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Recently I've been wondering why the Ampere is an SI base unit and not the Coulomb (which is derived). I read this answer https://electronics.stackexchange.com/a/23456/16834 , but I'm not sure I'm understanding the discussion.

To me it seems like the Coulomb would be a base unit, especially because of the manner it is mentioned in the Wikipedia page, and because it is defined as the change of charge with respect to time; velocity, the change in displacement with respect to time is a derived unit.

Are there reasons that the Ampere is a better base unit than the Coulomb?

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hedgepig
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1 Answers1

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It's about what was a measurable quantity in the late 19th century. Counting ~1019 electrons would take a long time, but it's "straight-forward" to measure the force two wires exert on each other.

Also, consider that electric current was well-known and widely-studied for many years before the existence of electrons was known and their charge was measured. I don't know a date for the first observation of electric current, but Ohm's law was published in 1827, while the electron charge wasn't measured until 1908.

Since they were first established, we've changed our choice of fundamental units very little, and only as improved measurement technology has come along. At the moment it's still considered easier to measure the force on parallel wires than to count quintillions of electrons, so we still consider the ampere a fundamental unit and the coulomb a derived unit, defined as an ampere-second.

The Photon
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  • If it were technologically feasible to count out exactly (within e.g. 1ppb) one coulomb's worth of electrons, would kilogram need to remain as an artifact-based measure, or could one gram be defined as the amount of mass which, if subjected to the amount of force exerted by two parallel wires separated by some distance (defined by light and cesium) would have its velocity change by some fraction of a meter per second per second? – supercat Mar 26 '13 at 20:32
  • @supercat, It's possible to redefine the kilogram that way even without taking the coulomb as fundamental and the ampere as derived. See the Wikipedia article on the *watt balance*. – The Photon Mar 26 '13 at 20:36
  • Ah - so scientists needed an electrical unit, and chose to pick the simplest solution? http://en.wikipedia.org/wiki/File:SI_base_unit.svg shows that the SI base units are interdependent; by using the Coulomb instead of the Ampere would there be some sort of reduced interdependency? Also, wouldn't reduction of interdependency be beneficial? (Although maybe not nearly as advantageous as using something easy to measure.) – hedgepig Mar 26 '13 at 20:38
  • @inkyvoyd, I don't really understand the graph --- what do the arrows mean? To your other question, I think it's arbitrary what we choose as fundamental units, the main reason governments get involved in this is to establish weights and measures for commerce, so when you buy a kg of apples you really get 1 kg, etc. I personally do a lot more calculations with amperes than coulombs, so the choice of amperes as fundamental seems more convenient to me. – The Photon Mar 26 '13 at 20:45
  • The Avogadro project is in operation to replace the only remaining artifact based standard (mass) with Si sphere with a known purity and # of atoms (derived from measurement). – placeholder Mar 26 '13 at 20:46
  • Sorry, I meant to link to the article page instead of to the image - http://en.wikipedia.org/wiki/SI_base_unit. Besides what you just mentioned, are there any other reasons one would prefer using the Ampere rather than the Coulomb as a base unit? – hedgepig Mar 26 '13 at 20:48
  • @inkyvoyd, see the new paragraph I just added to my answer. – The Photon Mar 26 '13 at 20:52
  • @rawbrawb, Avagodro's number relates number of particles to mass, not to charge. A more exact number would be 6.24x10^(18) electron charges per coulomb. – The Photon Mar 26 '13 at 20:55
  • considering I measure electrons for a living you'd think I'd remember that. brain fart. – placeholder Mar 26 '13 at 21:00
  • @rawbrawb, no worries. When I saw the edit I thought you were going to capitalize "ampere" and "coulomb" like someone else did to me once. – The Photon Mar 26 '13 at 21:02
  • @ThePhoton: It sounds like the Watt Balance relates current to mass, and the Wikipedia article indicates that the present definition for an ampere is also based on mass (though measuring grams of AgNO3 seems strange--is someone playing a joke?) – supercat Mar 26 '13 at 21:18
  • @supercat, yes, the idea is to relate the kilogram to fundamental physical constants rather than an artifact in Paris. It's not exactly what you suggested. – The Photon Mar 26 '13 at 21:57
  • @ThePhoton: The definition of the ampere given in Wikipedia requires measuring something in units of grams; I would think an attempt to use a watt balance to measure mass would thus fail unless one had some other definition of current (e.g. number of electrons per second) which did not depend upon mass. – supercat Mar 26 '13 at 22:01
  • @supercat, From Wikipedia's *Ampere* article, the standard ampere "is in practice maintained via Ohm's Law from the units of electromotive force and resistance, the volt and the ohm, since the latter two can be tied to physical phenomena that are relatively easy to reproduce, the Josephson junction and the quantum Hall effect, respectively." But I'm not a metrologist so I may be misunderstanding it myself. – The Photon Mar 26 '13 at 22:26