# How To Determine a Kilogram **URL:** \ **Category:** Factual Questions\ **Created:** [June 30, 2016, 9:17pm UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828 "2016-06-30T21:17:59Z")\ **Posts on this page:** 20\ **Page:** 5
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [November 4, 2018, 1:21am UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/81 "2018-11-04T01:21:25Z")
> [@rat\_avatar](#): > > The round sphere isn’t being used for the new KG, this was the International Avogadro Project and was rejected this idea and went with the Planck constant. The mole is what is being defined with those spheres. I am sure they had good reasons for their decision but I gotta say the Kibble Balance (aka Watt Balance) method to measure Planck’s constant seems hugely complex. Just watching that video there is so much going on (e.g. measuring the length of wire in the contraption, measuring the gravity differentials every few feet in the chamber where the experiment is done, etc.). With so many bits and pieces it seems it would be impossible for different labs to ever get the same result. Sure they would be close but could they really replicate it all perfectly? The sphere approach seems simpler. A sphere made of silicon of exactly a given diameter is a kilogram. Another attempt won’t get exactly the same diameter but they’d get pretty close and be able to calculate the number of atoms difference pretty easily to get the right answer. I guess the thing here is that one experiment literally becomes THE definition of the kilogram and anyone else who replicates it will not be the kilogram and will have to note their offset from that one experiment. ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [November 4, 2018, 1:39am UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/82 "2018-11-04T01:39:51Z")
> [@Whack-a-Mole](#): > > I am sure they had good reasons for their decision but I gotta say the Kibble Balance (aka Watt Balance) method to measure Planck’s constant seems hugely complex. > > Just watching that video there is so much going on (e.g. measuring the length of wire in the contraption, measuring the gravity differentials every few feet in the chamber where the experiment is done, etc.). With so many bits and pieces it seems it would be impossible for different labs to ever get the same result. Sure they would be close but could they really replicate it all perfectly? > > The sphere approach seems simpler. A sphere made of silicon of exactly a given diameter is a kilogram. Another attempt won’t get exactly the same diameter but they’d get pretty close and be able to calculate the number of atoms difference pretty easily to get the right answer. > > I guess the thing here is that one experiment literally becomes THE definition of the kilogram and anyone else who replicates it will not be the kilogram and will have to note their offset from that one experiment. If you look past just the sphere, sure but the optical interferometers, machining and crystal growth complexities are hidden. But the real issue is that solution failed to compete in accuracy and reliability with the Kibble balance. They did use the best results from both, but the fractional uncertainties from the Kibble balance were much better. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 4, 2018, 1:51am UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/83 "2018-11-04T01:51:26Z")
F = ma is the same equation as m = F/a, and no matter how you write that equation, it makes no sense to call a unit of force by the same name as a unit of mass. And a mole is most certainly absolutely not “the amount of matter in an object”. A mole is a count, like a dozen. A count of what? It could be anything. You could have a mole of atoms, or a mole of ions, or a mole of molecules, or a mole of small burrowing mammals. Well, OK, you couldn’t have that last one, since it would be a sizable fraction of the mass of the Earth, but in principle, you could refer to a mole of anything. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [November 4, 2018, 2:09am UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/84 "2018-11-04T02:09:32Z")
> [@Chronos](#): > > F = ma is the same equation as m = F/a, and no matter how you write that equation, it makes no sense to call a unit of force by the same name as a unit of mass. > > And a mole is most certainly absolutely not “the amount of matter in an object”. A mole is a count, like a dozen. A count of what? It could be anything. You could have a mole of atoms, or a mole of ions, or a mole of molecules, or a mole of small burrowing mammals. Well, OK, you couldn’t have that last one, since it would be a sizable fraction of the mass of the Earth, but in principle, you could refer to a mole of anything. As you probably know, Randall Munroe talked about a mole of moles [A Mole of Moles](https://what-if.xkcd.com/4/) ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [November 4, 2018, 2:12am UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/85 "2018-11-04T02:12:10Z")
