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.
--
Vic Roberts
http://www.RobertsResearchInc.com
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