Richard Harrison wrote:
Tom, W8JI wrote:
"Many people vizualize current in a small loading inductor as starting
at one end and traveling through the conductor turn-by-turn.

That`s how the experts say the coul in a TWT works, and it is no
different from other coils.
. . .

I maintain that there's no such group as "other coils", but that coils
act quite differently depending on their physical sizes and the amount
of coupling between turns.

I believe that current traveling down a straight wire goes at nearly the
speed of light. I also believe that if you take that straight wire and
wind it into a helix with very widely spaced turns, it also travels down
the wire at nearly the speed of light. But if you wind a helix that's
short in terms of wavelength and with a reasonable length/diameter
ratio, the field from the current in each turn couples to all other
turns, which makes the propagation axially from one end to the other
close to the speed of light. That is, for the same length of wire, an
inductor with closely coupled turns has a much lower propagation delay
than one with the turns spread out in a loose helix.

Do I infer from your comments that you believe that the current
continues to flow along the wire at about the speed of light, so that if
the wire length stays the same, the propagation delay along the widely
spaced helix is the same as for the short one with close spaced turns?
That is, that both these cases fall into what you categorize as "other
coils", which act the same as the helix in a TWT?

Roy Lewallen, W7EL