Reginald Fessenden and his electric oscillator, the first acoustic device to receive echoes from the bottom as well as from an obstruction in the water. The oscillator was designed as an underwater signalling, communication, and obstruction avoidance device. In "Submarine Signaling," Scientific American Supplement, No. 2071, pp. 168-170, Sept. 11, 1915. Image courtesy of NOAA Photo Library.
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