Any scientists on MN? Why is the ampere rather than the coulomb considered an SI base unit?

[Tech]

I was just reading this story about the defnition of the kilo and wondered why the ampere (which appears to be defined as charge per second) is considered a base unit whereas a coulomb (which is a quantity of charge) is considered a derived unit.

Isn't charge more fundamental than current? Or is that not how they decide what is to be a base unit and what's to be a derived unit?

What is going on?

My guess is that it's because current is easier to measure than charge. To measure a charge you have to move it from one place to another, which effectively means you are measuring current. So although algebraically charge is more fundamental than current, as far as measurement is concerned, it's less fundamental.

Interesting question - researching this has diverted me for about an hour I really should have been doing something else, but that's what the internet is for isn't it!

Firstly, the ampere isn't defined as charge per second, the unit of charge (1 coulomb) is defined as the quantity of electric charge carried in 1 second by a current of 1 ampere.

In the 19th century there was the CGS (Centimetre, Gram(me), Second) system which had a number of subsystems for dealing with electricity including Electrostatic Units (ESU) and Electromagnetic Units (EMU). Both of these defined their units in terms of forces which could be measured using the existing units of length, mass and time.

So in ESU, 1 Franklin is the charge on each of two equal point charges spaced 1 centimetre apart when the electrostatic force between them is 1 dyne (1 dyne is 1 g.cm/s2). In this system, the unit of current is defined as 1 Franklin per second. If we had stuck with this one, it would have ended up 'your' way, with the unit of current defined in terms of Franklins.

In EMU, 1 Biot is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one centimetre apart in vacuo, would produce between these conductors a force equal to two dynes per centimetre of length.

Later, the CGS-EMU system evolved into the MKSA system with the only changes being decimal scaling (1 metre = 100 centimetres, 1 kilogram = 1,000 gram and (at last!) 1 ampere = 0.1 biots), whereas for reasons I haven't found out, the ESU system* became less favoured. When the SI Unit System was established in 1960, it was based on the MKSA system, and so we have 1 Ampere defined as the constant current which will produce an attractive force of 2E?7 Newtons per metre of length between two straight, parallel conductors of infinite length and negligible circular cross section placed one metre apart in a vacuum.

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• and other unit systems including Heaviside-Lorentz Units and Gaussian Units, which seems particularly unfair given that the whole CGS system was Gauss's idea in the first place!

Thanks both. I suppose I'd unconsciously assumed that a coulomb would be defined as the charge carried by X number of protons, or something like that. But I suppose that would be quite hard to measure, especially so in the 19th century. And I suppose defining the amp in terms of force, so in terms of mass - length - time, seems fairly fundamental as well. I must read up on some basic physics. The stuff I did at schools seems very hazy and long ago...

Well there was a system called Stoney Units, proposed at about the same time as the EMU system, which was based on the fundamental unit of charge. Unfortunately there was no way of measuring it at the time (which is probably why it never gained acceptance), and an estimate was made which was out by a factor of 16 (so that was probably just as well)!

The charge on the electron was not measured until more than 30 years later in 1909 (remember Millikan's oil drop experiment from school?). From this charge we have the Faraday which is the (absolute value of the) charge on a mole of electrons. It wouldn't be very useful to base an everyday unit system on this though: 1 Faraday is over 96,000 Coulombs so the laptop I am typing on would need a 30 microfaraday fuse!

Oops, that fuse would be a 30 micro[insert name of unit of current equal to 1 Faraday per second] one of course!