The Ampere and the Coulomb

The ampere is a measure of electric current, one of the seven base SI units. From the ampere, the coulomb, the volt, the ohm, the farad and a lot of others are defined.


For SI units, sensible values are usually chosen. The kilogram, for example, is defined so that the density of water is 1000 kg/m3, and the kelvin starts at absolute zero and has 100 degrees between the freezing and boiling points of water. (There are exceptions – more on that some other time). For the ampere and the coulomb, the natural benchmark for conversion is the charge on an electron. It would make more sense if one coulomb were equal to the charge on a billion electrons, right?

That’s not the case at all.

1 coulomb is equal to the charge on 6.241509324×1018 electrons, which looks like a completely arbitrary number. So, my question is – is it really arbitrary? Is there some other reason why the ampere and the coulomb are not based on the electron charge?

In trying to solve this, I went to Wolfram Alpha and typed in ‘1 ampere’. The definitions there either seemed to have no relation to the ampere, or defined it on the basis of units defined by the ampere. In short, recursive confusion.

Why is the ampere the base unit? Why not the coulomb? 1 ampere = 1 coulomb/1 second, but the only definition I can find for a coulomb is 1 ampere-second.

So, why is the ampere what it is?

And why is it a base unit?

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