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 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. See also References Bibliography Further reading Transducers Sonar Maritime safety Acoustics