On Wed, 22 Jul 2009 09:47:11 -0400, Victor Roberts wrote:

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


Ordinary incandescent lamps are made from soda lime glass, which has
enough conductivity at 100C to maintain a low frequency (DC or 60Hz)
discharge in T12 fluorescent lamps using electrodes on the outside of
the bulb. The place where the wires are sealed in an incandescent lamp
(the pinch seal) is made from lead glass, that has much lower
conductivity than soda lime glass. Vacuum tubes are usually made from
borosilicate glass (Pyrex(R)) that has even lower conductivity than lead
glass, and much lower than soda lime glass.

Aha! In the experiment I just posted a message about, I was using a glass
rod, perhaps for stirring mixed drinks. It might well have been soda-lime
glass. While typing, I was wondering about the conductivity of various
kinds of glasses. Thank you, kindly, for the enlightenment.

I s.t.r. reading, a long time ago, that Corning Glass used the
resistivity of molten glass to keep it hot by passing a lot of current
through it. I don't know how it mas heated to begin with, though.

Best regards,

--
Nicabod =+= Waltham, Mass.