On Tue, 21 Jul 2009 19:40:49 +0100, Arfa Daily wrote:

"Nicholas Bodley" wrote in message
news
On Thu, 16 Jul 2009 17:38:16 +0100, Arfa Daily wrote:

I don't think I have ever had a domestic incandescent light bulb
[...]

This commentary reminded me of a quite-unpleasant experience in late
boyhood. Apparently, an incandescent bulb in the cellar had a film of
moisture on it (it had just been turned on). When I touched the glass,
I got a nasty shock; damp concrete floor, etc. The socket must have
been wired with its threaded shell to the hot side of the line.

Not the bulb's fault, of course.

Btw, hot glass is conductive, just about certain.

Regards,
--
Nicabod =+= Waltham, Mass.

Well, if it is, and I rather suspect it's not, the amount of
conductivity is very small. I work a lot with power valves (tubes) which
run hot enough to take your fingerprints off in decimals of a second,
and these often have as much as 800 volts on the anode pins. Some years
back, I used to work with high power audio distribution amplifiers on a
community radio network. The (foot high internally assembled with nuts
and bolts) tubes on these 7 foot tall amplifier cabinets, often used to
run so hot that the glass was actually glowing dull red. I have seen
dents in the glass where the vacuum has started sucking on areas that
have got *so* hot that they have softened. The HT supply on these beasts
was about 2.5Kv as I recall, derived from mercury vapour rectifier
tubes.

Given these facts, if the glass had much conductivity when hot, you
would pretty soon be into having leakage paths between the pins, and to
the grounded retainer metalwork, that holds the devices into their
sockets, wouldn't you ?

Arfa

Sorry for the delayed reply. If the glass is hot enough to soften, it's
probably conductive to some extent. However, if surrounded by cooler
glass, its resistivity would be too high to matter.

Try this, but be aware that you're dealing with lethal voltages, high
enough to jump a short gap (1 cm or so). You, specifically, most likely
know how to protect yourself! If in doubt, others should not try this.

Take a [neon] sign or oil burner ignition transformer (the latter is
probably better) and arrange temporary electrodes (or use those in a
discarded oil burner) to create a steady arc. That arc is like a flame --
it will melt and ignite materials. place a piece of well-insulated glass
in the arc (safety glasses!), and let the arc heat the glass. When it
starts to glow, or possible a bit sooner, it will become conductive;
you'll see the arc divert so current passes through the glass.

As to your power tubes/valves, you might have been lucky.

Regards,

--
Nicabod =+= Waltham, Mass.