I can't see very well the insulators in the picture but sometime the
stacked wafers are to effectively increase the arc gap.
"why do they look like springs?" I don't know. Why do some clouds look
like Mickey Mouse?
The old ones were modular and one insulator section could be replaced
out of the string. Probably still are.
The shape of a single section is curved on top and prestty flat with
circular waves on the bottom. Worst leakage is on the surface when the
insulators are wet. The shape greatly increases the surface path
length. The curved top tends to keep the flat underside dry.
I can recall as a young man camping under the power line and on a
damp night we could hear arcing.
What you heard was "corona" It is a situation where there is local
ionization of the air -generally at points (even raindrops on the
conductors) but it is localized by weaker fields away from these points so
that a conductive path is not made between conductors or conductor to ground
and there is no arcing. However
It does cause glowing streamers etc and produces both audio and RF noise as
well as ozone.
If you test an insulator in a darkened lab- you will see a glow and hear
noise and this will increase as you raise the voltage- then you may get some
quick flashes over the insulator if humidity is high but there is a fine
line between this state and flashover leading to an arc which has as high a
current as the source will allow.
With a uniform air gap- the conditions that cause corona will cause a
flashover and an arc will follow.
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Another way of putting it is that there is no completed metallic
conductive path. The circuit is completed using Maxwell's displacement
current. Charge is mobile where the air breaks down but not over the