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Surface Charge
Hello,
I seem to recall some discussion on here a few days ago
that involved the static charge that tends to gather on the
insulating surfaces of tesla coils.
There is a simple explaination for this. It has to do with
the mass of ionized particles and the type of ionization they tend
to take on.
In a fluid, and air can be considered as a fluid medium
in some ways, conduction often takes place by way of ion exchange.
Positive and negative ions can exist in the same medium. For there
to be charge flow in this manner the ions themselves must travel
in the medium.
This is not at all like the break-down that produces
streamers, it is more like what happens in your car battery in the
electrolyte.
The interesting bit is that if the masses of the differently
charged ions is significantly different there will be a favoritism in
the charge flow due to momentum.
This effect is important if the medium is being subjected
to an oscillating current because it means the differing mass of
the ion charges will make charge conduction simulate the behavior
of a rectifier.
In fact I once saw a very old vacuum tube that was built
to take advantage of this effect. It was a large sealed pyrex envelope
with a pool of mercury at the bottom. There was a gas in the envelope
with the mercury. The gas would ionize with one polarity the mercury
with another and you got an industrial size crude rectifier.
It probably dated from the turn of the century or before.
On a tesla coil secondary the ionization is produced by the
corona from the terminal capacitance and the oscillating field around
the coil provides the conditions needed to separate the charges. The
most mobile ions gather on the insulating surfaces.
I have a coil that bites me with a quarter inch arc from the
varathaned surface of the windings whenever I turn it off and touch
the side of the secondary.
Hope I'm not repeating the obvious.
John