[Elecraft] Current Flow on Copper Strips - a Question

[Dauer, Edward]

    Since I don't know what precedes the word "thus" in the quotation below I would ask if someone could explain this phenomenon for me.

I am trying to visualize this with a thought experiment.  In an earlier post someone (Skip?) mentioned that early transmission lines were sometimes hollow copper tubes, to respect the fact that AC flows only on the outside of a conductor.  So, imagine a hollow tube carrying RF (which may approximate the fast rise and fall times of a high voltage strike).  Current is flowing all over the surface, I gather.  Now squeeze the tube along its length so that a cross section becomes an ever flatter ellipse.  At the last instant squeeze it so that the sides are in contact with each other.  What happens to the current flow as that squeezing occurs?  Is it still all around the squished tube until the instant the two sides join?  And then it all flows primarily along the edges of the now flat conductor?  Howcome?

Tnx, 

Ted, KN1CBR
    
    
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    Message: 3
    Date: Wed, 25 Apr 2018 18:45:39 -0700
    From: Wes Stewart <wes_n7ws at triconet.org>
    To: elecraft at mailman.qth.net
    Subject: Re: [Elecraft] OT: Grounding Question
    Message-ID: <a75f6274-fbad-d203-7874-245f94bf5f6e at triconet.org>
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    Radio Engineering and Radio Engineer's Handbook are two different critters.? My 
    copy of Radio Engineering is the third edition and the pertinent information is 
    on p.20.
    
    To partially quote: "Thus, with a flat-strip conductor, the current flows 
    primarily along the edges."