back to article Holy moley! The amp, kelvin and kilogram will never be the same again

As incredible as it may seem, until this week the definitive measurement of a kilogram was a cylinder made of an alloy comprising 90 per cent platinum and 10 per cent iridium sat under a glass dome in a room in Paris. The cylinder is one of six official exact masses of one kilogram, and it has been that way since 1889: The …

  1. #define INFINITY -1

    Re: Sad case of science ignoring the evidence

    It seems more that the journalism has been tailored for those less inquisitive and more willing to believe 'new tech'. I'm sure there is an explanation--I would guess when it was removed from the vacuum in order to compare to the other standards.

    Are there any physicists around who can argue that gold would not have been a better choice?

  2. A.P. Veening

    Gold no better choice

    The weight loss (small but measurable) is a result from the cleaning, necessary because stuff from the air will cling to it. As gold is much softer than the used alloy of platinum and iridium, more weight would have been lost.

  3. Lars Silver badge
    Happy

    Re: Sad case of science ignoring the evidence

    "Are there any physicists around who can argue that gold would not have been a better choice?".

    I don't know, but I would guess that the economists and others would had been against it much because 40 prototypes were produced to start with.

  4. ibmalone Silver badge

    Re: Sad case of science ignoring the evidence

    The solution is to move to better ways of measuring mass, which is what they've done.

  5. doublelayer

    Re: Sad case of science ignoring the evidence

    They redefined the kilogram because we already know what the kilogram is. We know this with things that can actually be calculated easily in a science lab, like a specific quantity of a substance with a known density, measured under a known pressure. That isn't infinitely measurable, so it can't be used by the SI people, but they didn't need to investigate why the kilogram chunks had different masses because they knew why and they didn't need to find the one true kilogram because they knew what a kilogram is. They just needed a math problem to give the mass of a kilogram so everyone else in labs can keep measuring mass exactly the same way.

  6. #define INFINITY -1

    @Lars Re: Sad case of science ignoring the evidence

    Except I have a suspicion that a Kibble balance costs more than 40kg of gold....

    Is the relative cost comparable to the relative difference in accuracy?

  7. A.P. Veening

    Re: Economists

    I suggest you look up some economy books, platinum is normally more expensive than gold. It is just a matter of a couple of physical properties.

  8. #define INFINITY -1

    @A.P. Veening Re: Economists

    In 1889?

  9. Voyna i Mor Silver badge

    Re: @A.P. Veening Economists - In 1889?

    It's interesting. In 1889 the gold price per troy ounce was about $20. Platinum was cheap - about $4. I don't have a price for iridium, but in 1911 it was $62, at which time platinum was $43 and gold was still about $20 (because of course the dollar was linked to it).

    The standard kilogram is only about 10% iridium so the alloy looks to have been much cheaper than gold, but by WW1 the standard kilogram material was much more expensive than gold.

    If I was a billionaire, I think I'd have a boat with the heat exchangers made out of platinum iridium alloy, because I could. No corrosion worries.

  10. Lars Silver badge
    Happy

    Re: @A.P. Veening Economists - In 1889?

    Why not the keel too, it's heavier than led. There was one well known and quite successful French yachtsman who had a keel of uranium in his offshore racing yacht for that reason.

  11. ridley

    Re: Gold no better choice

    or from oxidation etc making it heavier. Hence the need to clean it.

  12. agurney

    Re: @A.P. Veening Economists - In 1889?

    Why not the keel too, it's heavier than led. There was one well known and quite successful French yachtsman who had a keel of uranium in his offshore racing yacht for that reason.

    I trust you mean denser. A kilo of lead is far heavier than a gram of uranium.

  13. Voyna i Mor Silver badge

    Re: @A.P. Veening Economists - In 1889?

    " There was one well known and quite successful French yachtsman who had a keel of uranium in his offshore racing yacht for that reason."

    Wait long enough, of course, and it would turn to lead.

    But the ideal keel material would be osmium. You'd need 10 years of world supply to make a decent keel, but you'd be able to work up quite a clip around your volcanic island lair with the wind blowing the right way.

  14. Charles 9 Silver badge

    Re: Gold no better choice

    "The weight loss (small but measurable) is a result from the cleaning, necessary because stuff from the air will cling to it."

    Wasn't that taken into consideration by placing the prototypes under double bell jars?

  15. Charles 9 Silver badge

    Re: @A.P. Veening Economists - In 1889?

    "But the ideal keel material would be osmium."

    In terms of density, yes, but last I checked osmium's also very brittle, meaning a hard knock can break it: NOT a good thing when it comes to a keel.

  16. TRT Silver badge

    Re: Ship's keel.

    Unobtanium?

  17. Michael Habel Silver badge
    Mushroom

    Re: Ship's keel.

    I propose we build the Ships Keel out of Sodium.

