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 near by receiver could thus detect the wheel motion.