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Fessenden's underwater ice finder

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In 1913, Reginald Fessenden reshaped the maritime industry through his invention of the Fessenden Oscillator. This electromechanical device allowed ships to send underwater telegraph signals and deploy underwater echo ranging. The Oscillator proved to be the first practical form of shipboard sonar, and its underlying technology remains in use on modern vessels to measure depth and distance of submerged objects.

Fessenden was born in Quebec in 1866, educated at Bishop’s College, and became a teacher of mathematics before moving to New York City in 1886 to work for the Edison Machine Works. For a number of years, he was a tester and researcher for a number of emerging electrical power companies. By 1892, he moved into academic research, teaching at Purdue University and the Western University of Pittsburgh. He engaged in pioneering research in wireless telegraphy. In 1900, he left academia and developed commercial applications for wireless technology. He designed an infrastructure for a wireless network for weather station communication. From 1902 to 1911, he ran the National Electric Signaling Company (NESCO), where he performed pioneering research in radio broadcasting. The company was financially unsound, however, and Fessenden was dismissed from the company in 1911 and it went into receivership soon after.

After his tenure at NESCO, Fessenden was hired as a consulting engineer by the Submarine Signal Company of Boston to develop a maritime echo ranging and communications system. The demand for such a system was proven just months before, on April 15, 1912, when the RMS Titanic sank in the North Atlantic Ocean after colliding with an iceberg. Ship captains needed better technology for avoiding underwater hazards and communicating with other ships in the event of a disaster at sea.

Fessenden’s oscillator did not generate a repetitive pulse; rather, it was similar to a microphone or loudspeaker in design. It made contact with the water through a circular metal plate. The plate was attached to a copper tube, which linked to the circular gap within a system of magnets. These magnets drew on direct-current and alternating-current. They used direct-current winding to create a polarizing magnetic field within the circular gap. And they deployed alternating-current winding to induce currents in the copper tube. Force was generated as the induced currents in the copper tube made a magnetic field that reacted against the polarized field in the circular gap.

This force created acoustic vibrations in the water. Ships could use the oscillator to send out signals in Morse code through the water. But it was also reversible. The alternating-current winding could be connected to a head set, allowing a listener to hear underwater sounds and echoes. Fessenden was able to detect echoes from icebergs up to two miles away, and could also sometimes hear echoes from the floor of the sea.

Despite its revolutionary promise for shipboard safety and oceanography, the Fessenden Oscillator’s adoption as an echo ranger took many years. Historian Gary L. Frost writes that seafarers found the invention to be a “godsend” by making travel far less dangerous in iceberg-filled waters. Yet, for “personal, political, commercial, and institutional” reasons, the Submarine Signal Company initially marketed Fessenden’s oscillator as a telegraphy system, rather than an undersea detection device.

Further Reading

Gary L. Frost, "Inventing Schemes and Strategies: The Making and Selling of the Fessenden Oscillator," Technology and Culture, Volume 42, Number 3 (July 2001), pp. 462-488