The corridors in the Stasi Jail had a bizarre primitive alarm system. It was the long wire shown in the photograph that goes round the building at about chest height. The guards were instructed to raise the alarm by pulling on the wire so that it came apart at the connectors shown. I was thinking about how it works. It looks like a series circuit. It could be a light that is on all the time but pulling the wire breaks the circuit so it goes off. Someone would have to concentrate on the light all the time. A sound alarm would be better. But that would be on all the time and stop when the circuit was broken. It would be irritating. You'd really need a piece of electronics called a NOT gate which would make the loudspeaker do the opposite of the circuit. Or there could be a magnetic relay switch that holds the loudspeaker circuit open in a second circuit. Breaking the first circuit stops the current to the electromagnet and allows a switch to fall back in the second circuit, sounding the alarm. I was just amazed that such a sophisticated and powerful system of oppression had such limited technology.
