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Re: sparks

"John S. Denker" wrote:

The real story has to do with ions. Once the air is ionized, the ions stay
in the vicinity for many milliseconds before they recombine. If there is a
field, the ions carry a current. The moving ions undergo violent
collisions with air molecules, which produces more ions, so the arc can
sustain itself under a wide range of conditions. Under a fairly-wide
subset of these conditions, if you manage to extinguish the arc, it will
stay extinguished.

These phenomena have very great practical importance, because every switch
and every circuit breaker must deal with this. It does not suffice to move
the contacts apart; the hard part is to extinguish the arc. ... I am
fascinated by the tricks they use to switch the really high-power circuits at
power plants and substations.

I just talked to an electrical engineer friend about this topic. He had some
interesting things to say. In order to break the arc the contacts must be
separated and then moved an appropriate distance apart. The ionized air has
some resistance and therefore greater distance means greater resistance. In
order to increase the distance of the path of ionized air, sometimes fans are
employed to move the air. Sometimes magnetic fields are used for the same
purpose. The magnetic fields or fans it seems cause the arc to deviate from
its relatively straight line path and travel in an arced path increasing the
resistance because of the longer path without physically separating the
contacts as far as would otherwise be necessary.


Cliff Parker

Never express yourself more clearly than you can think. -- Niels Bohr