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. arctic_haze Silver badge
    Holmes

    Re: How easy it is to get an international deal

    They treated it as an embassy. The only problem is embassy of what exactly?

  2. Throatwarbler Mangrove Silver badge
    Trollface

    Re: How easy it is to get an international deal

    . . . until Brexit happens, and the UK decides to redefine the kilogram as the mass of a swallow carrying two coconuts.

  3. Yet Another Anonymous coward Silver badge

    Re: How easy it is to get an international deal

    Le kilogram Anglais ?

  4. Jellied Eel Silver badge

    Re: How easy it is to get an international deal

    . . . until Brexit happens, and the UK decides to redefine the kilogram as the mass of a swallow carrying two coconuts.

    Nah, we can go back to using pounds and ounces!

  5. Credas Silver badge

    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.

    Well they obviously didn't survey anyone who's actually been to England in a while, or they'd know that in practice you'd be hard pressed to find those old Imperial units anywhere (aside from the yard/mile on our roads for some strange reason, or a pint in a pub).

  6. Ken Moorhouse Silver badge

    Re: you'd be hard pressed to find those old Imperial units anywhere

    Shop space is still measured in square feet (very topical at the moment).

  7. Paul Crawford Silver badge

    Re: yard/mile on our roads for some strange reason

    Because of the extraordinary cost of changing all signs and then having to deal with most cars in the UK having mph as the only (or dominant) scale for a decade or two afterwards.

  8. tfb Silver badge

    Re: How easy it is to get an international deal

    The sad thing is that I have spoken to people who, apparently seriously, said this was an argument for brexit. It may be they were joking but I had to kill them anyway.

  9. Alan Brown Silver badge

    Re: yard/mile on our roads for some strange reason

    Having lived through such a change (Imperial to metric) in two countries in 1973 and 1974, the roadsign and car odometer change isn't that difficult to deal with, people just deal with it.

    For starters, all UK roadsigns are already positioned so they can be changed to 400m, 800m, 1/2km, 1km or 2km and have been for a very long time, despite being written in miles (1/4 mile and 400m are interchangable)

    Fahrenheit to Celcuis is far more annoying. Using a scale which choose frozen brine as zero and Ox blood as one hundred didn't make a heck of a lot of sense at the time and it really still doesn't. At least Celcuis stuck with the same substance at both ends of the scale.

  10. disgustedoftunbridgewells Silver badge

    Re: How easy it is to get an international deal

    Or the weight/height of people and their pets, or distances in general.

    Fortunately my Spanish vet can convert from French units into English so I don't have to.

  11. disgustedoftunbridgewells Silver badge

    Re: How easy it is to get an international deal

    The argument isn't that we should use Imperial but that we should be allowed to choose for ourselves whether we mandate French, Imperial or neither.

  12. Charles 9 Silver badge

    Re: yard/mile on our roads for some strange reason

    I think one big stumbling block for changing temperature will be dealing with cookbooks. OLD cookbooks, especially large collections handed down through generations. I doubt there will be a service on hand to convert all the measurements and not that many people have a head for converting temperatures (especially over-boiling temperatures) on the fly.

  13. Anonymous Coward
    Anonymous Coward

    Re: yard/mile on our roads for some strange reason

    Because of the extraordinary cost of changing all signs and then having to deal with most cars in the UK having mph as the only (or dominant) scale for a decade or two afterwards.

    ===================================================================

    There are ways of changing the speedometer, and it's easier now, given that most vehicles are moving to digital displays and have been for years.

    As for the signs, once you change the default speed limits, there aren't all that many of them on a per capita basis, and you get new signs that don't have to be replaced for a long time. Maybe even better signs.

    It's past time to see off the last of the obsolete and often ambiguous measurement systems that are the measurement 'barnacles' of the past.

    If we can get the Yanks dragged into the 19th century as well, maybe we'll lose fewer space probes as a bonus.

