After tens of thousands of fruitless X-rays, a technician noticed a small coil of wire inside the on/off switch of an IBM Selectric typewriter. Gandy believed that this coil was acting as a step-down transformer to supply lower-voltage power to something within the typewriter. Eventually he uncovered a series of modifications that had been concealed so expertly that they had previously defied detection.
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. more