> [@rat\_avatar](#): > > While it is frustrating really it is due to different needs wanting the simplest formulas for their uses more than anything else. Note: I was mostly just punning on “slug” and “pound” of course (kind of surprised that no one had done it before me). ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [November 4, 2018, 2:37am UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/86 "2018-11-04T02:37:11Z")
> [@Chronos](#): > > F = ma is the same equation as m = F/a, and no matter how you write that equation, it makes no sense to call a unit of force by the same name as a unit of mass. > > And a mole is most certainly absolutely not “the amount of matter in an object”. A mole is a count, like a dozen. A count of what? It could be anything. You could have a mole of atoms, or a mole of ions, or a mole of molecules, or a mole of small burrowing mammals. Well, OK, you couldn’t have that last one, since it would be a sizable fraction of the mass of the Earth, but in principle, you could refer to a mole of anything. The mole is the SI base unit for the amount of a substance, and yes it is a bridge between the atom and the macroscopic quantities we use. In the case of SI _mass_ is a measure of a bodies inertial property or the property of matter that measures its resistance to acceleration. We could get into the whole complexity about invariant mass, rest mass, intrinsic mass, proper mass, and relativistic mass but there is no need to. If you are going to argue that the [Unit of amount of substance (mole)](https://www.bipm.org/en/publications/si-brochure/mole.html) is not a mole under SI feel free to take it up with the BIPM. For common use, when you care about mass more than forces it makes sense to simplify. If I need to go on a diet or buy some coffee why complicate with densities etc…when the mass works out within my needs. I don’t care the force I am exerting on the ground when I am on a diet…nor do I care about how many atoms or particles I gain or loose. The needs just don’t require the accuracy or precision in this context, and I never plan to travel to another planet. But if you are arguing that the SI Kilogram is not a measure of mass, which in this context is a measure of resistance to acceleration, please provide a cite. In this context _mass_ is a quantity that only depends on inertia. While it is a useful approximation of the amount of “stuff” it is not a measure of the amount of _stuff_. But to answer your question, I would take the sample mass divided by the molar mass of the substance. Water is ≈ 18.015 g/mol as an example for molar mass. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 4, 2018, 1:22pm UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/87 "2018-11-04T13:22:14Z")
How do you define “stuff”, such that the amount of it is not mass? And a mole isn’t a measure of substance, or of anything else. You can’t say “What’s the mole of that box?”. You can’t even ask “How many moles are in that box?”. You could ask something like “How many moles of molecules are in that box?”. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [November 4, 2018, 1:56pm UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/88 "2018-11-04T13:56:04Z")
The entire point of the mole is that it is a number. Chemical compounds are ratios of the count of constituent element atoms, and in order to provide a bridge between the mass of reagents and products, and to provide a quantitative approach to stoichiometric chemistry a number was needed of a suitable size. The BIPM definition cited above uses the word “amount” to mean something very specific… > [@](#): > > Amount of substance is defined to be proportional to the number of specified elementary entities in a sample, the proportionality constant being a universal constant which is the same for all samples. The unit of amount of substance is called the mole, The mole isn’t a unit of measure of stuff, it is a count of constituent elementary components of stuff. If your stuff isn’t made up of elementary entities that can be counted, you can’t define its constituents in moles. If you describe a lump of stuff in moles, you must know what the elementary components it is made of are. Otherwise you can’t define the measure. Stuff that doesn’t have an easy discrete definition of structure gets hard. You can’t say you have a lump of a mole of steel. A lump of pure iron yes. ---
