January 11, 2024 • 4 mins

The kilogram used to be measured against an agreed-upon chunk of metal stored in France, but no longer. Learn the new measure of this standard unit of measurement in today's classic episode of BrainStuff, based on this article: https://science.howstuffworks.com/math-concepts/kilogram-is-dead-meet-kilogram-20.htm

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Speaker 1 (00:01):

Welcome to brain Stuff, a production of iHeartRadio. Hey brain Stuff,

I'm lorn vogel bomb, and this is a classic episode

of the podcast. In this one, we dig into the

weighty history of the kilogram. It turns out this standard

measure has been redefined over time. Hey brain stuff, I'm

(00:23):

lorn vogel bomb. And for nearly one hundred and fifty years,

the official weight of a kilogram was determined by a

shiny cylinder of platinum locked away in a French vault.

The kilogram, like the meter and the second, is one

of the seven fundamental units of measurement, also known as

the International System of Units or the metric system the

SI for short. These were first formalized in the eighteen

(00:45):

seventy five Treaty of the Meter. Back then, the best

way to agree on the weight of a kilogram was

to forge a single hunk of metal and dubit legrand k,

and for more than a century all scientific scales were

calibrated back to that one physical reference point, with copies

stored in a dozen countries. But even solid objects can

(01:06):

change over time. When La grand k was weighed in

the nineteen eighties, it was a couple of micrograms lighter,

meaning that all highly accurate scientific scales, not like the

one in your bathroom, had to be recalibrated, and that's

what nerds call a real pain in the mass. Luckily,

a team of metrologists was already on the case. Metrology

being the science of weights and measures. They were searching

(01:28):

for a universal constant that would generate a fixed value

for the kilogram that's true now and will still be

true a million years from now. They had already found

such a physics fix for the second, which was redefined

in nineteen sixty seven from its previous value of a

fraction of a day one eighty six thousand, four hundredth

of a day to be precise, to something much more confusing,

(01:48):

but much more constant. It takes nine billion, one hundred

and ninety two million, six hundred and thirty one thousand,

seven hundred and seventy oscillations of a special microwave beam

to excite atoms of the isotope caesium one thirty three

to a higher energy level. Since that number will never

change unlike the exact length of a day, that's your

(02:10):

new second. Same for the meter, instead of being defined

as the length of a single meter long metal pole

forged back in eighteen eighty nine. It was redefined in

nineteen eighty three as the distance light travels in a

vacuum in a particular fraction of a second one two

hundred and ninety nine million, seven hundred and ninety two thousand,

four hundred and fifty eighth of a second. It wasn't

(02:30):

until twenty seventeen that scientists working at the US National

Institute of Standards and Technology and similar bodies worldwide finally

agreed on a universal constant for the kilogram. The achievement

required solving one of the thorniest physics problems of the

last century, coming up with a numerical value for planks

constant without getting too technical. A physicist, Max Planck proved

(02:51):

in nineteen hundred that matter releases energy in discrete chunks

called quanta. His equation for measuring these packets of energy

is included a constant called H hitherto known as planx constant.

Thanks to Einstein, we know that energy and mass are

mathematically related, that whole e equals mc squared thing, so

physicists figured out the planks constant, being a fixed unit

(03:14):

of energy, could yield the world's most accurate measurement of mass.

Calculating the exact value of planks constant took decades and

some serious technological innovation, specifically a nifty device called a

kibble balance. But they did that work, and we now

know that planks constant is six point six two six

zero seven one five zero times ten to the power

(03:35):

of negative thirty four jeweles per second I mean duh.

In mid November twenty eighteen, at the annual meeting of

the International Bureau of Weights and Measures in Versailles, France,

representatives from more than sixty countries voted to approve a

new and everlasting definition of the kilogram as calculated by

the plank constant. No more hunk of metal. The kilogram's

(03:57):

mass is now tied to planks constant. Definitions were also

announced for SI units, the ampeer electrical current, the kelvin

for temperature, and the mole the number of molecules or

atoms in an element. These new definitions will take effect

on May twentieth, twenty nineteen. The original platinum kilogram prototype

will remain in that underground French vault, while countless generations

(04:18):

of scientists will make world changing discoveries using the Kilogram

two point zero. Today's episode is based on the article

the Kilogram is Dead Meet the Kilogram two point zero

on how stuffworks dot Com, written by Dave Ruse. Brain

Stuff is production of iHeartRadio in partnership with HowStuffWorks dot

(04:39):

Com and is produced by Tyler Klang. Four more podcasts

from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever

you listen to your favorite shows.

