About using reed switches in a roll reader -- just in case someone
might waste their time, reed switches are the ones you do not want to
use because they're like little "buzzers", and the worst of all at it.
They vibrate on actuation and release -- both! It's the reason more of
them are not used in quick solid-state circuits. Burglar alarms is
their market, today. Also, their sensitivity varies all over the map.
Where one reed switch may actuate with a magnet 1/2" away, another
requires the same magnet much closer. Change the angle slightly, and
sensitivity changes drastically. And if buzzing isn't enough problem,
the hysteresis problem would be totally uncontrollable.
As far as "no arcing" due to a lack of air inside the tube, that really
has little to do with striking an arc. It's just that in a vacuum you
won't be able to use air to generate a plasma, but you still have metal
ions which will do it, and quite well. As long as a space has
dielectric value, you can zap it. When the points separate, you'll get
a nice "burn" if you're doing it with anything over 18-20 volts.
You might also be surprised to find how much a handful of similar
magnets vary, by weighting their pull until they release and tabulating
the results. You will be disappointed if you were expecting some kind
of consistency, I think.
[ John Malone uses Hall-effect micro-switch units in the pouch reader
[ at Play-Rite. I guess they have proven stable; I don't know about
[ sensitivity and hysteresis characteristics. -- Robbie