**Author:** ![peccavi](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peccavi/32/388_2.png) [@peccavi](https://boards.straightdope.com/u/peccavi)\ **Post date:** [November 4, 2018, 5:23pm UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/89 "2018-11-04T17:23:34Z")
> [@Crafter\_Man](#): > > To expand a bit on what **nate** said… > > Your measurement instrument must be _traceable_ to NIST. This means there must be an unbroken chain of calibrations between your measurement instrument and NIST. > > [snip]… > > So… my voltmeter is calibrated by PMEL. PMEL’s voltage standards are calibrated by AFMETCAL. AFMETCAL’s voltage standards are calibrated by NIST. Hence the unbroken chain of traceability to NIST. If I cannot prove to an auditor that my voltmeter is traceable to NIST through an unbroken chain of calibrations, then I will get in trouble and an investigation will occur. This is a good description of how standards are used and maintained in a working lab, but not the best example. For a couple hundred thousand dollars (not a big number if you’re manufacturing systems worth hundred’s of millions of dollars) you can own a [primary voltage standard](https://www.hypres.com/hypres-primary-voltage-standard/). It will be as accurate as any other primary voltage standard and if your lab participates in the periodic comparisons of national standards, it will actually be a traceable source standard. The voltage standard is not only an example of a standard that depends only on physical constants (e, h) and universally measurable quantities (f), it is also an example of reducing the lab setup to an off-the-shelf turnkey product. I had to pick the nit on this one, even though its a great description otherwise… ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [November 4, 2018, 6:20pm UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/90 "2018-11-04T18:20:02Z")
> [@peccavi](#): > > This is a good description of how standards are used and maintained in a working lab, but not the best example. > > For a couple hundred thousand dollars (not a big number if you’re manufacturing systems worth hundred’s of millions of dollars) you can own a [primary voltage standard](https://www.hypres.com/hypres-primary-voltage-standard/). It will be as accurate as any other primary voltage standard and if your lab participates in the periodic comparisons of national standards, it will actually be a traceable source standard. > > The voltage standard is not only an example of a standard that depends only on physical constants (e, h) and universally measurable quantities (f), it is also an example of reducing the lab setup to an off-the-shelf turnkey product. > > I had to pick the nit on this one, even though its a great description otherwise… That’s a good example of an intrinsic standard, but not the best IMO. 😉 Because [for $1045 you can own a standard that realizes a defined temperature of 0.01 °C](https://www.isotechna.com/Water-Triple-Point-Cell-p/b8-fslash-30-fslash-130.htm). ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [November 4, 2018, 6:35pm UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/91 "2018-11-04T18:35:15Z")
> [@Chronos](#): > > How do you define “stuff”, such that the amount of it is not mass? > > And a mole isn’t a measure of substance, or of anything else. You can’t say “What’s the mole of that box?”. You can’t even ask “How many moles are in that box?”. You could ask something like “How many moles of molecules are in that box?”. This is just a side discussion anyway, but not all things that have _mass_ are _massive_ and so _matter_ simply doesn’t count here. I didn’t choose the SI base quantities, nor did I choose their definitions. The problem with visualizing a mole of something because Avogadro’s constant is extremely large, not because it isn’t the unit of quantity which my cite provided it is under SI. Moles of substance (mol) \* Avogadro’s constant (atoms/mol) = Atoms (or molecules) To solve the same problem knowing the mass requires knowing the Molar mass, or similar to the following. g \* (1/(mol/g) \* (atoms/mol)) = Atoms (or molecules) While the SI system fails in both quantum and relativistic domains, explain to me how a hypothetical weightless box of mirrors full of photons will have “mass” or other situations when the term “matter” doesn’t quite work when trying to use kilograms as a measure of “stuff” The kilogram and mass in SI is purely dependent on inertia, and this is true despite issues with usability of the chosen base unit for for _quantity_. > [@](#): > > The Seven SI base units, which are comprised of: > [ul] > [li]m - Meter - Length.[/li][li]s - Second - Time.[/li][li]mol - Mole - Amount of Substance.[/li][li]A - Ampere - Electric Current.[/li][li]K - Kelvin - Temperature.[/li][li]cd - Candela - Luminous Intensity.[/li][li]kg - Kilogram - Mass.