what sonar device let morse code messages be sent underwater from a submarine in 1915
Fessenden oscillator 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.[3] 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.[4]