    And with no more non-metric cookbooks, you can stop trying to guess which kind of pint the receipe expects you to use.

  14. Anonymous Coward
    Anonymous Coward

    Re: How easy it is to get an international deal

    "we should be allowed to choose for ourselves whether we mandate French, Imperial or neither."

    I'd certainly go for neither. Degrees Réaumur is just annoying.

    Celsius and Kelvin all the way! Standard international units is the only reasonable choice.

  15. Anonymous Coward
    Anonymous Coward

    I am surprised...

    they didn't redefine the kg to make the Mars bar in your pocket half the size, but the same 'weight'

  16. #define INFINITY -1
    Coat

    I despair

    ... that switching away from ounces made it easier to shave of 'grams'.

    Would you accept a change from a 4 once chocolate to 3? From 100g to 90g?

    Say, shall we take a survey of which of the two has actually happened?

  17. Notas Badoff Silver badge

    Re: I am concerned...

    In all these comments the weightier, larger question has been ingored: Will this make me fat?

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  20. Charles 9 Silver badge

    Re: I despair

    "Would you accept a change from a 4 once chocolate to 3? From 100g to 90g?"

    Wouldn't matter. They'll do whatever they damn well please. A half-gallon of ice cream in America isn't really a half-gallon (as in 64 fluid ounces) anymore, anyway (for years it's been as little as 48 and no more than 56).

  21. Fruit and Nutcase Silver badge
  22. #define INFINITY -1

    Re: Reg Standards

    Good idea. Reg standards go relative and us commentards are found killing off Great White Sharks or Adult Badgers when the average ratio goes out of whack.

  23. Fruit and Nutcase Silver badge

    Re: Reg Standards

    Looking through that list...

    https://www.theregister.co.uk/Design/page/reg-standards-converter.html

    Amongst others, it lists the following two for Volume

    "Bulgarian airbag (C-cup Posh Spice)"

    "Bulgarian funbag (DD-cup Jordan)"

    I was thinking that a "Bulgarian goody-bag" would be a handy addition. Could the Reg Standards Bureau form a working party to look into the proposal please.

  24. John Brown (no body) Silver badge
    Coat

    Re: Reg Standards

    "I was thinking that a "Bulgarian goody-bag" would be a handy addition. Could the Reg Standards Bureau form a working party to look into the proposal please."

    Does that mean someone will have to be sent out with a tape measure and a Kibble balance to visit Miss Chesty Morgan?

  25. Totally not a Cylon
    Black Helicopters

    All very well until some bright spark discovers a tiny flaw in one of these constants and EVERYTHING changes!

  26. A.P. Veening

    That bright spark will actually be a dim wit in the period between publishing and burning at the stake.

  27. CiaranA

    My calculator is out of date

    The number wedged in my head for Avogadro's constant is 6.022045E23 - because it was a constant in my calculator at school, and you see that number quite a lot during physics lessons.

    Now I find it's changed over time! Proves the old adage that variables won't and constants aren't, I suppose.

  28. #define INFINITY -1

    Re: My calculator is out of date

    Well... there's a thought. The redefinition will make 'physical ratios' as constant as Pi. How many decimal places will Planck's be? It's a matter of choice right now.

  29. AIBailey

    Re: My calculator is out of date

    The only constant on my calculator was 58008

  30. John Brown (no body) Silver badge

    Re: My calculator is out of date

    Mine must have been the next model up from yours. Mine also included 5318008.

