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Re: TC Electrosattics Revisited II
From: Greg Leyh[SMTP:lod-at-pacbell-dot-net]
Sent: Friday, September 12, 1997 4:22 PM
To: Tesla List
Subject: Re: TC Electrosattics Revisited II
John Berry wrote:
> >From : Essentials of electromagnetism
>
> The proportionality constants km and ke in the laws expressed by (1.1) and
> (1.2) can be fixed only when a unit of electric charge is decided upon. It
> would be quite natural to choose a unit of charge that makes ke equal to one.
> Since electric charge and electric current are directly related, no choice
> remains for the proportionality constant km. In the early history of
> electromagnetism this connection between the electric and magnetic forces was
> not appreciated. A confusing duplication of unit systems then developed by
> assigning values to both ke and km independently. The electric units for charge
> and current, based on (1.1), were then different from the corresponding
> magnetic units based on (1.2). Further confusion arose by the use of other
> practical units, having a size considered more suitable for measurement
> purposes.
> This confusion of electromagnetic units was a powerful impetus to the
> development of modern SI units based on the use of metres, kilogrammes and
> seconds rather than centimetres, grammes and seconds as in earlier systems. In
> SI units, electric charge is regarded as a new basic physical entity whose unit
> is the coulomb (C). Electric current is then measured in the unit of the ampere
> (A), which is equal to the passage of one coulomb per second. In practical
> situations, electric current can be measured much more accurately than electric
> charge. For this reason the choice has now been made to define the constant km,
> leaving ke to be determined empirically. By international agreement:
>
> "A constant current of one ampere maintained in two parallel, infinitely long,
> straight conductors of negligible cross-section separated by one metre in
> vacuum, produces a force between these conductors of 2 x lO-7 newtons per metre
> of length."
John,
So is it correct to conclude that, under the SI unit convention, the Ampere is
determined empirically, and the Coulomb is defined as one ampere-second?
-GL