I heard this story from Jim Pomerene.
Sometime in the late 50’s there was an embassy with a rotor crypto machine.
These machines had several wheels which sporadically turned one notch as a letter was encrypted.
Whether the wheel turned was a deep secret lest the machine fail to keep secrets.
There was a small continuously turning motor whose torque would turn the wheel as needed.
When turning, the motor current would increase and the voltage would decrease.
The cypher signal was transmitted by a transmitter on the same power supply.
A lower voltage would cause the carrier frequency to change a very small amount.
A nearby receiver could thus detect the wheel motion.