I hit an interesting sort of "bounce" while working with Litton glass break
sensors years ago..

The symptom was that the sensors would randomly trip for no apparent reason.
When you'd arrive and check resistances, everything would be fine.

The sensors were gold plated rings sitting on gold plated pins in a sealed
cavity.  Through many hours of work, we would isolate a defective sensor,
send it back, and they would report that it was just fine.

After much frustration, I decided to sit and watch one system till it
tripped. I was scoping the voltage across the loop, which would increase
with increasing loop resistance.  This took several days of constantly
watching the scope, but it finally happened.  The voltage would rise up in a
very erratic fashion, and then fall back down, more or less at random, till
the slightest vibration happened, then everything would go back to quiet.

This brought me to understand a little-known property of mechanical
switches, called "wetting current".  This is the minimum current required
for the switch to actually be closed.   When I found out about this, I
increased the loop current used in the system from about 2mA to about 20mA
and the problem dissapeared.    This mod was reported back to Litton, and
incorporated into the design of the control interface, apparently we weren't
the only ones having problems.

The switches we sent back weren't exactly defective, but they had slightly
higher values of actual wetting current than normal. The act of removin them
from the system, and the pounding of shipping erased any evidence that might
have been present.

Ain't hardware fun?
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