<P> The Fessenden oscillator somewhat resembled a modern dynamic microphone or dynamic loudspeaker in overall construction . A circular metal plate, clamped at its edge, in contact with the water on one side, was attached on the other side to a copper tube, which was free to move in the circular gap of a magnet system . The magnet system had a direct - current winding to provide a polarizing magnetic field in the gap, and an alternating current winding that induced currents in the copper tube . These induced currents produced a magnetic field that reacted against the polarizing field . The resulting force was communicated to the membrane and in turn provided acoustic vibrations into the water . </P> <P> Unlike previous underwater sound sources such as underwater bells, the Fessenden oscillator was reversible; the AC winding could be connected to a head set and underwater sounds and echoes could be heard . Using this device Fessenden was able to detect icebergs at a distance of about 2 miles, and occasionally detected echoes from the sea floor . </P> <P> The device could also be used as an underwater telegraph, sending Morse code through the water . The Fessenden underwater signalling apparatus, or more usually just "The Fessenden", was fitted to Royal Navy submarines in World War I. British K - series submarines were equipped with Fessenden oscillators starting in 1915 . However, a submarine signalling the surface could be heard by any nearby (enemy) hydrophone, so the system had restricted utility during wartime patrols . </P> <P> During the First World War the Fessenden oscillator was applied to detection of submarines, but its rather low operating frequency of around 1 kilohertz gave it a very broad beam, unsuitable for detecting and localising small targets . In peacetime, the oscillator was used for depth finding, where the lack of directionality was not a concern, and Fessenden designed a commercial fathometer using a carbon microphone as receiver, for the Submarine Signal Company . </P>

What sonar device let morse code messages be sent underwater from a submarine in 1915