[/li][/ul] Irrespective of your concerns about practical visualization of a _mol_ I cannot defend or justify the SI design choices, but if this isn’t true you should be able to provide a cite. “Mass is the amount of matter in an object” simply doesn’t work post John Dalton’s atomic model. Mass being a quantity of matter and an inertial property works in Newton’s Philosophiæ Naturalis Principia Mathematica because he didn’t know about atoms etc… If you are making the claim that SI uses the density and volume form from Newton: > [@](#): > > The quantity of any matter is the measure of it by its density and volume conjointly > . . . This quantity is what I shall understand by the term mass or body in the discussions to follow. It is ascertainable from the weight of the body in question. > For I have found, by pendulum-experiments of high precision, that the mass of a body is proportional to its weight ; as will hereafter be shown. It should be pretty easy to show how the Kilogram uses density and volume, but BIPM and modern classical mechanics tends to define it according to Newton’s second law, where a body has a mass m if, at any instant of time, it obeys the equation of motion (F =ma). You are trying to re-debate operationalism, Mach did enough of that at the time to impact how SI was set up. To avoid recreating Mach’s arguments, which seem to have gained favor by the BIPM, here’s a link to Mach’s book and page. > **[The science of mechanics; a critical and historical exposition of its...](https://archive.org/details/sciencemechanic02machgoog/page/n261)** > > Book digitized by Google from the library of the University of Michigan and uploaded to the Internet Archive by user tpb. Please do point me to a source that shows that the BIPM uses the primitive form of mass as a “quantity of matter” as I obviously don’t know the Metric system very well if I need to consider anything else except inertia for the kilogram. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [November 4, 2018, 7:49pm UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/92 "2018-11-04T19:49:13Z")
Am I the only one who finds it odd _ampere_ is a fundamental unit, but _coulomb_ is not? 😕 I mean, ampere is coulombs per second, right? The second is a fundamental unit. One would think the coulomb would also be a fundamental unit, and therefore ampere would simply be a combined or derived unit (C/s). ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [November 4, 2018, 8:09pm UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/93 "2018-11-04T20:09:25Z")
> [@Crafter\_Man](#): > > Am I the only one who finds it odd _ampere_ is a fundamental unit, but _coulomb_ is not? 😕 > > I mean, ampere is coulombs per second, right? The second is a fundamental unit. One would think the coulomb would also be a fundamental unit, and therefore ampere would simply be a combined or derived unit (C/s). Other way around, 1C = 1A \* 1s But really it is probably because they could figure out how to derive the ampere and because moving from the old CGS system to the MKS system that SI adopted in 1960 was due to problems with Electric and Magnetic Units. If you want a real detailed history look here. > **[1506.01951.pdf](https://arxiv.org/ftp/arxiv/papers/1506/1506.01951.pdf)** > > 702.58 KB ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 4, 2018, 10:01pm UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/94 "2018-11-04T22:01:01Z")
**rat avatar** , what you’re talking about has nothing to do with relativity or quantum mechanics, and dragging them in just confuses matters for no good purpose. And no issues arise from Avogadro’s number being large; in fact, it would be useless if it were not large. But even if we consider a smaller unit of the same sort, the problem with your argument still become clear: One would never, for instance, ask “What is the dozen of the eggs in my refrigerator?”, which is how you’re claiming moles would be used. And what on Earth could you possibly mean when you say that “not all things that have mass are massive”? That’s literally the exact definition of “massive”. ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [November 4, 2018, 10:51pm UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/95 "2018-11-04T22:51:15Z")
> [@Chronos](#): > > **rat avatar** , what you’re talking about has nothing to do with relativity or quantum mechanics, and dragging them in just confuses matters for no good purpose. And no issues arise from Avogadro’s number being large; in fact, it would be useless if it were not large. But even if we consider a smaller unit of the same sort, the problem with your argument still become clear: One would never, for instance, ask “What is the dozen of the eggs in my refrigerator?”, which is how you’re claiming moles would be used. > > And what on Earth could you possibly mean when you say that “not all things that have mass are massive”? That’s literally the exact definition of “massive”. I EXCLUDED relativity or quantum mechanics to avoid that. But as the newton and the MKS system was added _because_ of [the needs of electromagnetic energy](https://www1.bipm.org/en/CGPM/db/9/6/) it cannot be ignored in this case. While it is a common claim, I will note you still haven’t shown any cites that “mass” in the context of SI was _intended_ to be a unit of the amount of substance. The SI unit of force is the newton, but that is a _technical_ unit of measure introduced in 1960 which replaced the dyn from