This British MK-IV 'fullerphone' field telegraph set was used by the British forces during WW-II. Similar sets have been used since WW-I. Its principles of operation were first designed by Captain A.C. Fuller in 1915 to make it impossible for the enemy to intercept telegraph communications. The set uses DC rather than buzzing AC voltages and converts them into audible signals by using a buzzer or interrupter.(40)
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