The US Department of State’s Countermeasures Directorate launched a new public exhibition titled “”Listening In: Electronic Eavesdropping in the Cold War Era.” Scientific American posted a slideshow of the exhibition and much of the text written about the devices, from old-school keyloggers to phone tap detectors. From SciAm:
IBM SELECTRIC TYPEWRITER
Because the Selectric coupled a motor to a mechanical assembly, pressing different keys caused the motor to draw different amounts of current specific to each key. By closely measuring the current used by the typewriter, it was possible to determine what was being typed on the machine. To prevent such measurements, State Department Selectric typewriters were equipped with parts that masked the messages being typed.
Spying on the Spies (SciAm, thanks JR Minkel!),
“Listening In: Electronic Eavesdropping in the Cold War Era” PDF brochure (US Dept. of State)