# What is the original definition of a Coulomb? **URL:** \ **Category:** Factual Questions\ **Created:** [January 9, 2024, 11:30pm UTC](https://boards.straightdope.com/t/what-is-the-original-definition-of-a-coulomb/995863 "2024-01-09T23:30:41Z")\ **Posts on this page:** 9\ **Page:** 2
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [January 10, 2024, 8:52pm UTC](https://boards.straightdope.com/t/what-is-the-original-definition-of-a-coulomb/995863/21 "2024-01-10T20:52:15Z")
OK, so what was the original definition of an Amp? I mean really original. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 10, 2024, 10:10pm UTC](https://boards.straightdope.com/t/what-is-the-original-definition-of-a-coulomb/995863/22 "2024-01-10T22:10:22Z")
The original definition was the one based on the force between two wires. You can tell because that’s the definition that doesn’t have any “magic numbers” in it: All of the numbers are nice and round. Similarly, the original definition of the meter was based on the circumference of the Earth. The definition of the meter got changed pretty quickly, because a standard based on the circumference of the Earth isn’t very practical. But when they made the new definition, they carefully picked numbers that made the new definition as close as possible, to the extent they could measure it, as the old definition. And likewise for each subsequent time they redefined it, and for all of the other units they’ve redefined. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [January 10, 2024, 10:25pm UTC](https://boards.straightdope.com/t/what-is-the-original-definition-of-a-coulomb/995863/23 "2024-01-10T22:25:04Z")
If we leave aside the problem of practical realization 🙂 recall that we start with standards of length, time, and mass. Then apply Coulomb’s/Ampère’s/Ørsted’s/… laws and you’re done 🙂 You may need to measure the speed of light at some point… For instance, in the cgs system of units, one unit of electric current is that which, along a circular path of radius one cm, produces a field of 2π oersteds at the center. Or, more simply \text{current} = \frac{\sqrt{\text{length}\cdot\text{mass}}}{\text{time}} in “electromagnetic units”, as explained here: [https://en.wikisource.org/wiki/1911\_Encyclopædia\_Britannica/Units,\_Physical](https://en.wikisource.org/wiki/1911_Encyclop%C3%A6dia_Britannica/Units,_Physical) ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [January 10, 2024, 11:34pm UTC](https://boards.straightdope.com/t/what-is-the-original-definition-of-a-coulomb/995863/24 "2024-01-10T23:34:43Z")
> [@beowulff](#): > > I remember hearing about how a Farad-sized capacitor would be as big as a milk bottle when I was in high school. Now you can get 10F capacitors that are the size of a AAA cell. When I was in college (mid 50s) 1 farad of capacitance occupied a small room. It consisted of 2000 electrolytic capacitors, each 500 microfarads. It was part of the power supply filter for the Univac I computer. I sometimes wondered if they couldn’t have used a large storage battery. A natural unit for charge would be the charge on an Avogadro’s number of electrons. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [January 11, 2024, 2:00am UTC](https://boards.straightdope.com/t/what-is-the-original-definition-of-a-coulomb/995863/25 "2024-01-11T02:00:55Z")
> [@Dorjan](#): > > very small resistance (say 0.1 ohms) That’s not very small compared to what 1 F electrolytic caps are capable of. Consider a 1 F, 6.3 V cap from this list: > **[CGS.pdf](https://www.cde.com/resources/catalogs/CGS.pdf)** ESR of 0.005 ohms (also, 3" diam and 5.6" length, so soda can is in the ballpark). That’s potentially 1000 A at 5 V, so getting interesting. Supercaps typically have much higher ESR. There’s a cost to being tiny. Of course, licking any of these wouldn’t do anything, because your saliva/tongue isn’t particularly conductive. ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [January 11, 2024, 2:16pm UTC](https://boards.straightdope.com/t/what-is-the-original-definition-of-a-coulomb/995863/26 "2024-01-11T14:16:11Z")
So the definition of an Amp was the current between the 2 wires 1 metre apart so that they have a force of 2 x 10-7 N between them. Was this force chosen so that the current would be such that J/C would give a volt answer that was a nice practical number? This is the idea I’m getting from reading this thread. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [January 11, 2024, 4:17pm UTC](https://boards.straightdope.com/t/what-is-the-original-definition-of-a-coulomb/995863/27 "2024-01-11T16:17:37Z")
The _ampere_ was chosen to be convenient (this applies to the volt and ohm as well; all the committee reports and electrical engineering conference proceedings and so forth are online in case we want to track down all the references), instead of a unit 10 times as big or 1000 times smaller. Wires 1m apart because SI uses metres. Force then comes from Ampere’s law; there is a coefficient there (technically the magnetic constant divided by 4π) that eats the powers of 10 resulting from this particular scaling of the units in case you were wondering what 10−7 is doing there ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [January 12, 2024, 12:48am UTC](https://boards.straightdope.com/t/what-is-the-original-definition-of-a-coulomb/995863/28 "2024-01-12T00:48:33Z")
> [@Cardinal](#): > > Was this before or after the definition mentioned by DPRK? After. It is a repeatable experimental definition. Doing the parallel wires measurement in reality is not going to be trivial if you want the level of accuracy a standard requires. Accounting for all the stray magnetic influences on your force balance is not going to make for an easy day. And we now find ourselves back at that end of the definition space, defining the ampere and coulomb in terms of the charge on an electron. Albeit with a lot more precision. ---
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [January 12, 2024, 3:33pm UTC](https://boards.straightdope.com/t/what-is-the-original-definition-of-a-coulomb/995863/29 "2024-01-12T15:33:08Z")
> [@Francis\_Vaughan](#): > > Accounting for all the stray magnetic influences on your force balance is not going to make for an easy day. And barely any force is there to begin with. Newtons to grams-force is about 100:1 so the force on the one meter, one ampere experimental apparatus is about 20 micrograms-force. [Previous page](https://boards.straightdope.com/t/what-is-the-original-definition-of-a-coulomb/995863.md?page=1)