CGS and the kilopond from the Metric system. It is absurd to claim that the committee restricted their ideas to 1687 and Newton’s Principia. The base units of candela and mole were added on purpose and the form of “mass” they use is related to F=ma. Resolution of the 3rd CGPM (1901) > [@](#): > > Declaration on the unit of mass and on the definition of weight; conventional value of gn\* > > Taking into account the decision of the Comité International des Poids et Mesures of 15 October 1887, according to which the kilogram has been defined as unit of mass; > > Taking into account the decision contained in the sanction of the prototypes of the Metric System, unanimously accepted by the Conférence Générale des Poids et Mesures on 26 September 1889; > > Considering the necessity to put an end to the ambiguity which in current practice still exists on the meaning of the word weight, used sometimes for mass, sometimes for mechanical force; > > The Conference declares > > [ul] > [li]The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram;[/li][li]The word “weight” denotes a quantity of the same nature as a “force”: the weight of a body is the product of its mass and the acceleration due to gravity; in particular, the standard weight of a body is the product of its mass and the standard acceleration due to gravity;[/li][li]The value adopted in the International Service of Weights and Measures for the standard acceleration due to gravity is 980.665 cm/s2, value already stated in the laws of some countries.[/li][/ul] Matter being any substance that has mass and takes up space by having volume does not include electromagnetic radiation in non-quantum/relativistic contexts. But as you keep arguing for 1687 I did error to saying that “massive” was related to “matter” and thus light etc… wouldn’t take up space. But as I have provided cites and you have not provided a single one, but lets see about a coherent system of measure. Explain to me how you account for changes in chemical energy changing inertia with your model of mass being a sum of all parts and a measure of quantity. Then show me where density is defined in Newton Principia. I get that you find this convenient, and it may be acceptably precise for your needs but it is not precise enough for a coherent system of measurement. Density is the mass per unit of volume and the SI derived unit for volume is cubic meters, explain how you can match the Newtonian “amount of stuff” definition of “The quantity of any matter is the measure of it by its density and volume conjointly” while still having mass be a base unit? There is a good reason that they got rid of the volume of water version of the kilogram from the Metric system and created the IPK in the first place. As this is GQ I am not going to abandon the evidence that says that kilogram is only tied to the inertial properties without citations. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 5, 2018, 12:56am UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/96 "2018-11-05T00:56:56Z")
OK, I can’t keep up with all of this, but where did I ever say that mass was “a sum of all parts and a measure of quantity”? A mole is a quantity. Mass is an amount. You’re the one who is saying otherwise. And who’s talking about density? ---
**Author:** ![PastTense](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pasttense/32/14550_2.png) [@PastTense](https://boards.straightdope.com/u/PastTense)\ **Post date:** [November 18, 2018, 4:48pm UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/97 "2018-11-18T16:48:10Z")
> [@](#): > > In the biggest overhaul of the international system of units since 1875, countries have voted to redefine four basic units of measurement — the ampere, the kilogram, the kelvin and the mole. > > At a meeting on 16 November, 60 delegates from governments around the world voted unanimously to change how these units are defined — which will come into force on 20 May 2019. [https://www.nature.com/articles/d41586-018-07424-8?utm\_source=twt\_nnc&utm\_medium=social&utm\_campaign=naturenews&sf202502753=1](https://www.nature.com/articles/d41586-018-07424-8?utm_source=twt_nnc&utm_medium=social&utm_campaign=naturenews&sf202502753=1) ---
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [November 28, 2018, 3:35am UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/98 "2018-11-28T03:35:58Z")
I suppose _Le Grande K_ is made out of Platinum-iridium and kept under 3 bell jars-worth of inert gas to prevent oxidation…adsorbing atmospheric oxygen would cause a microscopic addition of mass. ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [November 28, 2018, 4:11am UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/99 "2018-11-28T04:11:00Z")
Even with all the bell-jars hydrocarbons were still building up and adding to the most of the IPK’s mass. Less than 100µg but it still is happening. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [November 29, 2018, 3:07am UTC](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828/100 "2018-11-29T03:07:51Z")
