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The ampere is still a base unit, according to the SI brochure. However, in my perception the recent redefinition of units effectively defines the Coulomb as e/(1.602 176 634 × 10^−19), and the ampere is derived as 1 A = 1 C/s. Why did they not make the coulomb a base unit, instead of the ampere, last year?

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It appears that they have deemphasized the concept of "base units". They did not remove the term, but mention that all units are now defined in terms of constants. As wikipedia puts it:

With the 2019 redefinition, the SI is constructed around seven defining constants, allowing all units to be constructed directly from these constants. The designation of base units is retained but is no longer essential to define SI measures.

From the 9th edition of the SI Brochure

The choice of the base units was never unique, but grew historically and became familiar to users of the SI. This description in terms of base and derived units is maintained in the present definition of the SI, but has been reformulated as a consequence of adoption of the defining constants.

With that position, there seems to be no strong desire to modify which units comprise the set of base units.

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    $\begingroup$ Just to expand on the non-uniqueness of the choice of base units: We use the kilogram as the base unit of mass, which is 1000 grams. Weird, however, the use of the kilogram is well established over time. So, while we may change the definition of our base units, we keep them around to cause no trouble. $\endgroup$ – Dohn Joe Jan 14 at 9:05
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    $\begingroup$ @DohnJoe Well, I always thought the base units were chosen at a scale in which it would be reasonable to use them. I heard a podcast yesterday where they quipped about having the base measurement of volume be "a barrel" and then proceded to hilarity with cooking recipies having .0014 barrels of milk. It was funnier than the justice it gets in this retelling, though. Point being, a part of being "base" was being scaled to use. and most people encounter kilogramme-heavy things everyday, most appliances use ampere-ish current and most objects you see clearly can be found within some meters. $\endgroup$ – Stian Yttervik Jan 14 at 9:39
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    $\begingroup$ @Stian you're right. This is what is referred to in the SI Brochure as "... grew historically and became familiar ...". In german-speaking news, areas are often compared to soccer-fields, this gave rise to people using all kinds of "base units" in the comments of some news stories. For more unit-conversion fun: the unit converter from theRegister $\endgroup$ – Dohn Joe Jan 14 at 11:45
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It's also important to add, that it's hard to change.. All the textbooks and education. If it was easy, then for better consistency they would apply the metric system in the US too. It could be hard for many, many wouldn't care or wouldn't understand why!

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  • $\begingroup$ We do use the metric system in the US for some things, but when you have, for instance, legal definitions of property lines based on a system of mile-square grids surveyed over a century ago, it's impractical to change them to SI. $\endgroup$ – Monty Harder Jan 14 at 23:21
  • $\begingroup$ Just change them everytime a land dispute is handled? $\endgroup$ – Dr_Bunsen Jan 15 at 7:48
  • $\begingroup$ It's interesting, thank you for the comment. This is funny though, when nasa and esa collaborates they have to use the same units also. Once a Mars lander crushed because of some unit misunderstanding in a common project... $\endgroup$ – Kregnach Jan 15 at 17:16
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The redefinition of the SI was to untangle all the different units from each other, and have nature constants for basis(so no lumps of metal anymore).

And ampere has a good definition, so why change that? It is not going to change, like the other elements, and if the natural constants are meta-stable, we have bigger things to worry about than some measurements.

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    $\begingroup$ "And ampere has a good definition, so why change that?" Err, but they did change the definition of the ampere. It used to be "that current which when flowing in two infinite parallel wires one metre apart produces a force between them of 2 x 10-7 N/m" (and the coulomb was "that charge carried by one ampere in one second"). Now the coulomb is defined in terms of the charge on the electron, and the ampere is "1 C/s." $\endgroup$ – Martin Bonner supports Monica Jan 14 at 12:53
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    $\begingroup$ @MartinBonnersupportsMonica +1 though it seems (as the OP says) that the ampere is defined in terms to the charge on the electron and the second, and only then is the coulomb defined as 1 C = 1 As $\endgroup$ – Henry Jan 14 at 13:43
  • $\begingroup$ There are multiple ways to define ampere to a very small number. I do not know exactly, a colleague told me once(working at an NMI). But indeed the low newton force is not how they do it. $\endgroup$ – Dr_Bunsen Jan 14 at 15:48

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