How a Selectric gave away its typing
An IBM Selectric did not print letters with separate keys striking the paper. Pressing a key set a mechanism in motion: a small, spherical type element—often called a typeball—tilted and rotated until the chosen character faced the page. The machine’s internal movements therefore carried a record of what was being typed.
Soviet eavesdropping devices exploited that mechanical pattern. Sensors detected changes in the magnetic field associated with the Selectric’s moving parts. Electronics could interpret those changes as the motions used to select particular characters, then reconstruct the text. The bug was reading the machine’s physical behavior, not recording the sound of its keys.
From motion to a covert message
The process turned an ordinary office machine into a source of digital information. A character selection produced a sequence of internal movements; the electronics converted that sequence into data and sent it out in short radio bursts. The typewriter did not have to be plugged into a computer or connected to a network. The added device supplied the sensing, encoding, and transmission.
That distinction mattered for surveillance: a machine could appear to be an isolated piece of office equipment while still revealing what was typed. The surprising weakness was not a flaw in a network connection, but the fact that the Selectric’s precise mechanism made each keystroke physically legible to a hidden observer.
