Re: Experiment - Displacement Current's Magnetic Fields

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

I wrote:
> Reliance on belief is nearly always fatal for
> progress - Malcolm's ruler, ...

Malcolm wrote:
> I hope a crude and early attempt at modelling the resonator
> through a mechanical analogy is not taken by anyone as a
> doctrine.

Well I didn't mean to suggest that the flexing ruler analogy was
cranky or doctrine, but it works well as an illustration of how
these beliefs become established. The ruler voltage profile would
not have passed some fairly basic sanity checks, yet it was
beginning to find itself used as an input assumption in computer
programs, etc, in a way similar to the use of 1/4 wave wire length
and DC inductance in Fres calculations, which become established
simply by going unchallenged.

If you need a voltage profile and you don't have one, you have no
choice but to guess one.  This often happens in physics and you just
have to do the best you can to qualify your guess with whatever
cross checks that you can.

In the case of the ruler analogy, the rising voltage gradient 
towards the top of the coil is immediately suspicious, since it
implies not only a lot of current in that part of the coil (which
is incompatible with a reasonable guess at the capacitance profile),
but also implies a vastly increasing coil current as you approach
the top, which defies explanation.  A safer and justifiable guess
would have been a sine.
--
Paul Nicholson
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