[lyner]

sorin

Does a direct current flow only at the surface of conductor?

Are you trying for a reductio ad absurdum argument here?
Current never flows just 'at the surface' - there is always a skin depth.
There is no such thing as a true direct current - it will have a finite period of flow (it's been switched on and off for the 'experiment' and the reactive properties of the circuit will impose finite rise and fall times). That implies a non-zero frequency content. That implies a finite skin depth. Nothing absurd at all.

[Pumblechook]

I think skin depth is inverse prop' to the sq rt of the frequency so at very low frequencies the depth tends towards infinitity so will be much deeper than the diameter of any  practical conductor. 

[sorincosofret (OP)]

It is premature to discuss about alternate and continuous currents. For the moment actual theory does not provide why a alternate current tends to flow on the surface of the conductor (with a certain depth in the conductor volume) and why this depth is decreasing with frequency.
In fact for actual theory is quite indifferent if a electron is oscillating with 50 Hz or with 50 MHz.
In a second in both situation the electron should make 50 or 50 M oscillations around a equilibrium position. In the same time in absence of ,,alternate" current a electron run 1 mm in a second. So from macroscopic point of view a oscillation with 50 Hz or with 50 MHz should have the same effects on the electron movement. More precisely the electron does not follows the electric field variation. In this case how the alternate current propagate in the conductor?

[lyner]

Of course an electron does not follow the em field exactly - there is a  phase lag as the electron needs to accelerate and this takes time. Is that new to you? Have you done any work on the behaviour of oscillating masses under periodic forces?

It really would be a good idea if you actually read something substantial about this and didn't try to become an expert by skim-reading some simplified text. You claim to have understood and rejected what advanced texts have to say but you ask questions which demonstrate that you haven't understood them.
Have you not read anything about skin depth? What 'actual theory' have you been reading?
Is your knowledge only skin deep?