). And that 1/10 of a meter is a decimeter (deci means 1/10th). And if you form a cube whose sides are 1 decimeter, you get a volume that is 1 liter. And if that liter volume is filled with pure water at 0 degrees Celsius (the temperature at which water freezes, but in the liquid rather than the solid state), and weigh that water, it weighs 1 kilogram. I always thought this is how metric units of measure were created. Temperature was Celsius, with water being the arbitrary stuff used as reference, and where it freezes is 0C and where it boils is 100C and there are no offsets. Fahrenheit arbitrarily uses water too, but they have 180 degrees between boiling and freezing (I suppose its easy to have 180 marks on a half circle), except they have this weird 32 degree offset (I don't remember why), so that water is declared to freeze at 0+32 rather than 0, and boil at 180+32=212 (again, I don't know why). Using whole numbers, Fahrenheit is 1.8 times as accurate as the Celsius. When you use a single digit decimal with Celsius, its 5.5 times as accurate as Fahrenheit.

Re:Water? Kilograms are based on water! (1)

Goaway (82658) | about 3 years ago | (#37557206)

And if that liter volume is filled with pure water at 0 degrees Celsius (the temperature at which water freezes, but in the liquid rather than the solid state), and weigh that water, it weighs 1 kilogram.

This was the definition for about five or so years in the eighteenth century.

Re:Water? Kilograms are based on water! (1)

White Flame (1074973) | about 3 years ago | (#37559498)

0 degrees Fahrenheit was the freezing point of a particular brine, which was the lowest freezing point liquid material available at the time.

Re:Water? Kilograms are based on water! (1)

Relic of the Future (118669) | about 3 years ago | (#37559714)

Oh god. Okay, hold on.

One: meter. Originally, the meter was suppose to be 1/40,000 of the circumference of the earth as measured from poll-to-poll and back. They actually tried to measure this, with surveyors and crap, and they were a bit off on it. Now, yes, it is based on the distance light travels in a vacuum in a specific (very short) amount of time, although it is still very close to that original (inaccurate) measurement. However, you don't need to "worry" about relativistic effects: any measuring device you apply will shrink in the same proportion as what is being measured (because the difference is only visible to an observer in a different reference frame.)

Two: kilogram. They originally did it by a cubic centimeter (which is why the base unit name was the gram) not a cubic decimeter; when the changed to cubic decimeter/liter though they didn't redefine gram, they just said they were defining the KILOgram. And yes, it was at zero degrees, but it was also--unspecified but assumed--at one atmosphere of pressure. Later, it was changed to be at 4 degrees, since that's when water is at its most-dense (again, at 1 atm.) and that's the mass that the reference mass was based on, but since then the reference mass is the ONLY thing the measure is based on. Which is handy, because (as you've surely guessed) the value would change at different pressures (or if the definition of a meter changed) but is bad because you can't just describe a kilogram; you have to physically use the reference mass.

Three: Fahrenheit. Fahrenheit also used water, yes, but it was salt water (he was interested in international shipping, which goes through oceans, which are full of salt water). Salt water freezes at 0 F and boils at 100 F; nothing to do with degrees in a circle.

Now, bonus question: why is the pressure measurement "1mm of water" not equal to 1/10th of the pressure measurement "1cm of water"?

Re:Water? Kilograms are based on water! (1)

belg4mit (152620) | about 3 years ago | (#37562550)

Salt water does not boil at 100 Fahrenheit at any normal pressure.
BP of fresh/sweet water is 212, salt increases that (Raoult's Law).

Beyond redefinition of the meter, a matter-based unit of mass is
sensitive to the isotope ratio of the matter used e.g; deuterated (heavy) water

Definition of Horsepower (1)

Shompol (1690084) | about 3 years ago | (#37557892)

One Horsepower is the power of a big white dead horse kept at the headquarters of the International Bureau of Weights and Measures in Paris

Re:Definition of Horsepower (1)

White Flame (1074973) | about 3 years ago | (#37559438)

Unfortunately, its value is slowly losing its accuracy from its use in the recalibration of the standard unit for redundant futility.

Is it a constant? (1)

tconnors (91126) | about 3 years ago | (#37561340)

I hope the Plank Constant is not found to vary over the life of the universe, as alpha has been conjectured to.

I am a little surprised that the several spheres of silicon scattered around the world hadn't already redefined kg standard. I saw one of those balls 10 years ago, and understood then that the work was almost complete - the deviations from a perfect sphere were negligible, radius well determined, and purity excellent.

I'm also a little surprised that these versions of the kg standard need exist at all. I thought it was exactly 1L of water. Which is exactly 10cm*10cm*10cm of water. And a metre is exactly the distance light travels in 1299,792,458 of a second (it had formerly been 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the 2p10 and 5d5 quantum levels of the krypton-86 atom).

I guess "water" isn't sufficiently well defined or reproducible.

Re:Is it a constant? (1)

belg4mit (152620) | about 3 years ago | (#37562486)

No, "water" is not, even assuming its "pure." What hydrogen and oxygen isotope ratios?
What temperature and pressure? 1 gm/ml is an approximation, water's density is typically
a little less than one in everyday conditions.

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