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."

Counting Si

[brokenbeaker]

The problem with the single crystal of silicon method, a few years ago, was that there were all these lattice vacany defects cropping up. The formation of such point vacancies is so entropically favoured that I don't think they can ever eliminate them...

How do they measure it?

[LX.onesizebigger]

My question is, how do they measure it? Using a non-decaying meter stick? How do you measure the definition of a measure?

Best units of measure

[Alien54]

I sort of like the idea of a universal unit of measure.

One nominee that is amusing is to have the basic unit of distance based on the speed of light.

One light nanosecond = roughly 11.1 inches, kinda close to a foot.

I remember how Grace Hooper used to pass out wires that were that long, just to make the point.

Any other nominees?

Re:That's why I like the pound

[krisp]

A pound is a measure of weight. A kilogram is a measure of mass. here [nyu.edu]
is a page describing the difference between weight and mass.

But for those of us who don't like to click:

1) Mass is a measurement of the amount of matter something contains, while Weight is the measurement of the pull of gravity on an object.

I wonder if the new guy is wiping the kg too hard

[Anonymous Coward]

There used to be a custodian of the kilogram whose job it was to wipe the dust and oxidation off the mass with a chamois before it was used. It had to be done just right-too little pressure and the kg would have some stuff left on it and weigh more. He retired a few years back, so i wonder how it's done now. Also, apparently there had to always be two people to handle the kg, one to carry and another to catch it.

Why not use diamond?

[Jeremy Erwin]

Is there any physical reason (other than that small matter of cost ) that crafting a new kilogram (or more likely, gram) out of diamond would not be an ideal solution?

BTW, theNational Physical Institute [npl.co.uk] has a FAQ on its Pl-Ir standard kilo.

Define Plank's Constant?

[Anonymous Coward]

I thought if they would "define" Plank's constant instead of measuring it, the kilogram would fall out of this. This is similar to how they now "define" what the speed of light is instead of measuring it.

it is all silly anyway -- there's a better idea

[Anonymous Coward]

Max Planck came up with an idea (in 1899!) for a system of measurement that really avoids all of these silly restrictions.

His idea: base all measurements of fundamental CONSTANTS like Planck length, c, etc.

The place where I saw this: www.planck.com.

Re:Kilogram?

[Enraged_jawa]

Hah, yeah, true, probably due to Autodesk/Autocad defaults. When I need a metric equavalent, I just measure it with my digital calipers in english, then hit the in/mm button to see the metric equavalent.Metric is much easier to work with, though.

Re:Why not...

[wass]

The answer is another question : How would you define a standard energy?

c is a constant, of course. In fact, it's used to define the meter as how far light travels in a vacuum in 1/(299,792,458) of a second. Second is defined as the time for a certain number of vibrations of a Cesium atom to occur. As per your question of relating mass to Joules, note that high-energy physicists do this all the time. They usually refer to masses of particles as MeV/c^2. And they usually work in units where c=hbar=1, thereby making distance, time, and energy all essentially the same units (easier to do calculatins that way).

One thought that jumps to mind for a standard energy interval is the lyman alpha energy width (the jump of the electron in a hydrogen atom from n=2 to n=1 where n is the energy quantum number). Or, for mass, use a standard mass of a well-defined particle such as an electron. In fact, I'm surprised that NIST doesn't do this. It might be that isolating electrons for mass measurements are too difficult (gravity is weak), but electron mass does show up in many other calculations (specific heat of degenerate electron gases, for instance). Or isolating ultra-pure hydrogen gas and spectroscopically measuring Lyman alpha is more difficult than it seems. I guess NIST wants [relatively] easy methods for measuring these quantities.

Okay, I just found this site [unc.edu] which answers the question. They quote

This one physical standard is still used because scientists can weigh objects very accurately. Weight standards in other countries can be adjusted to the Paris standard kilogram with an accuracy of one part per hundred million. So far, no one has figured out how to define the kilogram in any other way that can be reproduced with better accuracy than this. The 21st General Conference on Weights and Measures, meeting in October 1999, passed a resolution calling on national standards laboratories to press forward with research to "link the fundamental unit of mass to fundamental or atomic constants with a view to a future redefinition of the kilogram." The next General Conference, in 2003, will surely return to this issue.

It all boils down to ability to measure the standard units to the highest precision possible. I'm actually stunned that the mass of that bar can be weighed to that precision.

