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Re: Skin Effect... and More(croft
Subject: Re: Skin Effect... and More(croft
Date: Sat, 03 May 1997 21:42:44 -0700
From: Bert Hickman <bert.hickman-at-aquila-dot-com>
Organization: Stoneridge Engineering
To: Tesla List <tesla-at-pupman-dot-com>
References:
1
Tesla List wrote:
>
> Subject: Re: Skin Effect... and More(croft
> Date: Mon, 28 Apr 1997 21:57:12 +0500
> From: "Alfred A. Skrocki" <alfred.skrocki-at-cybernetworking-dot-com>
> To: Tesla List <tesla-at-pupman-dot-com>
>
> On Mon, 28 Apr 1997 17:40:06 +1200 Malcolm Watts
> <MALCOLM-at-directorate.wnp.ac.nz> wrote;
>
> > > Malcolm, Alfred, and all,
> > >
> > > There's even more wierdness associated with Skin Effect! It turns out
> > > that some current actually does penetrate the conductor's interior. With
> > > increasing depth, the magnitude of this current decreases, AND the phase
> > > is continually retarded. What this really means is that it's possible
> > > that after a certain depth of penetration, current may actually be
> > > flowing in a direction OPPOSITE to that on the surface(!). In such a
> > > case, the effective AC resistance of the conductor would be decreased by
> > > removing the inside part of the conductor - a copper tube will show less
> > > AC resistance than a solid copper rod! This effect _actually occurs_ in
> > > high current busbars coming off the generators at 60 Hz in big power
> > > plants
> >
> > That is very interesting. I settled on a rule of 3 skin depths for
> > secondary wire diameter as it seemed that using much larger copper
> > was not only running into diminishing returns resistance-wise but also
> > reducing inductance unreasonably. For example, I calculated that a
> > total diameter of 8 skin depths was just about the practical limit
> > for reducing resistance but using that wire gauge, inductance is
> > slashed to around 1/4 of its value. Now it seems from what you are
> > saying that one would actually be going backwards by using wire that
> > large.
> >
> > Fascinating post. Any chance a copy of the book could be procured for
> > me?
>
> Book? What book?
>
> Alfred A. Skrocki
The book I was refering to was the very excellent "Principles of Radio
Communication" 3rd Edition, by John Morecroft, John Wiley & Sons, 1943,
pages 177-178. This 1084 page gem is definately worth getting if you
can. I'll repeat a bit of my previous post since this book is really
interesting...
"There's lots of other interesting things in this book as well...
excellent explanations of proximity effect, "resistance" of sparks and
arcs, "best form" for a single-layer solenoid (Diameter = 2.45 x Length
for maximum inductance), spark-gap oscillator characteristics, and a
whole lot more.
For example... on page 302, Morecroft references work done by Zenneck
and J. S. Stone which confirms Malcolm's observation that the decay of
current is, indeed, linear when a spark gap is used in an RLC circuit.
He presents a formula for the tank circuit current which is a function
of the maximum current multiplied by the factor (1-Rt/2L) where R is the
initial resistance of the spark gap, t = time, and L is the tank
inductance.
Morecroft also covers disruptively excited coupled tuned circuits and
their behavior versus k in one of the most lucid manners I've seen thus
far. He also covers the case of "impulse excitation" with a relatively
low-inductance primary - this interesting variation results in a
single, unidirectional pulse in the primary (NO oscillations!), causing
the secondary to ring at its characteristic frequency. Sort of like a
Marx-excited primary circuit. He also show an interesting variation
where the initial energy is stored in the primary inductor, instead of
the capacitor, and the input DC current supply is suddenly openned.
I'll follow-up on some of the other interesting things in this book in a
later post (once I've had a chance to really read more of it!). Add this
book to your list of Tesla Coiling references - its a definate winner!"
I've since located, and ordered the 2nd edition, but am still looking
for a copy of the 1st. Morecroft left even moer interesting
radio-teklegraph material in the earlier editions. Will post som eof the
more interesting things later!
-- Bert --