  31. DropBear Silver badge

    Re: My calculator is out of date

    You aren't really paying attention, are you. There's no choice involved (or rather, it's already decided that there won't be any, really soon):

    "On 16 November 2018, the International Bureau of Weights and Measures (BIPM) voted to redefine the kilogram by fixing the value of the Planck constant, thereby defining the kilogram in terms of the second and the speed of light. Starting 20 May 2019, the new value is exactly h = 6.62607015 × 10^-34 J ⋅ s"

  32. Alan Brown Silver badge

    Re: My calculator is out of date

    Mine said 5318008618 (until the teacher saw and asked to have the joke explained. She pretended not to be amused)

  33. disgustedoftunbridgewells Silver badge
    Holmes

    Re: My calculator is out of date

    8Ɩ9 8008ƖƐϛ

    (For anybody else who couldn't be bothered working out what that was supposed to say )

  34. Charles 9 Silver badge

    Re: My calculator is out of date

    But will h STAY exactly 6.62607015 × 10^-34 J ⋅ s? Or will the increased use of Kibble balances reveal more significant digits that can result in a redefinition of h, which will in turn alter everything dependent on h?

  35. A 15
    Boffin

    A few comments

    It seems a little peculiar that they were quite happy to use the charge on an electron to define the Coulomb, but didn't wan't to use the mass on the electron to define the kg (it is known to an appropriate precision).

    In regards to the comment about someone else coming along and finding a flaw... well that isn't very likely; the definitions were not created on the grounds that science now knows these units perfectly so we'll just set them in stone. They were defined so that all the units that can be derived from the base units can be updated (slightly), when our measuring improves. The new way leaves a field with fewer moving goals.

    There is still a bit of awkwardness with certain units, like the definition of the second is based upon being measured at sea level (as general relativity plays a role).

    Also I will kind of miss Avagadro's constant being the number of atoms in 12 grams of carbon 12. Still, at least I have a set of numbers that I can learn and won't change now :p

  36. Charles 9 Silver badge

    Re: A few comments

    "It seems a little peculiar that they were quite happy to use the charge on an electron to define the Coulomb, but didn't wan't to use the mass on the electron to define the kg (it is known to an appropriate precision)."

    Except the rest mass of an electron is calculated from the Planck constant (and the Rydberg constant): any uncertainty in its measurement comes from the Planck constant, so you could say this is closer to the source.

    "There is still a bit of awkwardness with certain units, like the definition of the second is based upon being measured at sea level (as general relativity plays a role)."

    Took a bit of reading to figure out what's involved (time dilation caused by the differing scalar speeds at differing altitudes). I guess in the end you have to pick something because there are no real absolute points of reference in the universe.

    "Also I will kind of miss Avagadro's constant being the number of atoms in 12 grams of carbon 12."

    Probably because that would have to depend on the definition of the gram (or in this case, the kilogram). That puzzles me. Why does SI use the kilogram instead of the base gram? Might this change in future now that the standard-bearer has changed as well?

  37. A.P. Veening

    Re: A few comments

    "Why does SI use the kilogram instead of the base gram?"

    Because it was more convenient for a number of calculations.

  38. Yet Another Anonymous coward Silver badge

    Re: A few comments

    Of course it should have redefined the gram as the kilogram, or we should speak about milli-kilograms

  39. Carpet Deal 'em
    Boffin

    Re: A few comments

    "Why does SI use the kilogram instead of the base gram?"

    The kilogram was originally called the "grave", but the name was dropped for various reasons and, in the process, some genius decided to base the default on the centimeter. Unsurprisingly, the original grave was the more convenient measure, but by then "gram" had stuck, so they popped the kilogram in its place.

  40. ibmalone Silver badge

    Re: A few comments

    It seems a little peculiar that they were quite happy to use the charge on an electron to define the Coulomb, but didn't wan't to use the mass on the electron to define the kg (it is known to an appropriate precision).

    One reason is related to a rather intractable debate I ended up with a computer scientist in. They didn't really see a difference in measuring things in relative units and SI units, they're both ratios right? But the reference ratio is only one part of the system, you need a reproducible way to calibrate other measurements, which is why these SI standards have two parts, one is the thing you're using as the basis of your definition and the other is the measurement process.

  41. John Brown (no body) Silver badge

    Re: A few comments

    "There is still a bit of awkwardness with certain units, like the definition of the second is based upon being measured at sea level (as general relativity plays a role)."

