Fessenden oscillator
Encyclopedia
A Fessenden oscillator is an electro-acoustic transducer
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...

 invented by Reginald Fessenden
Reginald Fessenden
Reginald Aubrey Fessenden , a naturalized American citizen born in Canada, was an inventor who performed pioneering experiments in radio, including early—and possibly the first—radio transmissions of voice and music...

, with development starting in 1912 at the Submarine Signal Company
Raytheon
Raytheon Company is a major American defense contractor and industrial corporation with core manufacturing concentrations in weapons and military and commercial electronics. It was previously involved in corporate and special-mission aircraft until early 2007...

 of Boston. It was the first successful sonar device. Similar in operating principle to a dynamic voice coil
Voice coil
A voice coil is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it...

 loudspeaker
Speaker driver
A speaker driver is an individual transducer that converts electrical energy to sound waves, typically as part of a loudspeaker, television, or other electronics device. Sometimes the transducer is itself referred to as a speaker, particularly when a single one is mounted in an enclosure or as...

, it was an early kind of transducer, capable of creating underwater sounds and of picking up their echoes
Hydrophone
A hydrophone is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates electricity when subjected to a pressure change...

.

The creation of this device was motivated by the RMS Titanic disaster of 1912, which highlighted the need to protect ships from collisions with icebergs, obstacles, and other ships. Because of its relatively low operating frequency, it has been replaced in modern transducers by piezoelectric devices.

Oscillator

The oscillator in the name referred to the fact that the device vibrated and moved water in response to a driving AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 current. It was not an oscillator
Electronic oscillator
An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in innumerable electronic devices...

 in the electronic sense that it generated a repetitive signal, in fact electronic oscillators did not yet exist when this device was created. Since the design of the device does not depend on a resonant response, it should not be considered a harmonic oscillator
Harmonic oscillator
In classical mechanics, a harmonic oscillator is a system that, when displaced from its equilibrium position, experiences a restoring force, F, proportional to the displacement, x: \vec F = -k \vec x \, where k is a positive constant....

 either.

Operation

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.

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.

The device could also be used as an underwater telegraph, sending Morse code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...

 through the water. The Fessenden underwater signalling apparatus, or more usually just "The Fessenden", was fitted to Royal Navy
Royal Navy
The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

 submarines in World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

.
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.

Application

During the First World War the Fessenden oscillator was applied to detection of submarine
Submarine
A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

s, 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
Carbon microphone
The carbon microphone, also known as a carbon button microphone or a carbon transmitter, is a sound-to-electrical signal transducer consisting of two metal plates separated by granules of carbon. One plate faces outward and acts as a diaphragm...

 as receiver, for the Submarine Signal Company.

See also

  • Underwater acoustics
    Underwater acoustics
    Underwater acoustics is the study of the propagation of sound in water and the interaction of the mechanical waves that constitute sound with the water and its boundaries. The water may be in the ocean, a lake or a tank. Typical frequencies associated with underwater acoustics are between 10 Hz and...

  • Underwater acoustic communication
    Underwater acoustic communication
    Underwater acoustic communication is a technique of sending and receiving message below water . There are several ways of employing such communication but the most common is using hydrophones...

  • Hydrophone
    Hydrophone
    A hydrophone is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates electricity when subjected to a pressure change...

  • List of Reginald Fessenden patents
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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