  18. jmch Silver badge

    Re: Sad case of science ignoring the evidence

    "somehow the various copies of the kilogram made over the years, with the best equipment of the time, ended up being different to each other"

    I'm somewhat confused by the graph showing "Le Grand K" mass changing over time, together with the mass change of all the copy Ks. After all, 1kg was BY DEFINITION the mass of "Le Grand K", so if the other copy masses changed relative to it, I would have to say that teh other cylinders were gaining or losing mass.

    Of course I understand that it's possible that a physical object can have very slight mass gains or losses... but if I measure K at 1kg, then 10 years later I find it weighs 9.9999kg or 10.00001kg, how can I make the claim that "Le Grand K" has lost or gained mass rather than that my balance is no longer well calibrated / my copy cylinder has gained/lost mass?

  19. defiler Silver badge

    Re: Ship's keel.

    I propose we build the Ships Keel out of Sodium.

    Seconded.

  20. Loyal Commenter Silver badge

    Re: Sad case of science ignoring the evidence

    I don't know, but I would guess that the economists and others would had been against it much because 40 prototypes were produced to start with.

    Except both platinum and iridium are more expensive than gold.

  21. tfb Silver badge

    Re: Gold no better choice

    I think you have to take them out of the bell jars to use them.

  22. DropBear Silver badge
    Trollface

    Re: Ship's keel.

    Oh, a lunacy competition...? Woohoo, I'm in! How about... sawdust and ice?!?

  23. Loyal Commenter Silver badge
    Coat

    Re: @A.P. Veening Economists - In 1889?

    it's heavier than led

    That's because LEDs are light...

  24. Alan Brown Silver badge

    Re: Sad case of science ignoring the evidence

    "At one time this would have been enough to warrent an investigation"

    It did. Several. And then several more. Some variances were explainable (fingerprint contamination, etc) and others just seem to be random noise of the universe

    The things may have been under jars, but it's not an inert space and they were periodically cleaned. The opportunity to pick up or lose a few molecules of contamination arose, but even when the greatest of care was taken to ensure no contamination took place, successive measurements on the same balances would give different results on different days.

    Even le grand K has varied a few nanograms between measurements - sometimes when measurements were taken within weeks of each other. This variance and the continued variances between the lesser Ks is what drove the search for a better reference (which started about 40 years ago)

  25. cray74 Silver badge

    Re: @A.P. Veening Economists - In 1889?

    If I was a billionaire, I think I'd have a boat with the heat exchangers made out of platinum iridium alloy, because I could. No corrosion worries.

    If the rest of the boat is made of non-platinum group metals then you'd need to be careful of galvanic (dissimilar metal) corrosion. Platinum's at the far end of the galvanic series so it's a threat to most other metals. It's relatively easy to address if you can separate the metals with non-conductive barriers (e.g., paint), but I wouldn't say "no worries."

  26. cray74 Silver badge

    Re: @A.P. Veening Economists - In 1889?

    But the ideal keel material would be osmium.

    Osmium oxidizes too easily and its common tetroxide is poisonous. It also only offers a slight density gain over platinum, which is more common and better behaved chemically, and only a modest gain over tungsten and uranium.

    Another approach to enhance sailboat performance is switch to a multi-hull so you're not burdened with a heavy keel.

  27. cray74 Silver badge

    Re: Sad case of science ignoring the evidence

    Except both platinum and iridium are more expensive than gold.

    Now, yes. However, platinum used to be considerably cheaper than gold.

  28. Ken Moorhouse Silver badge

    Re: the search for a better reference (which started about 40 years ago)

    Now that's what I call a long weight.

  29. John Smith 19 Gold badge
    Thumb Up

    Handy if you want to send a description of some technology to another civilisation.

    The thing is once you need maximum precision (maybe because you want to build something very big, or very small) it turns out that a lot of physical "constants" (like the length of an Earth day for example) actually aren't.

    Very impressive.

  30. #define INFINITY -1

    Re: Handy if you want to send a description of some technology to another civilisation.

    Wouldn't the other civilization be inclined to assign the correlation to coincidence?

  31. Charles 9 Silver badge

    I have a thought, though. How can one define the mole by Avogadro's Constant when the constant is defined in terms of moles? Sounds circular if you ask me.

  32. Justin Case

    There's only one way to get rid of a mole

    Blow his bloody head off!

  33. bombastic bob Silver badge
    Boffin

    not sure why it says '1023' either - I thought it was 6.02nnn x 10^23

  34. Aleph0

    A mole of X

    As I understand it, the definition of "mole" as a measuring unit is simply a fancy way of saying "In a mole of X there are 6.02#### * 10^23 units of X".

    As always there's a relevant XKCD.

  35. ratfox Silver badge

    Re: A mole of X

    I didn't get that part. I thought that the Avogadro number was already the definition of a mole. What was it before?

