Fessenden oscillator

Similar in operating principle to a dynamic voice coil loudspeaker, it was an early kind of transducer, capable of creating underwater sounds and of picking up their echoes.

The oscillator in the name referred to the fact that the device vibrated and moved water in response to a driving AC current.

The resulting force was communicated to the membrane and in turn provided acoustic vibrations into the water.

However, a submarine signalling the surface could be heard by any nearby (enemy) hydrophone, so the system had restricted utility during wartime patrols.

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.