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View Full Version : The Definition of The Kilogram Is About to Change. Here's What That Really Means.



sharpiefan
11-14-2018, 09:32 PM
Finally, 130 years after it was established, the kilogram as we know it is about to be retired. But it's not the end: a new definition will be put in place - one that's far more accurate than anything we've had until now.
Tomorrow, on 16 November, the General Conference on Weights and Measures (CGPM) in Versailles is going to vote to officially make the change. Le kilogramme est mort, vive le kilogramme.
Most people don't think about metrology - the science of measurement - as we go about our day. But it's vastly important. It's not just the system by which we measure the world; it's also the system by which scientists conduct their observations.
It needs to be precise, and it needs to be constant, preferably based on the laws of our Universe as we know it.
But of the seven base units of the International System of Units (SI), four are not currently based on the constants of physics: the ampere (current), kelvin (temperature), mole (amount of substance) and kilogram (mass).
"The idea," explained Emeritus Director of the International Bureau of Weights and Measures (BIPM) Terry Quinn to ScienceAlert, "is that by having all the units based on the constants of physics, they are by definition stable and unaltering in the future, and universally accessible everywhere."
For example, a metre is determined by the distance light travels in a vacuum in 1/299792458 of a second. A second is determined by the time it takes for a caesium atom to oscillate 9,192,631,770 times.
A kilogram is defined by… a kilogram.
No, literally. It's a kilogram weight called the International Prototype of the Kilogram (IPK), made in 1889 from 90 percent platinum and 10 percent iridium, and kept in a special vault in the BIPM headquarters.
In fact, the kilogram is the only base unit in the SI still defined by a physical object.

(CONT'D AT LINK)


The Definition of The Kilogram Is About to Change. Here's What That Really Means (LINK) (https://www.sciencealert.com/after-130-years-the-definition-of-the-kilogram-is-finally-going-to-change)


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David G
11-14-2018, 09:36 PM
https://www.youtube.com/watch?v=NzrkDGxZexA

Don't touch my scale if you please Mr. Customs Man...

sharpiefan
11-14-2018, 10:04 PM
https://www.youtube.com/watch?v=NzrkDGxZexA

Don't touch my scale if you please Mr. Customs Man...

Y> :D

gypsie
11-14-2018, 10:13 PM
A second is determined by the time it takes for a caesium atom to oscillate 9,192,631,770 times.

Say that number out loud, then go back and read the words 'oscillate' and 'second' and then have a think.

Phil Y
11-15-2018, 02:11 AM
I thought it was the weight of a litre of water. Which is 10cm cubed.


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Peerie Maa
11-15-2018, 03:36 AM
I thought it was the weight of a litre of water. Which is 10cm cubed.


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At what temperature and pressure?

Phil Y
11-15-2018, 04:52 AM
20C. Sea level I suppose but don't know quite what pressure


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P.I. Stazzer-Newt
11-15-2018, 05:18 AM
It was at 4 Celsius.

At the hat time water was considered to be incompressible, so pressure made no difference.

Peerie Maa
11-15-2018, 08:04 AM
It was at 4 Celsius.

At the hat time water was considered to be incompressible, so pressure made no difference.

And both above and below that temperature water stars to expand. Not to mention the volume of it's container.

sharpiefan
11-15-2018, 08:44 AM
For the curious.....



On April 13, 1668 the metric system was born. It had been devised by the English scholar, John Wilkins (1614-1672). The system was elegantly defined. One would take a length of string with a mass attached, and adjust that length until the pendulum had a period of two seconds (one second each direction). Now that you had a standard length you would divide that by ten, and use that length to make a cube, which became the liter. Fill the cube with rain water and the mass is a kilogram.

(CONT'D AT LINK)


The Americans Who Defined The Meter (LINK) (http://themetricmaven.com/?p=1530)


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willmarsh3
11-15-2018, 09:18 AM
It will be interesting if we find these natural constants change and what metrologists will do about it.

Norman Bernstein
11-15-2018, 09:24 AM
The most surprising thing I learned, when I heard this story, was the fact that although the meter had been defined in terms of physical constants, years ago... the kilogram had not yet been defined that way. I would have thought that this issue was resolved a long time ago.

sharpiefan
11-15-2018, 09:46 AM
I can't help but wonder what those folk have against nice, easy, round numbers. :)

Would it be too much to ask to define the metre as the distance light travels in 1/300,000,000 of a second?

sharpiefan
11-15-2018, 09:53 AM
The most surprising thing I learned, when I heard this story, was the fact that although the meter had been defined in terms of physical constants, years ago... the kilogram had not yet been defined that way. I would have thought that this issue was resolved a long time ago.

<tongue-in-cheek>
I think they've been looking for a 'fudge factor' -- they've decided what value they want it to have, now they have to conjure up something from the real world to make it so. It's why the numbers in the other definitions are so weird.
</tongue-in-cheek>

Too Little Time
11-15-2018, 10:23 AM
I can't help but wonder what those folk have against nice, easy, round numbers. :)

Would it be too much to ask to define the metre as the distance light travels in 1/300,000,000 of a second?
That would have resulted in a different length. No one who does precise measurement wants that.