Well before the first world war, Reginald Fessenden, an eminent scientist, had discovered what he called his Fessenden Oscillator and the Royal Navy, surface vessels and submerged vessels took the technology to heart, pocket and practical advantage.

The early submarines, dating from the 1900 period, had, for twelve years, been extremely vulnerable to being rammed or run-over by surface vessels.  Virtually at all times when in busy navigable waters, a submarine was escorted by a ship of sloop size to ensure its safety.  When this occurred, the highest safety factor was when the boat was on the surface, the two vessels in sight of each other and fully aware of what each was doing.  When the boat dived the trouble, or potential for trouble started, because the submarine could not hear the ship in terms of underwater noise [HE = hydrophone effect] nor could it communicate with the ship.  Its only small measure of comfort that all was reasonably well up top [on the surface] was by having an occasional glance through the periscope. When dived to a depth below periscope depth, the chances of hearing anything other than the internal machine noises were impossible.

Fessenden had discovered that a diaphragm bolted flush to the vessel's side could be made to vibrate at some definite frequency. If the vibrations could be tailor-made to represent known signals then any vessel could talk to another vessel by underwater means, meaning ship to ship, submarine to submarine, and certainly, ship to submarine and vice versa.

The vibrations made were emitted as omni-directional sound waves so there could be no security attached to the device because any ship with a bolted on oscillator could hear what was being transmitted between two ships in the same area. Moreover, because of the noises occurring in the ship and submarine due to machinery and the noises in the water due to turbulence of the propellers, rudders and ships movement through the water, the quality of the 'communications link' was very poor. 

By the year of 1912, submarines of course had W/T equipment.  However, because the aerial had to be rigged by hand, which meant that it had to be un-rigged by hand, the submarine could not use it as effectively as it wanted to do - note, some submarines would dive with the aerial rigged rather than be caught and trapped on the surface {see the 1900 - 1913 PRE WW1 Files for more detail}.  When the telescopic W/T aerial arrived, boats could use their W/T to communicate when at periscope depth and so the need for the Fessenden Oscillator was of less significance, and the next stage on that front was ASDIC and subsequently UWT [under water telephone].

So now we know that there was such a device, where it was fitted and why, we need to find out what it looked liked and how it functioned.  We will start by telling you that it became one of the many jobs of the telegraphists to operator what became the S/T SYSTEM   when in the two way communicating mode.  The SYSTEM was the OSCILLATOR [already introduced]; the S/T TRANSMITTER and the S/T RECEIVER or the HYDROPHONE, meaning water telephone, or, and nearly a full circle, the UWT which was a Type 185 in the 1960's fitted into all ships and submarines. S/T meant 'Sound Telegraphy, which,  in the communicating sense, meant UNDERWATER MORSE CODE.

The weight of the original Fessenden Oscillator {1914} was 11 cwts [11 x hundredweight = 1232lbs = 559kg] but by 1930 a new Oscillator was in use which weighed in at just 70 lbs, 32kg. The early diaphragm/microphone was engineered to operate {vibrate} on and around the 540 c/s mark,  but as time progressed they had moved up to just over 1000 c/s {1 kc/s} which was a good AF tone for most telegraphists. We will mention the devices from 1914 until 1930. 

Throughout the pages of the PRE WW1 1900 -1913 files we have mentioned, countless times, the SILENT CABIN.  In this cabin, a part of the W/T office, the Morse Code operator sat ostensibly there to protect himself  from the ravishes of high power RF from the W/T circuits he would have to key. However, he also operated the S/T circuit from the Silent Cabin, and the routes for his Morse Key were clearly marked before him.  When communicating with submarines [or with other ships via S/T] his key would have been diverted to the Oscillator/ST Transmitter and away from the high power RF transmitters/wire aerials aloft.  Likewise, all receivers notwithstanding {thus including those of the ST Receivers/Hydrophones} culminated in the Silent Cabin where the telegraphist would read his signals.

This drawing shows the Silent Cabin Morse key {No 23} and earphones {No 39} [known then as telephones] but the proverbial route to an aerial is missing and in its place, there is a route to the underwater hull mounted Oscillators/Transducers {Nos 1 and 2}. See ST CIRCUIT.pdf.

In this model there are only two transducers, the norm then to be one PORT  and one STARBOARD. Later, as you will see a THIRD Oscillator was added, and for Reception, the ST Receiver/Hydrophone, four transducers were fitted.

The SYSTEM Transducers were for all intents and purposes transmitter and receiver aerials. The difference was that, [a] there were no high powered voltage/wattage outputs involved; [b] the operator had no receiver dials which he could operate to clear interference etc; and, perhaps most of all, the speed of operating was reduced to approximately 5-10 wpm no matter how agile one was on a W/T circuit using  the Morse key.  The underwater environment was really a sleepy old world, filled with natural noises from sea creatures as well as man-made noises.  All noises would and did vibrate the diaphragm of the hydrophone and obviously they would vie with the Morse signal, effectively causing interference.

An operator, be he in a submarine or in a ship and using only the sea as a  propagation path, used the Fessenden system principally  to transmit More Code messages along underwater corridors which manifestly allowed all audio waves to travel, particularly those of vocal mammals  [whales, dolphins, porpoise and others] and yet which we ourselves could not use for voice for communicating.  That was to come much later. 

By the late 1920's the S/T distance was 30 odd miles in the right conditions [cold noise free seas] and this allowed a new freedom of manoeuvring for the boats.  It also opened the flood-gates to developing ways and means of listening for submarine activity in the area.

Before I show you the hardware which is almost totally given over to how the transducers worked with little or no regard [graphically anyway] as to WHERE it was fitted, I will show you the following drawing.  It is not to scale nor is it a masterpiece, but it does show the concept in enough detail for you to understand what follows.

The Oscillator [or vibrator] and of course the microphone, were fitted immediately behind and to the diaphragm pressure plate which was fully exposed to the sea.

This file shows a picture of the original Fessenden Oscillator and a suitable text explains its function. The top picture is an explosion to show the workings in detail, and the bottom picture shows you what the device actually looked like FESSENDEN'S OSCILLATOR.pdf. Note that the plate marked No1 [although not obvious] has the sea acting directly upon it.

Sixteen years on, a new lightweight Oscillator was introduced and this file shows how it functioned LIGHTWEIGHT OSCILLATOR.pdf.

The SYSTEM Transmitting Sets {Transmitter Types 102, 104, 105, 105A and 106} were a suite of sets part of W/T Branch Transmitter Portfolio and as such formed part of the technical training of the telegraphists.

Now we move to the ST Receivers. The earliest set were the Mk IV, Mk V, Mk VI, the TANK Hydrophone and the R.D.H. Mk VIII. Some of these were not directly open to the sea, and one was a directional transducer which could be rotated for best signal.

Finally, we look at the Hydrophone Installation which no longer involved the telegraphist but introduced other players as you will see from the simple schematics shown. Again, some involved direct contact with the sea, whilst others were inside the ship. In some cases four transducers were fitted. The series were known as the Hydrophone Installations Types 700, 702, 703, 706, 707 and 709.

By the time the Type 707 system was introduced the control positions were in the Chart House, and in the D/F Office.

Now, as a bonus and  just in case you ever want to know about underwater jargon, the following file has ALL the answers including some nautical conversion charts and tables - fathoms to millimetres and all that type of thing UNDERWATER MEASURES.pdf.  It is an unusual glossary and of general Naval interest.