Re: GB> Chaffee Spark Gap

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

Original poster: "Michael H Nolley by way of Terry Fritz <twftesla-at-qwest-dot-net>" <nolleym-at-willamette.edu>

>
>
> Hi
> sorry to play the dummy here, but this all sounds really interesting,
> could you run down a couple of things to a non-expert? like describe
> basically what a poulson arc is and does, is it similar to a quench gap?

    The Poulsen Arc was an early attempt at a continuous wave radio, using
a specialized arc.  The vacuum
tube, which was developed only a short time later, totally eclipsed the
Poulsen Arc, being far more
efficient and reliable.  The fact that the Arc never entered into major
usage by an amateur community also
led to its relative obscurity.
    The arc itself is relatively simple in construction, and it has certain
resemblances to the tesla tank
circuit we know of, in that it involves a spark gap and a capacitor.  The
Poulsen arc emerged out of the
chance union of spark gap radio technology and Arc lighting technology,
which utilized carbon rods and very
high currents.
    Arc modulation emerged as a possibility by accident, when Duddel
discovered that carbon electrodes,
when connected in series with a DC power supply,  capacitor and tank
circuit, made a "whistling"
noise--indicating that the usual disruptive action the "rattatat" of the
spark gap was not occurring.
Apparently, the use of carbon as an electrode intensified what is called
the "negative resistance"
characteristic of arcs.  Effectively, this means that the voltage and
current are inversely proportional.
If the voltage becomes high, the current falls, and if the current rises,
the voltage falls.  This creates
an oscillatory condition which can, under the right circumstances, be used
to create an rf signal in the
hudreds of kilohertz.  The whole goal of the Poulsen arc is to capitalize
on this effect and to create
various means by which the oscillation can be supported at very high power
levels.
    Typically, this involves using large, water-cooled electrodes of carbon
and copper (the carbon being
only necessary for the negative electrode) a magnetic quenching system
using large inductors which are
powered by the source current, (typically 500V at high amperages) and an
alchohol atmosphere, which when
heated by the arc, releases hydrogen.  Hydrogen is by far the best
insulating gas for use in spark gaps,
because of its extremely high thermal conductivity and good dielectric
properties.  The arc also uses
additional charging inductors which both prevent runaway current and
reinforce the oscillatory effect.  I
don't know very much else about the exact workings of the Poulsen Arc--
i.e, what the exact train of event
is (perhaps someone versed in microsim etc. might look at this).

>
> And the probable environment in the gap in the article? It said moist
> hydrogen in nitrogen, if I remember correctly? Is that at sea level pressure,
> or more, or less? what would be purity concerns, (besides eliminating
> the o2!)

    Purity isn't a huge issue, and ironically, neither is the O2.  In most
Poulsen systems, the main gap
would be enclosed in a metal container into which the alchohol vapor was
introduced.  At the start of the
arc, the minor explosion that resulted was let off by a small safety valve,
at which point the alchohol
vapor could build up again without reacting with oxygen.  Apparently, the
reason hydrogen is less dangerous
than other gases when reacting with oxygen is its low atomic number.  In
other words, it is extremely
light.  If one were to try to use acetylene instead of hydrogen in the
Poulsen arc gap, one would have a
very dangerous situation, because Acetylene is highly saturated with
carbon.  It has much more potential
energy and potential byproducts per cubic volume.
    If one doesn't want to use an enclosed container, this can be avoided
by using an alchohol lamp.
Arrange the alchohol flame so that the unburnt section (the blue part) is
orthogonal to the arc.  This is
only practical in small arcs.

> Would the alcohol vapor have to be pure alcohol, or would this too be
> in nitrogen?

Ethanol is what is usually used--denatured alchohol for us.

> Could you "prime" such a gap by running  a spark through
> alcohol and air with an outlet hole and burn off the 02, then seal it
> and run it?

    Yep.


> It seems that if small enough a cavity is made, then the
> amount of alcohol needed would be very small (teaspoon at a time)? If
> list members are shooting hair net into the backs of spud guns for four
> seconds (tee hee), it couldn't be that dangerous, could it?

Yep, usually the arcs would introduce the alchohol a drop at a time.
    If you are interested in Poulsen Arcs, type it into your average search
engine.  There is a fair amount
of information out there--and its where I got all the information I just
told you.  If you would like
details about how I designed my arc, you can email me at
nolleym-at-willamette.edu.
    --Mike