Fullerphone

A Fuller phone is not actually a telephone, but a Morse telegraph. Captain Fuller had the answer, when in WWI it was discovered that the German enemy could eavesdrop on Morse communications of the British army. The strong Morse pulses propagated through the earth and could easily be detected with induction coils or by earth-tapping. He devised a modified Morse telegraph system that relied on a local battery and a 500Hz buzzer. That buzzer runs continuously, but its tone is only passed to the headphones when a DC current is present in the line, or when the local Morse key is pressed. The unit is sensitive enough to pick up a DC signal of microAmps. A double choke and capacitor filter ensures that the keyclicks of the outgoing Morse signal are suppressed, to prevent detection. A variable compensator is built in to reduce the effect of any stray currents in a long cable. The Fuller phone was further improved, and many units were used during WWII. My apparatus dates from 1941.