Re: primary solid/ tube

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

Original poster: "Sean Taylor by way of Terry Fritz <twftesla-at-qwest-dot-net>" <taylorss-at-rose-hulman.edu>

> For copper, the depth of the RF currents is "generally" given as:
>
> D = 2.602 / SQRT (F)
>
> Where:
>
> D = The depth in a copper conductor RF currents travel in inches.
> F = Is the frequency in Hz.
>
> So if we have a typical Tesla coil operating at around 200kHz the depth of
> the RF currents is only:
>
> 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.

I would agree with the use of copper tubing, don't know about cost, but for
equal diameter, there is less weight, less material, and its easier to work
with.

Sean Taylor