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Re: Magnetic N and S poles



Ludwik Kowalski wrote:

On the next page the unit Ampere is "DEFINED
AS A CURRENT WHICH, WHEN FLOWING THROUGH
A CIRCULAR COIL OF 10 TURNS AND 10 CM RADIUS,
WILL PRODUCE AT ITS CENTER A MAGNETIC FIELD
OF STRENGTH EQUAL TO 2*PI DYNES." I suppose that
"the unit pole" was implied in this definition.

1) No, it doesn't imply a unit pole. All it requires
is another loop to measure the effect of the field
created by the first loop.

Such a setup is called a Rayleigh current balance,

http://nvl.nist.gov/pub/nistpubs/sp958-lide/056-058.pdf

2) The last I knew, it was the main operational determinant
of what an ampere is. All the other electrical units
are, in turn, derived from the ampere; for instance,
a coulomb is an amp*second.

http://physics.nist.gov/cuu/Units/units.html

At this level of detail we can see that it is clearly
wrong to think that the magnetic field is defined in
terms of the ampere. The opposite is closer to the
truth! But even that's not the whole story.

There is no cosmic significance to this way of
determining things. It is just that the Rayleigh
current balance turns out to be more
accurate than the alternatives, such as determining
the coulomb using a Faraday precipitation cell and
deriving the ampere as one coulomb/sec.

In fact, metrology is more complicated than this.
What they really do is make a number of different
measurements that provide multiple interlocking
constraints on the various unknowns. Then they
do a big multivariate error analysis (chi square
and all that).

Imagine a huge geodesic dome with all sorts of
extra struts and cross-braces. Everything is very
highly overdetermined and overconstrained.

The notion of one quantity (e.g. magnetic field)
being "defined" in terms of one or two others
by some special formula is untenable. That is
why I keep saying: If somebody asks for the
"definition" of magnetic field, suggest a better
question, such as: "what are 3 different ways
of measuring a magnetic field in terms of something
else you know".