    And for that matter, what is "sea level"? Tides, gravitational variation at different points across the surface of not perfectly spherical Earth due to the non-even distribution of mass inside the wonky shaped Earth.

  42. Charles 9 Silver badge

    Re: A few comments

    They took that into consideration. The actual point of reference is MEAN sea level, which averages out variations and puts all the atomic clock calculations at the same height (it also specifies a target temperature so as to be more consistent).

  43. disgustedoftunbridgewells Silver badge

    Re: A few comments

    If they were going for convenience they'd have gone for a kg weighing 0.453kg

  44. Charles 9 Silver badge

    Here's a little food for thought. It seems one big reason they wanted to redefine the kilogram was because the different prototype kilograms were diverging from each other, and they were a little puzzled as to why: was one losing mass or the other gaining mass, and so on. That and the research into using the Kibble balance meant they could move away from prototypes.

    Same for the temperature redefinition, as using the triple point basis wasn't consistent at extreme temperatures, and the Boltzmann constant was a better fit.

  45. #define INFINITY -1

    I certainly hope not; that would be a total fail on the part of scientists.

    If they are diverging and they DO know why, then the redefinition has merit. Otherwise they're just bloody lazy and lack inquisitiveness.

  46. swm

    The meter bars were slowly warping which is why the meter is now defined in terms of physical experiments and not physical artifacts.

  47. Ken Moorhouse Silver badge

    Re: The meter bars were slowly warping

    I'd say your monitor needs degaussing.

  48. ibmalone Silver badge

    "Here's a little food for thought. It seems one big reason they wanted to redefine the kilogram was because the different prototype kilograms were diverging from each other, and they were a little puzzled as to why: was one losing mass or the other gaining mass, and so on. That and the research into using the Kibble balance meant they could move away from prototypes."

    I certainly hope not; that would be a total fail on the part of scientists.

    If they are diverging and they DO know why, then the redefinition has merit. Otherwise they're just bloody lazy and lack inquisitiveness.

    No, as alluded to in the comment you're replying to, how do you investigate this? You need a mass reference to do it. Enter the Kibble balance. Now you have a way to measure mass that doesn't depend on a chunk of gold alloy. If it turns out to be something mundane like tiny amounts of wear or adsorption then you can watch it happening, but if the only way to measure mass is by comparing to another chunk of alloy which is also changing in some way then you're rather stuck.

    Similar answer to the question of what happens if these universal constants aren't actually, you find some related processes that should be the same under your assumptions and start comparing them.

  49. DrBobK
    Headmaster

    The candela is odd.

    The candela is a measure of human perceived brightness. As people's perception of the brightness of a light source depends on the wavelength of the light (we are most sensitive to wavelengths corresponding to green (around 550nm) and less efficient to shorter and longer wavelength) the definition of the candela has to include a multiplier representing the relative luminous efficiency as a function of wavelength - this function is known as V-lambda. The definition of V-lambda is based on people subjectively matching the brightness of lights with different wavelengths. The standard V-lambda defined by the CIE (Commission Internationale de l'Éclairage) is, in fact, based on measurements from a very small number of observers. There are different versions of V-lambda for daylight adapted (photoptic) vision, dark adapted (scotopic) vision, and the intermediate state - mesopic vision. The candela is defined in terms of the physical power of a light over an illuminated area at a single standard wavelength (and so is entirely physical at that wavelength), but to use the candela as a measurement of luminous intensity at any other wavelength one has to use a subjectively defined multiplier from the appropriate V-lambda. All pretty weirdly subjective for an SI base-unit!

    (I study human vision for a living.)

  50. #define INFINITY -1

    Re: The candela is odd.

    X-Windows has a few functions based on TekTroniks's research into HSV spaces--can't find much else on their theories relative to 'modern' ones that Adobe use. I understand the 30-59-11 of RGB is the values which make colour-to-gray TV's 'work'.

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