Re: novelty bulb

[William Beaty <billb@ESKIMO.COM>]

I have several of these, purchased at Spencer Gifts at the local gigantic
mall.  A search on "flamingo bulb" finds this collection of bulbs:

  http://www.prnovelty.com/Flamingo_Bulb.html


On Wed, 24 Jul 2002, Barry Gragg wrote:

> I find it curious that
>
> 1. the discharge glow is produced at such a low voltage, compared to the
> 2000 volts in a gas tube ballast.

It's like a large version of a neon pilot light.  With neon signs, the
larger the distance between electrodes, the larger the voltage required,
but there is a fairly low minimum voltage to strike an arc. The base of
these bulbs becomes warm, so they probably have a current-limiting
resistor in the base.


> 2. the orange glow around the elctrodes is uniform across the length of
> the 40 mm wide electrode, even though its separation from the other
> electrode varies from about 4mm to 20mm.

I find gas discharge tubes fascinating.  They have some analogy with
electrochemical cells in that the various layers of glowing gas have
potential difference across them, and the ionized gas behaves somewhat
like an electrolyte.  In a DC Geissler tube, the cathode is enveloped by a
thin glowing layer called "Cathode Glow", which in turn is enveloped in a
larger dark layer called "Faraday's Dark Space,"  and the Cathode Glow is
separated from the cathode by a very thin "Crooke's Dark Space."  The
Cathode Glow supports a constant P.D. called the "Cathode Fall" (of
potential,) as if the cathode glow itself was a high-resistance material.
I think it's roughly analogous to the Helmholtz layer in a battery.  As
long as the positive electrode doesn't intrude into the cathode-glow
region, the cathode glow can uniformly cover the entire negative
electrode.  If the current of the power supply is severely limited, then
the cathode glow self-organizes into a tiny disk-shaped region called the
"cathode spot."   See PIRA 5M20,
http://www.physics.ncsu.edu/pira/5eandm/5M20.html

With these glowing bulbs, by applying AC we make the "cathode glow"
occupy each electrode surface alternately.  Try connecting a 200V power
diode in series with the bulb and see what happens.


> 3. there is no discharge 'ray' between the orange electrodes at their
> point of least (4 mm)separation.

I think that a "ray" would exist if you could remove the series resistor
and crank up the current to a very high level.  But I'm not sure.  If you
could separate the electrodes and then increase the voltage, a "ray" would
certainly develop.  That glowing region appears when electrons have enough
space that they can accelerate to a speed where they ionize gas molecules
during collisions.  In a DC Geissler tube, this "ray" is called the
"positive column" or "striated positive column."  In a neon sign, it's the
positive column which lights up the tube, and during a spark it's the
positive column which lights up the arc channel.

When observing sparks from VandeGraaff machines, I think you can see the
entire "Geissler tube" plasma structure, including a cathode glow, a
Faraday dark space, then a long bright positive column.


(((((((((((((((((( ( (  (   (    (O)    )   )  ) ) )))))))))))))))))))
William J. Beaty                            SCIENCE HOBBYIST website
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