# Re: Skin Effect & Primary Current?

```Kevin wrote:
---------------
Gentlemen,

> I'm not trying to be PC, but -no women on this list?

I recall someone figureing how deep the penetration was of
the primary current into the conductor. Could someone who
knows or remembers how to figure it, please tell me.

>
The skin depth is defined as:

d= square root [ 1/(pi*mu*c*f)]

where (SI units)

d=depth in meters
pi=3.1415...
mu=permeability of material in Henries/meter
c= conductivity in 1/(ohm-meters) (not a DVM!)
f= frequency in Hertz

For copper mu is about the same as for vacuum since
copper is not very magnetic. So use mu=4*pix10^-7 H/m

The conductivity of copper can vary considerably
depending on purity and temperature, but one reference
suggests c=1/(1.7x10^-6 ohm-cm) = 5.9x10^7 1/(ohm-meters)

for a typical f=100kHz

I get

d=2x10^-4 meter
=0.02cm
=0.0082 inches

Of course, it is not a good design rule to assume
no currents flow below this depth. Nor is it true
that currents always flow uniformly along the surface of
a conductor with this given depth at the prescribed