Spark-gap sparks vs. solid-state sparks

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Kennan C Herrick by way of Terry Fritz <twftesla-at-uswest-dot-net>" <kcha1-at-juno-dot-com>

A number of people have commented, on the List and in separate papers, on
the seeming fact that spark-gap coils produce longer sparks than
solid-state or vacuum-tube-driven coils.  There has also been the
observation that central to that phenomenon is the rapid initial rise of
secondary voltage that is produced in spark-gap coils.  In those coils,
the voltage rises to spark break-out in a few cycles at most whereas in
the other coils, tens of cycles are required.

What I have not seen commented upon, however, is what I now think I see
as being the fundamental reason why this is so.  It appears to me that
the long sparks are engendered utilizing the same phenomenon that makes
nuclear bombs work; and that is, physical inertia.  And...a comment on
the bombs later.

In brief and inexactly put since I am not an expert, a spark, when it
breaks out of the top electrode, must push away air molecules, by heating
them, before it can progress.  Those molecules possess inertia: it takes
a significant amount of time to push them away and the energy that must
do the pushing comes from the top electrode.

It is the existence of that time interval that is the key:  The higher
the rate-of-rise of top electrode voltage prior to break-out, the higher
the electrode's voltage will be enabled to rise during that
inertially-created time period.  That voltage will so rise because the
spark is prevented from proceeding due to the air's inertia.  That it is
prevented from proceeding means that, briefly, its resistance does not
get added to the intrinsic resistance of the secondary, thus briefly
maintaining the secondary circuit's Q in its high state.

While the secondary's Q is thus relatively high, its current is
relatively high because its equivalent series L-C-R impedance is low, at
its resonant frequency.  Thus, its current efficiently continues to flow
into the top electrode and acts to elevate the voltage there far above
that which would otherwise appear at spark break-out.  By "otherwise" I
mean under the conditions imposed in solid-state or vacuum-tube coils for
example, where the secondary's rate-of-rise of voltage can be nowhere
near so high.

It is the much higher electrode voltage, accumulated during one or more
leading-quarter-cycles of excitation that occur during the
inertial-containment time(s), that accounts for most of the spark.  It
may well be that several consecutive quarter cycles are involved, and
that the spark grows step-wise during a number of cycles of
excitation--until the spark's added circuit-resistance diminishes the
secondary's current too much for that process to continue.

For a riveting account of such a use of inertia, read Richard Rhodes'
"Dark Sun", about the making of the hydrogen bomb.  In Chapter 24, Rhodes
provides a microsecond-by-microsecond account--complete with construction
diagrams--of the process by which the better part of 82 tons of solid,
gaseous and liquid material was turned into photons in the space of a few
microseconds.  It was only possible because of the inertia of its
components: they stayed together long enough for the numerous consecutive
nuclear processes to occur.

I see now that the challenge for s.s. designers such as myself lies in
attempting to emulate, to a degree at least, the rate-of-rise capability
of the common, garden-variety 19th-century spark-gap.  One has to chase
that spark and well-overtake it, so to speak.

Ken Herrick
________________________________________________________________
GET INTERNET ACCESS FROM JUNO!
Juno offers FREE or PREMIUM Internet access for less!
Join Juno today!  For your FREE software, visit:
http://dl.www.juno-dot-com/get/tagj.