skin effect...Re: Riding t-loads, and other dangerous stuff.

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

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

 As a Tesla Coil
> finale, they brought in a faculty member who sat on an insulated platform
> (shown being erected:
> http://sprott.physics.wisc.edu/photos/wop/2000/016.JPG) and performed the
> "flaming fingertips" demo, as well as held a flourescent bulb,
> etc(http://sprott.physics.wisc.edu/photos/wop/2000/017.JPG).  Recall that
> this was for a crowd of 8-14y/o kids, primarily.  I don't recall if they
> used the "skin effect" bulloney (bulogna) or not......

Generally well respected authors have written about the "safety from skin
effect" in peer reviewed journals.  They should know better, but obviously
didn't actually do the calculations. Even worse, the reviewers and referees
should have caught it, but, they didn't.

As someone who works with designing and implementing RF systems on a daily
basis, skin effect is something that I deal with pretty much continuously.
However, I'm always working with good conductors, and pretty high
frequencies (30 MHz on up), and the "skin" really is a "skin" and very thin.
We agonize about surface oxide layers, how thin can the silver plating of
carbon fiber waveguides be, the effects on 25 micron wide traces in thick
film hybrids and PC boards, and stuff like that.  This is probably true of
most other folks who do this professionally: there just isn't much work on
hundred kHz signals in resistive media... and certainly true of the average
academic, except for folks doing subsurface radar or ELF propagation, no
research is being done on this area (for all intents and purposes, EM
propagation can be modelled very very well with analytical and numerical
models, and with low frequencies (long wavelengths), the simulation times
are very reasonable). For an academic (taking some license.. it's really not
this bad): No cutting edge research => no cutting edge publications => no
tenure or academic fame.

The result is that you (as an RF engineer) hear the words skin effect, and
immediately think in terms of your own experience: current in a thin skin in
a conductive thing, and never go and check to see if the assumptions are
really valid.  Add to this a bit of empricism (I tried it and I didn't feel
a shock (although, it wasn't skin effect, but the low pass nature of your
nerves)), and the error is cast in concrete...

Perhaps someone should write a peer reviewed journal paper (I don't know
which journal... perhaps something aimed at Physics teachers...) or even an
editorial column, about "The fallacy of "skin effect" and human skin"

It just goes to show... even "experts" in a field may not be right... Check
the math and be sure, especially if it is your own life on the line...

I should point out that an interesting discussion on this came up in an RF
safety class I took... Folks servicing aircraft on the deck of a carrier
were getting strange aches and pains in their ankles and wrists...It turns
out that they were providing a capacitively coupled path for RF (VHF
frequencies) from the aircraft skin to the deck of the ship, and the wrist
and ankle are high resistivity (lots of bone, not much water) compared to
the rest of the body, so significant heating was occurring.  No shocks or
tingle, etc.

No skin effect there... and the frequency was >30 MHz...