To Jackie and Geoff -

Certainly it is a deviation from the normal behaviour. But that implies that an alarm is "always on" except when it's going off. Most other things are always off, except when they're turned on: light switches, television (at least in my house), computer, faucets, etc.

This leads me to my original suspicion. "Off" indicates an open circuit for household applicances, lights, etc. Maybe the original fire alarms normally had closed circuits, and began to sing when their circuit was opened (ie. when it was off) (instead of the other way around). I was wondering if anyone knew how the first fire bells worked, how they were activated. Maybe the clue is there.