On Sun, 08 Feb 2009 20:01:18 -0800, David Nebenzahl wrote:

Just one thing: did someone here say that switches like this were
typically N/O rather than N/C? And that such could be used for an alarm
circuit?

Think about it: if you have a N/O switch as an alarm sensor, all the bad
guy has to do to get in is to cut it out of the circuit. There's a
reason alarm switches are normally closed.

Phil Allison said:

** It's a better than even money bet Terry wants the switch to be closed
with a magnet nearby.

Which IS the NORMAL situation in an alarm installation.

IOW, it's a question of how you define "normally".

Reed switches are open in the absence of a magnetic field, and closed when
one is present.

For a relay, you would call such contacts "normally open".

But for a burglar alarm, the "normal" situation is for the magnetic field
to be present, from a permanent magnet in a door or window which is
"normally" adjacent to the reed switch. In this situation, the contacts
will be held closed.