> [@Doughbag](#): > > 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](http://www.exploratorium.edu/ronh/weight/) You didn’t read that page, did you? You are conflating the use of SI vs US customary with the difference between mass and weight. > [@](#): > > **Mass and Weight** > Before we get into the subject of gravity and how it acts, it’s important to understand the difference between weight and mass. > We often use the terms “mass” and “weight” interchangeably in our daily speech, but to an astronomer or a physicist they are completely different things. The mass of a body is a measure of how much matter it contains. [snip] > > Weight is an entirely different thing. Every object in the universe with mass attracts every other object with mass. The amount of attraction depends on the size of the masses and how far apart they are. [snip] > > If you are in a spaceship far between the stars and you put a scale underneath you, the scale would read zero. Your weight is zero. You are weightless. There is an anvil floating next to you. It’s also weightless. Are you or the anvil mass-less? Absolutely not. If you grabbed the anvil and tried to shake it, you would have to push it to get it going and pull it to get it to stop. It still has inertia, and hence mass, yet it has no weight. See the difference? > [@TipTapTwo](#): > > Side question, why did people decide to define kilogram super accurately, and then say a gram is 1/1000 of that? Why did we not do it the other way around with a super accurate gram and say a kilogram is 1000 of those? This [NIST](https://www.nist.gov/si-redefinition/kilogram-past) page talks about it. Essentially, they started trying to define the gram in terms of the cm, because they thought that size would be convenient, but decided it wasn’t convenient after all. So they reverted to defining the kg. > [@rat\_avatar](#): > > While it is frustrating really it is due to different needs wanting the simplest formulas for their uses more than anything else. > > Really the bias can be illustrated by the form of Newtons 2nd law of motion. > > If you prefer F=ma you will have kilograms and newtons/pounds and slugs but some applications prefer m = F/a which ends up with kilograms and kilograms-force (kilopond)/pounds and pounds-force. I came across this in college once, but SI doesn’t use the kilopond or kilogram-force. The SI unit of force _is_ the Newton. > [@](#): > > The kilopond was used in Germany until the late 1970’s. In the US the FPS system is still in use by Engineers and has the pound-force base unit that actually causes the problems with the slug. While not arguing the benefits or values, the official NIST pound has been a unit of mass forever and has been defined off the kilogram for just about as long as there was a kilogram. [snip] > > It would be really nice if Engineers would quite using the slug base unit as while that wouldn’t be as convenient as switching to SI for conversion at least everything would be defined off the same base units and coherent system. The root of this problem is the history of how the terms originated. The pound was originally a measure of force and mass - they were considered the same thing. With Newton came the understanding of the difference, and the need to separate the units because the relationship is offset by the acceleration of gravity (standard 32.2 ft/sec[sup]2[/sup]). Slugs aren’t the problem, the problem is the pound mass. Whenever you use that, you have to use a conversion factor, 32.2 ft-lbm/lbf-sec[sup]2[/sup]. > [@rat\_avatar](#): > > To clarify, Mass is actually not the amount of matter contained by a body a Mole is. A mole is a count of matter, which is a form of amount. Mass is a different measure of amount - the resistance to acceleration. > [@](#): > > If you are trying to check floor loading or how much rocket propellant you need, the _force_ or newtons is a better unit of measure. If you are trying to determine the amount of rocket fuel, you definitely need mass. You have to account for the mass of the fuel that is being used, and the momentum transfer. > [@Whack-a-Mole](#): > > I guess the thing here is that one experiment literally becomes THE definition of the kilogram and anyone else who replicates it will not be the kilogram and will have to note their offset from that one experiment. No. The whole point is to move away from having to measure a specific item each time you want to compare. The standard is being based on unchanging fundamental constants. The experiments are being used to refine the most precise values for those constants given the existing base standards. It is a cumulative result from numerous experiments. [Previous page](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828.md?page=4) [Next page](https://boards.straightdope.com/t/how-to-determine-a-kilogram/758828.md?page=6)