Welcome to brain Stuff, a production of iHeartRadio. Hey brain Stuff,

I'm lorn vogel bomb, and this is a classic episode

of the podcast. In this one, we dig into the

weighty history of the kilogram. It turns out this standard

measure has been redefined over time. Hey brain stuff, I'm

(00:23):

lorn vogel bomb. And for nearly one hundred and fifty years,

the official weight of a kilogram was determined by a

shiny cylinder of platinum locked away in a French vault.

The kilogram, like the meter and the second, is one

of the seven fundamental units of measurement, also known as

the International System of Units or the metric system the

SI for short. These were first formalized in the eighteen

(00:45):

seventy five Treaty of the Meter. Back then, the best

way to agree on the weight of a kilogram was

to forge a single hunk of metal and dubit legrand k,

and for more than a century all scientific scales were

calibrated back to that one physical reference point, with copies

stored in a dozen countries. But even solid objects can

(01:06):

change over time. When La grand k was weighed in

the nineteen eighties, it was a couple of micrograms lighter,

meaning that all highly accurate scientific scales, not like the

one in your bathroom, had to be recalibrated, and that's

what nerds call a real pain in the mass. Luckily,

a team of metrologists was already on the case. Metrology

being the science of weights and measures. They were searching

(01:28):

for a universal constant that would generate a fixed value

for the kilogram that's true now and will still be

true a million years from now. They had already found

such a physics fix for the second, which was redefined

in nineteen sixty seven from its previous value of a

fraction of a day one eighty six thousand, four hundredth

of a day to be precise, to something much more confusing,

(01:48):

but much more constant. It takes nine billion, one hundred

and ninety two million, six hundred and thirty one thousand,

seven hundred and seventy oscillations of a special microwave beam

to excite atoms of the isotope caesium one thirty three

to a higher energy level. Since that number will never

change unlike the exact length of a day, that's your

(02:10):

new second. Same for the meter, instead of being defined

as the length of a single meter long metal pole

forged back in eighteen eighty nine. It was redefined in

nineteen eighty three as the distance light travels in a

vacuum in a particular fraction of a second one two

hundred and ninety nine million, seven hundred and ninety two thousand,

four hundred and fifty eighth of a second. It wasn't

(02:30):

until twenty seventeen that scientists working at the US National

Institute of Standards and Technology and similar bodies worldwide finally

agreed on a universal constant for the kilogram. The achievement

required solving one of the thorniest physics problems of the

last century, coming up with a numerical value for planks

constant without getting too technical. A physicist, Max Planck proved

(02:51):

in nineteen hundred that matter releases energy in discrete chunks

called quanta. His equation for measuring these packets of energy

is included a constant called H hitherto known as planx constant.

Thanks to Einstein, we know that energy and mass are

mathematically related, that whole e equals mc squared thing, so

physicists figured out the planks constant, being a fixed unit

(03:14):

of energy, could yield the world's most accurate measurement of mass.

Calculating the exact value of planks constant took decades and

some serious technological innovation, specifically a nifty device called a

kibble balance. But they did that work, and we now

know that planks constant is six point six two six

zero seven one five zero times ten to the power

(03:35):

of negative thirty four jeweles per second I mean duh.

In mid November twenty eighteen, at the annual meeting of

the International Bureau of Weights and Measures in Versailles, France,

representatives from more than sixty countries voted to approve a

new and everlasting definition of the kilogram as calculated by

the plank constant. No more hunk of metal. The kilogram's

(03:57):

mass is now tied to planks constant. Definitions were also

announced for SI units, the ampeer electrical current, the kelvin

for temperature, and the mole the number of molecules or

atoms in an element. These new definitions will take effect

on May twentieth, twenty nineteen. The original platinum kilogram prototype

will remain in that underground French vault, while countless generations

(04:18):

of scientists will make world changing discoveries using the Kilogram

two point zero. Today's episode is based on the article

the Kilogram is Dead Meet the Kilogram two point zero

on how stuffworks dot Com, written by Dave Ruse. Brain

Stuff is production of iHeartRadio in partnership with HowStuffWorks dot

(04:39):

Com and is produced by Tyler Klang. Four more podcasts

from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever

you listen to your favorite shows.

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