What came first, the coulomb or the amphere?

  • #1

Main Question or Discussion Point

I'm struggling to understand why 'amps' is a base unit. An amp is defined as one coulomb per second, correct? Then surely it isnt a base unit! It clearly relies on the coulomb. Like, if the coulomb didnt exist, or the amp was discovered before the coulomb was discovered, what would the amp be defined as?
Likewise with the coulomb, right... One coulomb is defined as the amount of charge that passes a point in one second when the amphere is one. But the thing is, how can you know what an amphere is if you dont know what a coulomb is? How can you know what a coulomb is if you dont know what an amphere is? Surely one of the units had to have been created first, but how can that be if one cannot exist without the other?
I'm just very confused and very frustrated. Ive been thinking about this for days and i still dont get it. Please please could someone just explain to me. It would be very much appreciated.
 

Answers and Replies

  • #2
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The ampere is not defined in terms of coulombs, you are right that this would create a hopelessly circular system.

NIST gives us a different definition:

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per meter of length.
This then depends on the base units for the dimensions of length, mass and time, which I believe are also carefully defined in order to avoid any interdependence.
 
  • #3
OmCheeto
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The ampere is not defined in terms of coulombs, you are right that this would create a hopelessly circular system.

NIST gives us a different definition:

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per meter of length.
This then depends on the base units for the dimensions of length, mass and time, which I believe are also carefully defined in order to avoid any interdependence.
Thank you!
I don't think about such things much.
I always think of an ampere as a volt divided by an ohm. :blushing:

I wonder if anyone has ever attempted to build an ammeter based on the definition.

(googles 10-7 newtons)

hmmm.... per yahoo answers, a grain of salt weighs ~6e-4 newtons.

My guess is no.
 
  • #4
SteamKing
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You have to e careful with Yahoo answers: The answers are voted on.
 
  • #5
sophiecentaur
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Thank you!
I don't think about such things much.
I always think of an ampere as a volt divided by an ohm. :blushing:

I wonder if anyone has ever attempted to build an ammeter based on the definition.

(googles 10-7 newtons)

hmmm.... per yahoo answers, a grain of salt weighs ~6e-4 newtons.

My guess is no.
The Ohm is definitely not a base unit. It's just the ratio of PD and Current.

A current balance is the standard way to measure current from scratch, I believe.
 
  • #6
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This is not the end of the story, though. Proposal have been made to re-define some base SI units, including the ampere. The new definition, if passed, will be in terms of the number of elementary charges per second. I understand the vote will happen next year.
 
  • #7
OmCheeto
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The Ohm is definitely not a base unit. It's just the ratio of PD and Current.

A current balance is the standard way to measure current from scratch, I believe.
But have you ever tried to picture a device to measure amperes based on the definition?
Can you imagine Fluke trying to sell 100 meter long(infinity is a long way), 1 meter diameter vacuum tubes, with little springs and levers to measure the sub-micro Newton forces?

Hmmm.... Come to think of it.

337px-Current_balance.jpg

Ampere balance ... in the National Museum of Nature and Science, Tokyo, Japan.

That probably wouldn't fit on my tool-belt either.

And looking at the definition of a volt, I don't have a clue how to even start building a volt meter.

A joule per coulomb? Who the hell is going to count out that many electrons?!!!!!!

I think I'm starting to understand the OP's confusion.
 
  • #8
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And looking at the definition of a volt, I don't have a clue how to even start building a volt meter.
As soon as you figure out a way to create a meter stick from the definition of the SI meter, it will be all downhill.
 
  • #9
OmCheeto
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As soon as you figure out a way to create a meter stick from the definition of the SI meter, it will be all downhill.
:bugeye:

Thank you voko.

And on that note, I will recuse myself from this thread.

-----------------------
ps. My thanks to everyone involved in this thread. I greatly appreciate being reminded of how little I really know. Seriously. :smile:
pps. And yes, I looked up the definition of a Candela. Holy [expletive deleted]!
 
  • #10
sophiecentaur
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Afaik, the SI definition of the Amp is based on the Current Balance.
Metrology is all a matter of the 'possible'. In the end, everything hangs on everything else and some quantities are easier to measure than others. Time and frequency can be measured with greater accuracy / precision / repeatability than other quantities so, where possible, they are used in the definitions of other quantities.
Before poking fun at the way some units are defined and measured, you need to remember that we seldom, if ever, actually use the 'defined method' as a basis for measurement. We use a chain of sub-standards. Any piece of lab equipment will probably not use the original measurement basis. We accept that quite happily.

The meter, based on a length of platinum in Paris, is a bit lame as a standard because someone on Mars will not have access to it. A definition based on the wavelength of an atomic energy transition is far better because all atomic isotopes behave the same (under the same conditions) and it is totally 'portable'. Basing the kg on a lump of platinum is also a bit lame. Much better to 'count out' a given number of atoms of one isotope, for repeatability, but the details are not straightforward.
 
  • #11
Thanks everyone, this helped a lot :)
 

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