BrainStuff Classics: How Do We Define the Kilogram?

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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Episode Transcript

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.