Re: primary solid/ tube

["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>



> >
> > D = 2.602 / SQRT(200000) = 0.00581825 inches
> >
> > So even copper tubing is mostly wasted copper in conducting even LF
> > currents  ;-)
> 
> Well, the current doesn't just magically stop at that depth.  The actual
> skin depth is the depth where the current has dropped off by a factor of
> 1/e.  It does actually keep going.  Every skin depth more just decreases by
> another factor of 1/e.  The skin depth also tells you the "equivalent"
> conductor size to pass the current at DC.  i.e. a 0.00581825" conductor at
> DC will have the same resistance as a conductor operating at 200 kHz <IF>
> the depth of the copper is much larger than the depth of the copper, say
> approximately 10 times, the skin depth equivalence is just an approximation
> when that condition is true.
> This is all just for technical correctness, or at least what I remember of
> it.  The tubing wall is most likely greater than 0.05", and the
> approximation holds.  (this is all correct, isn't it? at least that's how I
> remember it all!)
> 
> > Basically, you want lots of external surface area and the interior copper
> > is of no use.  In all cases I can think of, go for copper tubing.

You are correct.. it is the depth at which the current has dropped to
1/e... And, if you were to integrate in an infinite slab, the contribution
of the part below the skin depth adds just enough to make it as if you had
a finite thickness equal to the skin depth with uniform thickness.

Hey folks, though, something else to bear in mind, particularly for thin
wire (where diameter is < 10 skin depths, give or take) is that the formula
above is for an infinite FLAT slab.  In a round wire, it's actually worse.
If you think of the current as trying to squeeze other nearby out, you can
see that in a round wire, the current on the "back side" of the wire is
also contributing, as is the wire in the sides, etc.

There are a bunch of tables around that give Rac/Rdc for round and tubular
conductors for various ratios of diameter to "infinite flat plate skin depth".