WWII
Under field conditions the range of the Fullerphone would not exceed 15-20 miles, but it is capable of working under severe interference from stray earth currents. The Fullerphone was designed to be used during the war to stop the interception of messages using conventional methods. When alternating currents are used for morse buzzer or telephone transmission, the resultant inductive effects may be detected by means of a special listening amplifier placed in proximity to the line. The Fullerphone was developed to avoid passing into the line or earth, rapidly changing alternating currents as used in straight line communication, but at the same time using a buzzer note as a means of reading the signal, confining the sudden changes of energy level to the instruments.
(More detailed info on section "Digital Documents")
