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How (does) voltage rise(s?).

Original poster: Chip Atkinson <chip@xxxxxxxxxx>

Greetings all,

My dad came over to help me work on a little tesla coil this afternoon.
I was explaining how the voltage rises in the secondary and kind of came
up with an explanation.  It seems to make sense so I thought I'd put it in
the pool of knowledge and see if it floats so to speak.

The first question was why one wants to loosely couple the primary and
secondary.  Why not just have a solenoidal primary enclosing the
entire secondary rather than a pancake or conical primary?

I drew out what I figured would be the magnetic field from a solenoid and
a pancake.  The solenoid field is more or less uniform within the coil
itself and loops from the ends of the coil back to the other end.  Kind of
like a "tall" torus.

With a pancake coil, the field is like a regular torus with the bulk of
the magnetic field affecting the base of the secondary, as opposed to a
solenoidal primary where the the field would entirely enclose the

With the uniform solenoid field the secondary is just subject to turns
ratio and the voltage doesn't rise any more than that.  There is no
"whipping action" where the voltage is more or less free to rise as there
is when only the base is driven as by a flat coil.

The next question in my mind is *why* does the voltage rise?  How can I
think of it?
Here's what I came up with -- the electrons are like a gas.  If you have a
column of air, say, and drive it at the base you can actually get louder
(greater voltage) output if you drive it at the resonant frequency.  For
example, a didgeridoo.  The thing though was that doesn't the oscillating
medium have to have momentum to resonate?  That's where I figured the
inductor of the secondary came into play.  An inductor conceptually gives
electrons momentum -- they keep flowing in their current direction and
resist changing directions.

The purpose of the capacitor at the top is mainly to store up this
pressure wave of electrons until the pressure (voltage) at the top reaches
the breakout voltage.  Then once it's broken out, the electrons are
suddenly released and produce a more energetic streamer or spark.  The
reason for not just having a bare wire at the top is that the point effect
of the bare wire allows the breakout voltage to be too low.  The reason
that we don't put a toroid the size of a box car on a 3" coil is that a
toroid that big would "soak up" all the surplus electrons and spread them
over such a large area that the voltage would actually drop from what it
was right where the wire attached to the toroid.

Does this make sense?  Is it right more or less?  What I'm trying to do is
figure out how to understand and thus explain it.  I'm not worrying so
much about accurately predicting the actual values but more in
understanding the concepts about what's going on.

I swear I haven't been smoking. :-)