Hi Scratch,
Thanks for dropping in!
I may not have made myself clear ....
I do realise that repeated operation of a reed switch,
even after many years would not cause it to retain
much magnetism, that wasn't quite what i meant.
One can magnetise a nail by stroking it with a magnet
so long as you repeatedly use the same end of the
magnet, it doesnt get very much magnetism transferred
to it, but usually after a while, it could pick up
another nail.
I doubt if one could magnetise a nail by moving a
magnet close to it, then away again on the same route,
which is what a reed switch gets.
When one is magnetising a nail in this way, the
stroking action is intended to 'line up' the molecules
in a way that does not happen by moving the magnet
towards, and away again, like the action of a door.
Now, i have never tried this with a reed switch,
but i am very curious as to whether or not it would
retain any magnetism, and thereby remain closed.
It may be that the reeds are made of some ferrous
material that does not retain magnetism very well
but still responds as required.
I may see if i can find one somewhere to give it a try.
Thanks for mentioning the 'resistor with the switch'
i had thought that something like that would be done.
In instrumentation, it is something that used to be
done quite a lot. The zero reading from a sensor is
often about 12mA (i think), this low current can be
'zeroed' on the meter at the readout by the pointer
adjustment.
If the meter is reading below zero then the repairman
knows that its not getting its 12mA, and that helps to
locate the fault.
If i cannot get a reed switch to do this, maybe i
could stick a little magnet behind it to keep it shut.
I would prefer it to close without me doing that.
Regards, John