At the same time that we were trying to cope with the 131-B2 mixer, we began to examine every other cipher machine. Everything tested radiated, and radiated rather prolifically. With rotor machines, the voltage on their power lines tended to fluctuate as a function of the number of rotors moving, and so a fourth phenomenon, called power line modulation, was discovered.

(…)

During this period, the business of discovering new
TEMPEST threats. or refining techniques and
instrumentation for derecting, recording. and analyzing
these signals, progressed more swiftly than the art of
suppressing them. Perhaps the attack is more exciting than
the defense-something more glamorous about finding a
way to read one of these signals than going through the
drudgery necessary to suppress that whacking great spike
first seen in 1943.