  36. Yet Another Anonymous coward Silver badge

    Re: A mole of X

    It's the number of atoms of C12 in 0.012kg of C12, change kg and you change the number.

    Personally I'm a bit sad that the way cooler, count the number of Si atoms in a kg sphere of Si, method didn't win

  37. TRT Silver badge

    Re: Avagadro's number...

    Is the ratio of mass of stone to flesh divided by the price of two slices of gluten free bread.

    Or was that Avocado's paradox?

  38. TeeCee Gold badge

    Kibble balance?

    So, the new standard is actually based on American pet food?

  39. DJV Silver badge

    Re: Kibble balance?

    "Unfortunately Kibble balances are a) insanely expensive, b) the size of a small room, and hence c) only exist in around five special laboratories around the world"

    They forgot: d) have to be replaced often due to cats eating them.

  40. Pascal Monett Silver badge

    So we've gone from six to five

    We have set aside 6 imperfect measurement units and now rely on 5 very expensive locations for a perfectly precise result.

    We may have gained in precision, but we've lost in numbers. I don't know how expensive one of those balances are, but I'm pretty sure we're not going to be building a whole lot more in any case. What else can a Kibble balance be good for ?

  41. Glen 1 Bronze badge

    Re: So we've gone from six to five

    Rather than 6 imperfect measurements, we have 5 places where *many* perfect replicas can be made.

    They can be periodically compared against the 5 kibble balances, and be shown to have a certain accuracy. If it drifts too much we can bin it and make another.

  42. bombastic bob Silver badge
    Devil

    Re: So we've gone from six to five

    apparently, a kibble balance can be used to measure Planck's constant.

    https://en.wikipedia.org/wiki/Kibble_balance

    Also I'd think it would require a VERY accurate definition of 'Ampere'.

    And if any two SI standards are recursively/mutually dependent (because of measurement techniques, etc.), could a converging solution be used to make them as accurate as possible?

    (I'd really hate for our measurement standards to end up as one big Catch 22 or chicken/egg paradox)

  43. Ken Hagan Gold badge

    Re: So we've gone from six to five

    "I don't know how expensive one of those balances are, but I'm pretty sure we're not going to be building a whole lot more in any case."

    On the contrary, I'd expect that in just a few years we'll have quite a number more. It's only technology.

  44. Voyna i Mor Silver badge

    Re: So we've gone from six to five

    "On the contrary, I'd expect that in just a few years we'll have quite a number more. It's only technology."

    The way things are going, if civilisation lasts that long, they'll be in mobile phones.

    I still can't quite get over the fact that my phone contains a magnetic compass, a barometer and a set of radios all of which would fit into the cathode of one of the valves on my first radio set.

  45. Will Godfrey Silver badge
    Angel

    Re: So we've gone from six to five

    But is' quite possible your first radio set would still work today (if there's still something being transmitted on those wavelengths).

    Your phone, however...

    Nuff sed?

  46. Voyna i Mor Silver badge

    Re: So we've gone from six to five

    "But is' quite possible your first radio set would still work today (if there's still something being transmitted on those wavelengths)"

    If you think I would be happy powering up something with 55 year old big electrolytics...

    Valves not a problem - I did test one of my old EF86s the other day and it seemed to work, no blue glow - but electrolytics are prone to degradation. I once worked with someone who had had a colleague die when a 68000 microfarad experimental capacitor exploded, after which they put in pressure relief valves.

  47. Alan Brown Silver badge

    Re: So we've gone from six to five

    "I'd really hate for our measurement standards to end up as one big Catch 22 or chicken/egg paradox"

    What, you mean like being dependent on something as parochial as the rotation period of a particular planet in a binary planet/satellite pair(*), in a particularly short window of a period of their 7-8 billion year lifespan?

    Or the circumference of that particular planet?

    (*) Earth/moon are frequently described as a binary due to the size of the satellite having an easily observable effect on the barycenter of the pair (it's about 75% of the way towards the surface from the earth's core). It's not a classic binary inasmuch as the barycenter is still below the surface of the earth but it does mean there's a hell of a wobble in there and it would have been a true binary originally)

  48. Anonymous Coward
    Anonymous Coward

    How easy it is to get an international deal

    ...when its not about power politics, but about making life easier in ways politicians don't even notice.

  49. Anonymous Coward
    Anonymous Coward

    Re: How easy it is to get an international deal

    ... interesting side-light on this was in the BBC documentaries on the fundemental scientific units a few years ago which, in the program about the history of weights and the kg, mentioned that when Germany occupied France in WWII they specifically did not make a claim over the bulding holding "Le Grand kg" as they maintained the convention that it was considered to be "International territory" and not part of France.

  50. flokie

    Re: How easy it is to get an international deal

    French news were reporting on this last weekend, and surveyed random people on the streets for their thoughts.

    "Obviously, everybody uses kilos. Well, except the English, but then they're not normal".

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