Re: Carbon dioxide bike tyre inflation (was Re: That was dumb..)

On Sun, 25 Nov 2007 02:39:16 GMT, nobody@xxxxxxxxxxxx (Scott) wrote:
On Sat, 24 Nov 2007 18:24:22 -0500, in alt.sysadmin.recovery,
<jferg@xxxxxxxxxx> wrote:

In case you haven't already worked it out - it was because the hotel
was too cheap to pay for 'three-way' bulbs. Many lamps on this side of the
pond are designed to take bulbs which have two filaments of different
wattage, so as to provide 3 different levels of brightness. (i.e.
30-watt, 70-watt, or (both on) 100-watt). When you put a normal (single-
filament) bulb in, you end up with 2 off positions and 2 on positions.

Considering the geometry of a 3-way switch base (an extra contact ring
between the threaded and tip contacts), I'm vaguely curious why seemingly
all 3-way switch bases demonstrate the same behavior when loaded with a
normal lamp, e.g. two positions off, followed by two positions on. I've
never encountered a switch base that features an off-on-off-on pattern for
the center contact, which would appear to be a much less astonishing mode of
degraded operation.

Hmm.. Well, consider the truth table, assuming you want brightness to go
up one notch each click, which is most user-friendly:

Low High Filament
1 0 0
2 1 0
3 0 1
4 1 1

From this we can conclude that the center contact is standardised to be
the high filament, probably also so that a 3-way bulb used in a standard
fitting gets the high filament lighting up rather than the less useful in
an emergency low filament.

Notably, neither degraded mode precludes the delightful possibility of the
socket containing an energized contact which cannot be positively identified
with a single-filament lamp.

If you're identifying energised contacts with a light bulb prior to,
apparently, reaching into the socket with your fingers (or you won't
notice) you've got bigger problems than not noticing a three-way switch
base. That's what neon-bulb-through-1MOhm[1]-ground-through-human
screwdrivers are for.


Jasper

[1] Replace with a resistor sized for 110V operation if necessary.

.

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