# How To Determine a Kilogram

The kilogram is the only fundamental measurement that is determined by reference to a specific object, the International Prototype Kilogram in Paris. As a recent article in Slate describes, there is a movement afoot to redefine the kilo using the Planck constant.

But why bother with the Planck constant? Why not simply use the Avogadro constant and define the kilogram with reference to one mole of some specified element and isotope?

In short: Everyone agrees that it’s bad to rely on an artifact. Everyone agrees that we should use some other, better method. Not everyone agrees on what the best method is.

…and the corresponding article on the approach using Planck’s constant (with some history on the choice included).

FTR, NIST-4 will be our new mass standard.

Thanks for the references to the NIST articles. So it appears that using Avogadro’s number has indeed been considered as the basis for defining the kilogram, and even with the use of the Planck constant it is expected to provide valuable additional data. I’m not sure why the Planck constant is considered more desirable, since it sounds like the two methods have comparable uncertainties.

Sure… yeah probably that would be a good way to ensure that the definition of the mass isn’t subject to unknown and random changes, such as NIST saying
“yeah well we know our reading of gravity has changed,that that means our kg now is different, but hey, they voted for our standard, so now the kg is what it says now !”
This in no way prevents the NIST 4 achieving accuracy of 3 parts in 10^8…
because the avagadro number is 4 .something 10^23… anyone with a fast electron microscope thingy can push carbon 12 atoms around until he has that many, and therefore 12.000000000000000 grams. (there may be a fudge factor required, as the carbon atoms may have bonding energy, and hence extra mass… the idea would be that at some point the definition is far more accurate than the NIST 4… Because even if the bonding energy mass is substantial the variation in it might by tiny… If the bonding energy in a practical collection of carbon atoms could be constrained ? )

I was taught in school that 1 kilo is the weight of one liter (i.e. one cubic decimeter) of water at sea level and 1°C. Isn’t that good enough ?

For the accuracy required - nowhere near.

First of all, it’s (to three decimal places) 6.02*10^23.

Second, and more importantly, that’s only three decimal places. Oh, sure, it’s known to more than that, but is it known to enough more? You don’t just need a new standard; you need for your new standard to agree with the old one to within the precision of measurement of both.

Third, and more importantly yet, just how do you propose to “push carbon 12 atoms around until you have that many”? Go ahead, calculate just how long it would take. Use any assumptions you consider reasonable. We’ll wait (for the calculations, that is, not for the actual pushing).

but it’s 4°C… and that’s just an approximation.

That’s good enough if you bake one loaf bread in a household or even in a commercial kitchen on earth in the normal living area for people.
However, calculating the weight (mass) of a planet, it gets wrong very fast or accelerating that 1 liter to let’s say 12500km/s or getting closer or further away from the sun, the mass and volume remains the same, but the weight is different.

Don’t confuse it with “weight” see “Mass vs weight”

Avocados get all shrively and dry up. What a lousy constant!

The kilogram is mass, not weight, so doesn’t depend on the force of gravity. The pound is a unit of weight. So my mass on the moon is still 90 kg, but my weight would be only 33 lb.

Don’t confuse the issue with using kg and lb… If your earth weight is 90kg, you would weight 14.9kg on the moon. Cite

Yuck yuck…I’ll be here all week.

You know this is quite good. I can’t think offhand of a more cromulent sciencey pun chain.

In July 2009 you chose your username in anticipation of this one post? Respect.

It’s more correct to say the pound is a unit of force. And “weight” is what we call the force when it is due to gravity.

You can’t have a weight of 90 kg. The kilogram is a unit of mass, not weight. If something has a mass of 90 kg on the Earth, it will have a mass of 90 kg on the Moon. Of course, the weight of the object on the Moon will be approximately 1/6th the weight it is on Earth.

It doesn’t have to be that hard. Just push around half as many (3.01*10^23) and multiply your answer by 2. That should be about twice as easy (and fast).

Obviously a “fast electron microscope thingy” can push an atom every 10[sup]-17[/sup] seconds. Duh!