During the 1980s, American security engineer Charles Gandy knew that the Soviets had found a way to electronically spy on the US Embassy in Moscow. Americans had already discovered antennas hidden inside a false hole in a chimney. But Gandy couldn't figure out what signals these antennas were relaying to the KGB. After much searching, He discovered that a typewriter's ball had been hacked through an extraordinarily sophisticated method to relay whatever it typed to Soviet agents.
A point of historical clarification: inside many electric typewriters there is a metal ball that the machine uses to print text. It looks like this. IEEE Spectrum describes how the Soviets tracked the movements of that ball:
A solid aluminum bar, part of the structural support of the typewriter, had been replaced with one that looked identical but was hollow. Inside the cavity was a circuit board and six magnetometers. The magnetometers sensed movements of tiny magnets that had been embedded in the transposers that moved the typing “golf ball” into position for striking a given letter.
Other components of the typewriters, such as springs and screws, had been repurposed to deliver power to the hidden circuits and to act as antennas. Keystroke information was stored and sent in encrypted burst transmissions that hopped across multiple frequencies.
Perhaps most interesting, the transmissions were at a low power level in a narrow frequency band that was occupied by intermodulation overtones of powerful Soviet TV stations. The TV signals would swamp the illicit transmissions and mask them from detection by embassy security scans, but the clever design of the mystery antenna and associated electronic filtering let the Soviets extract the keystroke signals.