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Re First Tesla coil, skin effect formula and wire guages

Subject:  Re First Tesla coil, skin effect formula and wire guages 
   Date:  Wed, 23 Apr 1997 22:09:06 -0400
  From:   "Owen Lawrence" <owen-at-iosphere-dot-net> 
    To:   "Tesla List" <tesla-at-pupman-dot-com>


> You might try a neon sign transformer instead of the 2N3055 ocsillator
> driving a flyback xmfr.  This setup will only produce 1-2 ma of drive
> current.  Usually you need around 30-60 ma of drive at 12 Kv which 

Thanks for your response.  What assumptions are being made to arrive at
these specs ("need 30-60ma at 12KV")?  I intend to build a small (but
hopefully effective) coil to begin with.  If I know ahead of time I only
have 1-2ma to work with, I should be able to design around that.  

> A good suggestion not to use a pole pig until you have built a few
> smaller
> units first.  Pole xmfrs are extremely dangerous and will fry you in an
> instant.  

I intend to stay very far away from them until I know what I'm doing. 
At
this point I consider neon sign transformers very dangerous.  I'm just
curious how you folks go about procuring one of these bigger ones.  If I
don't know what they're worth I won't know if I stumble across a good
buy
and should grab it for one of you, if not for myself.  

        I called a motor shop yesterday for a price on wire.  I was told
it's
$16(Cdn)/lb, no matter what guage.  So I found some tables of values
adopted by AIEE in January, 1914 and still in effect (as of 1977)
showing
the following:

AWG     ft/lb   ohms/1000ft  diameter(mm)
32       5160   162           .203
30       3300   104           .254
28       2080    65.3         .320
26       1310    41.0         .404
24       818     25.7         .511
22       516     16.2         .643
20       323     10.1         .813
18       256      8.05       1.02
16       128      4.02       1.29
14        80.4    2.52       1.63
12        50.6    1.59       2.05
(for amusement:)
56     1 380 000  43.2k       .012  (over 261 miles/lb!)

Sorry I can't quote the reference, I just have photocopies.  I CAN get
it
from a friend if you're interested.  The table includes all values from
46
down to 4/0, and even values up to 56.  I thought these might be
pertinent.
 If you're interested in a guage not shown let me know.  There are lots
of
other formulas such as relation between temperature, conductivity and
resistivity of copper, but this is getting pretty picky for me.

        While looking up this table, there on the previous page was the
formula
for skin effect, which I recall someone asking about recently.  
Here it is:

"The current intensity falls off exponentially with depth.  The depth of
penetration (s=sigma) is the depth at which the current intensity has
fallen to 1/e of its value at the surface, where e equals 2.718...

At three- and four-times the depth of penetration, the current intensity
is
5 percent and 2 percent of that at the surface respectively.

Where the diameter of the conductor is large compared to the depth of
penetration, the total current is the same as if the surface current
intensity were maintained to a depth of penetration.

    s = 503.3sqrt(rho/(urf)) millimeters

    rho = resistivity in ohm-meters
        = 1.72x10e-8 for copper
        = 2.83x10e-8 for aluminum
    ur  = mu r = relative magnetic permeability
        = 1 for both copper and aluminum
     f  = frequency in magahertz

For example, for copper the depth of penetration is as follows:

   MHz        Depth of Penetration sigma (mm)
    .1          .209
    1.          .066
    10.         .021
    100.        .0066
    1000.       .0021

Sorry, .1 is the lowest frequency it gives. Anyone know the quanities of
skin effect for skin?

I hope this information is of use.  

  - Owen -