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In article <4622@sfsup.UUCP>, dcm@sfsup.UUCP (David C. Miller, consultant) writes:
> In article <13355@cup.portal.com> mmm@cup.portal.com (Mark Robert Thorson) writes:
> >
> >At the risk of enormous flames, both here and hereafter, the fine wire inside
> >a flashbulb is magnesium wire.
> 
> If I remember correctly, the wire is simply aluminum and the bulb is
> filled with pure oxygen.  That way if the bulb is broken (either by
> accident or intent) it is rendered harmless.

	All of this recent speculation about the composition of flashbulbs is
interesting, but it is incorrect.

	The most common filament wire used in current flashbulb production
is an alloy of tungesten and rhenium.  Pure tungsten has been used in the
past.  Aluminum or magnesium wire would never be suitable for a filament;
such wire is too soft or brittle for use as a filament, in addition to having
too high a resistsnce for a 3 volt firing circuit.

	The filament is coated with a small amount of "primer" material,
generally a perchlorate mixture.

	The combustible "filler" material inside a flashbulb is generally
finely-shredded zirconium.  Magnesium and aluminum have been used in past
years for the filler material, but they do not possess the spectral power
distribution that is today ideal for color photography.

	During manufacture, a flashbulb is evacuated and heated to drive
out moisture (which is the #1 killer for flashbulb failure).  The bulb is
then filled with pure oxygen under pressure - typically around 100 psi.
The bulb is then coated with a protective lacquer to entrain any glass
should the envelope burst.  This lacquer is also used for color correction
purposes, which is why it is blue in color. Following lacquer coating, the
bulb is then coated with an anti-static material (yes, flashbulbs can be
discharged by static electricity, and in early years people were injured
when a flasbulb would fire after just being touched).

	When current is passed through a flashbulb, the filament heats
and ignites the primer.  Particles of the primer rapidly propagate through
the flasbulb, igniting the zirconium filler material, which then combusts
rapidly in the atmosphere of pure oxygen.

	Those of you with photographic expiernce probably know that a
flashbulb (like an AG1) typically reaches peak illumination in about
15 milliseconds, with the effective photographic illumination being
extinguished after an additional 25 or so milliseconds having elapsed.

	In case you are wondering, the Magicube flash lamp used in the
Instamatic X cameras are fired _mechanically_ by means of a torsion spring
which causes a striker arm to hit a percussion primer.
 
<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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