Historically, what came first: coulomb unit or ampere unit?
This is a bit murky, because the first charge and current units weren't SI.
If you mean the SI coulomb and ampere, then I believe the ampere was chosen, and then the coulomb defined as 1 amp-sec. At some point, the ampere was chosen to be one tenth of the "absolute ampere" (now called the abampere in the electromagnetic unit system), probably becaause some engineer thought the absolute ampere was too large a unit.
I have been trying myself for some time to find out who and when this change was made. (It was sometime in the 19th century.) SI units would make more sense if if the ampere had been kept equal to the abampere. Then, in cgs, mu0/4pi would equal 1.
Thank you for your attention, Meir.
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