As a side note, if you can come up with a better way of measuring fundamental constants, you might win a Nobel Prize. The guys that discovered the integer quantum hall effect initially published their results as a better way to measure some of the fundamental constants.

Re:Kilogram?

[jayratch]

That's a very silly point.

He's not saying to replace car speedometers and such things like that. He's saying to replace the real tough stuff, like the 1/4-20 bolt standard that's holding together just about everything.

Reality: Hold on to your English socket set. But with every car, bike, etc, now on metric, its time to start switching. There's no need to replace infrastructure with "metric pipes" because the size of your pipes makes no difference until they need replacement.

Though in construction... it will certainly be tough having to install 2-meter doors in place of the six foot ones, especially when you have a room with doors at both ends. So keep everything that doesn't require daily measurement.

Shouldn't a kilo be measure in energy?

[el-spectre]

I'd think the way to be most accurate (albeit rather unwieldy) is to quantify it as the mass equivalent to XXX units of energy, no?

We have one decent "metric"

[tuxlove]

I agree, it's lame that the US does not use metric, or at least the non-scientific community. Having a science background, I'm perfectly comfortable with both English and metric.

But we do have one thing right, at least. We measure the fuel efficiency of cars by miles/gallon (or kilometers/liter, if you like). I couldn't believe it when I first discovered that some countries use liters/100 km as a measure of efficiency. Talk about a bass-ackwards way of describing a car's efficiency. It's completely counterintuitive. Bigger should always be better, not smaller. What's the point of having a wonderful measurement system like the metric system if you can't even apply it usefully?!

:)

Aaargh.

[Grendel Drago]

I was about to ask the same thing, but you beat me to it.

Avogadro's number [wolfram.com] is a defined constant, so far as I can tell.

And since a molecule of C-12 is defined to be 12 amu, and since 1 mole of x-amu molecules masses x grams... isn't this already settled?

--grendel drago

Re:Kilogram?

[Anonymous Coward]

Somebody tell those non-technical folk to stop making interplanetary satellite probes!

Congress mandates the contractors who build the probes shouldn't be forced to go metric. It drives the engineers crazy!

Re:Kilogram?

[Stuart Gibson]

But in the continued spirit of imperialism, we still don't really use them. Legally, supermarkets and the like label their products metrically, but make sure there are plenty of conversion guides about for everyone to refer to. We still have pounds of butter (just another pound we won't get rid of), talk about people's weight in stones and pounds, height in feet and inches and distances in miles. This isn't just older folk either.

Goblin

Re:I Agree - We should go metric

[MyGirlFriendsBroken]

admittedly road signs are still in miles / mph

Even these are starting to change now. Some of the signage for road works is in meters now a days, some not (in yards) and I've no idea how they decided, its not region dependent or anything. Speeds are always in mph though or else I think things would get very confusing.

On a personal note does anyone know what happened to the bhp (break horse power) and mpg (miles per gallon) figures in car promotional liturature I don't have any point of reference for the new l/km and PS (i think) figures and units?

Pipe-sizes are not that simple...

[Ashtead]

Threaded pipe dimensions in inches were based on the internal diameter at some point, just to make things even more interesting. Thus, a so-called 1/2" pipe actually has an external diameter of approximately 20 mm, which translates to somewhat more than 3/4". All the common Pipe Thread sizes are this way. There has been some attempt at metricising these, at least within Scandinavia, instead of referring to terms like 1/2" or 3/4" pipe threads, terms like R15 and R20 have been seen instead. That seems to be neither here nor there.

Just to confuse the matter more, in the 1970s, it was common to use metric sizes of threaded copper pipe, which had external diameters in sizes approximating common fractions of inches: 13mm = 1/2", 16mm = 5/8" and 19mm = 3/4" just to mention some of them. These appearently were all threaded with 1mm pitch threads.

Later, these were replaced by true metric pipe sizes with compression fittings or capillary solder fittings. Now the sizes changed again, common ones are 8, 10, 12, 15, 18, 22, and 28 mm. And of course, one needed compression fittings made for 16mm and 19mm also, so as to fit the older pipes...

That's Europe. What I have seen in the US are the commonly found so-called 1/2" copper pipes with solder fittings, this is about 16mm (5/8") in diameter, so I guess they are still using internal diameter measurements. Similarly, the so-called 3/4" pipes appear to have about 21mm outside diameter.

I guess the easiest way to turn these into metric sizes would be to redefine them as 16mm and 21mm and leave it at that. At least the traditional inch-units pipe thread sizes are roughly the same everywhere!

