Re: SI and electric charge

["John S. Denker" <jsd@MONMOUTH.COM>]

Ludwik Kowalski wrote:
> SI resulted from
...
> and the idea of making electromagnetic units absolute
> (rather than standard-based).

There seems to be a persistent desire to identify some things
as "absolute" or "fundamental".  This is an understandable,
human desire -- but it's not good physics.  I can think of
an "absolute" unit of charge (the charge on one electron)
but it is not the SI unit.

>  Non of the above changes when
> C is introduced before A

Of course not -- but all this is a non-sequitur.  I hope we can
agree that
 *) Good metrology is not necessarily good pedagogy, and
 *) vice versa.


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I also think the history of SI is of marginal relevance at
best.
 *) The historical development of science does not parallel
the logical pedagogical development of science.

Furthermore, much of what has been said in this forum about
this history of SI is not correct.  But we don't need to
go there.

===================

> How many of us think that defining C before A is harmless?

Seems harmless to me.

> without changing the SI.

Again:  pedagogy is not the same as metrology, or history,
let alone the history of metrology.

====================
====================

> the idea of using four basic units instead of three,

Tangential remark:  The fundamental laws of physics don't
require any particular number of units.  In MTW (Misner,
Thorne, Wheeler _Gravitation_) they use centimeters for
almost everything:  cm of distance, cm of time, and cm of
mass.

Also:  Metrologists speak of "base units" in contrast to
"derived units".  There are various units that form the
"base" of SI, but these cannot be considered "basic" in
the sense of "fundamental" or "absolute".  They are
more-or-less arbitrarily chosen, which makes them about
as non-fundamental as they could possibly be.
 -- You could argue that the Rydberg is in some sense
fundamental ... but the volt is related to it by a
non-fundamental fudge factor.
 -- You could argue that the Bohr radius is in some sense
fundamental ... but the meter is related to it by a
non-fundamental fudge factor.
 -- You could argue that the Joule or the Joule per mole
is the logical SI unit of temperature ... but then SI turns
around and defines the Kelvin, which is related to the
Joule by arbitrary fudge factors.

To my way of thinking, derived units are derived from
the "base" units _without_ introducing any new fudge
factors in the derivation.  That means that the number
of base units is roughly equal to the number of arbitrary
fudge factors.  By this reckoning, SI has a whole lot
more than three or four "base" units.

======================================

Bottom line:  Here's how I would explain it:

The convenient and conventional unit of charge is the
Coulomb.  There's nothing fundamental about it.