Hi everyone! Today in SI we are going to talk about the kilo. It’s a prefix that means “thousand”.
Yes… but no, I mean, in this context we are going to talk about the kilogram, which is equal to one thousand gram Yes, in the SI though, the unit used for the mass Yes! But in the SI the chosen unity of mass isn’t the gram, It’s the kilogram Isn’t that a bit stupid? Alright, but there is one thing even worse I mean, the current definition of the kilogram is the mass of an object that weigh one kilogram Yeah… Yeah… The international system We have seen it when we were talking about the meter but after the French Revolution the constituent assembly was seeking to find . the most precise way to measure new base units to make a new units of measurement system.. … Because a new system was required. Because before the french Revolution everything was crap And in particular to choose the mass unit a commission is created with, on board, Antoine Lavoisier The one that said “Nothing gets lost, nothing gets created” and everything else, the one we already talk about in the Lagrange’s episode. -But also René Juste HAUY.
-This is pronounced HAÜY. René Juste HAÜY, who, in addition of being a abbot, is the father of the geometric crystallography.
And the geometric crystallography that’s… -Nobody cares
-No not me !
-Who said that ? In 1794, HAUY is imprisonned and Lavoisier is beheaded.
-YOLO ! French Revolution in your faaace ! Remember, Langrange will say about this “They only needed a moment to cut his head off, and… maybe hundred years won’t be enough to reproduce a similar one. Nevertheless, only eighty five years later, Albert Einstein will be born so well… that’s an other story That being said, it allows me to make you do a little relativity exercice, here, like that, real quick. helter skelter, well, so…
Albert Einstein was a contemporary of your parents. Or maybe even of yourself, say? Well Albert Einstein had the opportunity, when he was a child, to meet people who were contemporary of the French Revolution… Voilà! I leave you a couple second to think about it. For the commission of mass determination, we then substitute Hauy and Lavoisier by…
By two other guys, but we totally don’t care because in fact they’ll simply go allong with Wilkins’s propositions which we already talked about in the meter episode. Wilkins had propose as mass unit,
the mass of a universel water volume. Being approximately 1 cubic meter of water. Well, the fellas of the academy did had some precision to the thing well, to begin with it has to be pure water. Wilkins simply suggested rainwater Witch was clean at the time, but wasn’t pur either. Then, they suggest, pur water, but… At 4° Celsuis. Well, why 4 ? to start with they were right to settle on a temperature, because well, it fluctuate but, as a matter of fact 4° Celsius is the temperature at which liquide water is the densest at the condition that we are at atmospheric pressure. They should have mention at atmospheric pressure. They then determined several standart mass. That is a cubic meter of water’s mass, it’s the ton which is a usefull measure at ship’s scale. The mass of a cubic decimeter, as well, which is usefull at market’s scale, I mean a cubic decimeter, is a liter, so you can imagine that it is actually usefull in daily exchanges between people. They also determine a cubic centimeter of water’s mass Which is comparable to the mass of coins And the mass of a cubic millimeter of water So here it’s really for the very fine measurements They call the mass of a cubic centimeter of water the gram Then the mass of a cubic decimeter of water which is 1000 times bigger A decimeter is ten times a centimeter, but … to the cub, it is 10 times for the length, 10 times for the width and 10 times for the eight Yes, it is 10 times 10 times 10, so it is 1000 times bigger Thus the mass of a cubic decimeter worth 1000 times 1 gram, i.e one kilogram. And for those who wonder where “gram” comes from… At the times there was already something existing wich was approximately of the same weight called the “gram” One “gram” was one twenty-fourth
of the roman ounce Roman ounce wich was 27,264g of today So one “gram” from then was approxymately 1,136g of today So we are close Roman ounce was called roman ounce because ounce comes from oncia and oncia means twelfth And roman ounce was a twelfth of the roman pound Btw the roman ounce should not be confused with the french ounce from the old regime wich was 30, 594g. Ounce notation is “oz” and for people who still uses the imperial system or the American system there is a unit called liquid ounce noted “fl oz” wich isn’t a mass but a volume qui fait 28,413 ml regarding the imperial system. and 29,573 ml for the Americans. So when you have “12 fl oz” written on a bottle… don’t worry about it it represent the volume inside but it’s in stupid units… In 1795 we have a precise definition of a kilogram: it is the masse of 1dm^3 of pure water at 4°C But not to worry about water we are going to melt objects out of metal that we are stupidly going to call “weights” wich are going to make precisely 1kg. They are used as “Standard” (metrology). Then we are going to get more accurate in the production of those Standard first in platinum and then in an platinum-iridium alloy, until today. And at the first General Conference on Weights and Measures kilogram is defined as the mass of one of those standards which were melted ten years sooner in 1979 From then on, this standard is conserved Under a cloche, in sterilized things -anyways- at the Breteuil in Saint-Cloud in France. And of course, we are going to melt in the same way some more standards which are going to weigh the exact same mass as that original standards and that we are going to send across the globe for everyone to have their reference and that’s when everything will go wrong. Voilà ! The important thing to know is that the kilogram is the only SI unit defined from a physical object -an artefact- and not from a matter property nor a physical phenomenon independant of anything object. And the problem is that the different kilogram standards all around the world don’t weigh the same mass anymore. It varies mildly but it’s not the same mass. And when we know that we actually link the kilogram to other SI units which we are able to precise – to mesure with a precision close to the 10 billionth. Even a little bit, it’s already way too much ! Thus the wish to define the kilogram in different way than the physical object And I’ll stop there for this episode because actually Veritasium has already made two video on this subject – One presenting the method allowing to approach the kilogram by counting the atoms – Yep ! – And the other one about the extremely precise calculation of the Plancks constant (h) – constant from which we will be able to redefined the definition of the kilogram. – Links are in the description – That’s all for this episode. I hope you enjoyed it, And that it wasn’t too heavy. (In french, he makes a pun about “relou” which is a slang word for “lourd” (used as “boring” but really means “heavy”)) That’s a shitty joke If you enjoyed it feel free to put a thumb up to comment, like and share, to do whatever you want. – But honestly sharing is really cool – You can also subscribe to the channel and click on the bell to be notified about the new videos and you can donate to help the channel. See you soon for a new episode and by then, stay curious And take your time to think about it. (Last sentence isn’t really clear but he played on word with the name of his channel “E-penser”=”Y penser”=” Think about it” )