Changes to the International System of Units

The Changes to SI Units announced in the fall of last year, take affect tomorrow. It won’t change your everyday experience of the liter, or the kilogram, but it will make scientists’ lives easier.

Shamelessly stolen from myself…..

The International System of Units (or SI – it did start in France) underwent some changes. Most people think of the bits they run into everyday as the metric system, and I know Americans hate the metric system. Why? Because they learned one thing, and believe they are incapable of learning something new.

Scientists just ditched the single lump of metal that defined the kilogram for 129 years for a better measure. Everyone is hung up on the kilogram, but there were several other parts of the SI units that were changed. I guess reporters aren’t too up on physics to know what those are. Some (maybe most) articles ONLY mention the kilogram. (More on that later.)

Scientists in Paris on Friday approved a change which will alter how a kilogram is defined, the first change to the system of measuring mass which has been in use since 1889.

So originally a kilogram was supposed to be the weight of 1 liter of water (1000, cubic centimeters) taken just at the freezing point. (O degrees Celsius, that is.) That was a nightmare. So in 1889, they created a cylinder of platinum-iridium alloy and 6 copies. (The original was known officially as the International Prototype Kilogram (IPK) and unofficially as “Le Grand K.”) This was also a nightmare, but good enough to be going on with. Except that it seems to have been changing over the 129 years. (Dust makes it heavier. Cleaning it makes it lighter. It seems to be absorbing some pollution.)

So anyway, the new definition (effective May 20, 2019) of the kilogram defines the Planck Constant h to be 6.62607015 x 10-34 Joule-seconds. (Where a Joule-second is equal to kg⋅m2⋅s−1. where the kilogram, meter and second are defined in terms of h, c and ΔνCs.) The upshot of this is that anyone with a device known as a Kibble balance can work out exactly how much the weight in front of them is, without referencing some cylinder in Paris.

OK, so most Americans know what a liter is, if only because your favorite soda comes in a liter or 2 liter bottle for game-day. And most are familiar with a Kilogram. But is that any call to ignore the other changes to the SI units that you don’t run into everyday?

The ampere will be defined by the elementary electrical charge (e,) the kelvin will be defined by the Boltzmann constant (k,) and the mole will be defined by the Avogadro constant.

Since I’m sure there are folks who are confused on this issues. A mole is measure of how many particles there are in a sample. One mole is exactly 6.02214076 x 1023 particles. (Avogadro’s Constant) An ampere is the base unit of electric current. It’s new definition is in terms of the elementary charge (charge carried by a single proton) e=1.602176634 x 10-19 when expressed in Amp-seconds. And the Kelvin is defined in terms of the kilogram, meter and second, which are all defined in terms of h, c and ΔνCs. I guess I should note that ΔνCs refers to the unperturbed, ground-state, hyperfine transition frequency of a Caesium-133 atom. That frequency is 9,192,631,770 Hz.

Maybe reporters don’t want to try to explain any of that. (I only realized what I was getting into after I started typing) but they could at least mention it, and most of them didn’t.

One thing that is really striking in these equations and constants is just how good the human-race has gotten at measuring things like electric charge, and vibration frequency.


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