Re:Mass not weight guy

[Anonymous Coward]

so why do they say somebody weighs 80 kg, for example? Should weight really be kg.m/s^2? Like 784 kg.m/s^2?

-- not trying to be an ass, just curious.

That's kind of cool...

[interactive_civilian]

Since the speed of light is 299,792,458 meters per second (taken from here [utk.edu]), I am roughly 5.67 light nanoseconds tall. Interesting in a useless fact kind of way...

Re:if _kilo_gram is base

[amorsen]

At first the gram was the base unit. However, try deriving the other units from that, and you will see that units like Volt come out with inconvenient sizes for everyday measurements. So they changed to the kilogram without inventing a new name for it. This is quite unfortunate for several reasons, including the fact that everyone abbreviates it to just kilo. Also, what do you call 1000 kg? A kilokilogram? A megagram? No: A (metric) ton.

Incidentally, there will always be some units that end up with inconvenient sizes. Try going to your local electronics store and asking for a 1F capacitor.

Re:Wrong subject... yet...

[Citizen of Earth]

The Japanese had to surrender theirs after World War II.

I'm just curious about why. They couldn't be trusted with the technology of measuring mass?

Re:if _kilo_gram is base

[eht]

My local electronics store has 1F capacitors, huge things they are and on the expensive side, and quite a number of car audio places have them so you can make your car sound more annoying to other people.

Acrylic Sounds [acrylicsounds.com] even has 10 Farad caps for sale.

Re:Kilogram?

[sphealey]

Then maybe america should move out of the dark ages sometime.

Funny how when the topic is software or food supplies, everyone jumps in with comments about the dangers of monoculture and the value of diversity in supply, but when the topic is the metric system there can be no deviation from the ONE TRUE FAITH.

Personally, having gone through school at a time when the US was considering a change, and having spent some time in Europe, I have no problem with the metric system. It is more convenient from some tasks, particularly in the chem lab.

But there is nothing inherently superior about a measurement system based on powers of 10. For many tasks, such as woodworking, metric measurements are far more difficult to work with than inches and 1/16th. In fact I would argue that the most "natural" base for a measurement system is 12 as it is evenly divisible by 2, 3, and 4; whereras base 10 is only divisible by 2 and 5. Thirds and fourths are very common divisions of stuff; fifths are not, so a base 12 system is more user-friendly.

That's my 0.02 euro anyway.

sPh

oh oh! a funny too!

[ShinmaWa]

Replying to myself --

There's one sign on I-19 that I find absolutely hilarious though. It says something along the lines of:

Ajo Rd - 1000 m
Irvington Rd - 3000 m
Valencia Rd - 5000 m

The theory - Either they
A) ran out of 'k'.
B) had a whole bunch of '0's to get rid of.
C) don't quite get the concept.

Re:Kilogram?

[radish]

I do get it. This already happens in the UK, it's not a problem at all. We have lots of houses which are older than the metric system (and the USA for that matter). They use imperial stuff. We have lots of new houses - they use metric. And yet I can still call a plumber and he can figure out how to fix my pipe, and my electrician is able to fix a light. Amazing.

If there was any will to do it you'd do it, which indicates there's no will. Which is fine, I don't give a toss what you measure your wooden houses in, but don't come over all "it's too haaaaaard" - you sound like a whinging kid.

Measurement conversion

[alchemist68]

Reading through the responses to this post brings back fond memories of Dr. Wade's DiffEQ class back in 1996. I recall a homework assignment that required solving a differential equation, then plug-n-chug to get the numerical answer. I was the only non-math major in the class and the only student who had the correct answer. The reason I had the correct answer was that I was a chemistry major familiar with systems of measurement, and the problem specifically stated to find the MASS of the object in the English system. Everyone thinks the measurement of mass in the English system is the POUND, which is completely incorrect, the POUND is a FORCE unit. The mass unit of measure in the English system is the SLUG.

Defeating the purpose

[Captain_Stupendous]

The watt balance solution seems to be linking the Kilogram (mass) with force (weight). This is not entirely desirable, since something that masses a kilo on earth will still mass a kilo in space, or on the moon, or on jupiter. It's mass doesn't change, only it's weight. The Watt balance then, would not only be impractical (imagine having to construct a "3-story structure" every time you want to accurately weight something?), but downright useless for many aerospace applications. Any system of measurement that's dependant on the phase of the moon for it's accuracy should immediately be discounted, in my opinion...