Eavesdropping in the Former Soviet Union
The phone’s ringer is a pretty simple thing: there’s a coil, a magnet and a hammer controlled by the magnet that hits the gongs when there is AC current in the coil. The ringer system is connected directly to the phone line when the phone is on hook. (Actually through a capacitor that protects the ringer system from DC current normally present in the line.)
If you haven’t figured yet, the coil with the hammer is a speaker, not a perfect one, but a speaker anyway, and that also means that the system can be used as an electrodynamic microphone. Any ordinary speaker is an electrodynamic microphone at the same time, if you hook it up to an audio amplifier using normal microphone input.
So this was how actually they, the KGB, did their eavesdropping, I thought. They didn’t need to freeze outside or put bugs in our homes, because they had a nice wiretapping device in every single home in the country. The shocking part of it was that they didn’t just eavesdrop phone conversations – that one was kind of obvious. They were able to hear everything. The PSTN switching stations were considered strategic objects, they were under KGB’s control and surely it was no problem for them to get a few powerful amplifiers hooked up to certain lines leading to homes they needed to eavesdrop. Simple!