Great War Telephone Equipment 1914-1918:

The Fullerphone Mk 1 (Great War)

The Fullerphone Trench S (Great War)

   

 

The Fullerphone Mk 3 (Great War)

Great War DC Morse Telegraph Set or Fullerphone,

used within 3000 yards of the front line from 1916 onwards.

The properties possessed in common by the vibrator and the buzzer which were the cause of overhearing and induction set up by them, were the comparatively high potential and the rapid alternations of the currents sent out along the line. These set up similar currents in the adjacent circuits either by induction or by earth leakage and this was a difficulty which could not be overcome without radical alteration in the circuits used.

No time was lost in endeavouring to provide a substitute for these instruments and the genesis of the experiments which resulted in the invention of the fullerphone (named after its inventor, Maj. Gen. A.C.Fuller ) is interesting. As early as August 1915, the Director of Signals asked the Signal Service Training Centre to experiment on methods of signalling without wires. Earth induction telegraphy proved suitable for this purpose, and this was the independent reason for the attraction of attention to the subject of overhearing. In the late summer, the officer conducting the experiments was struck with the idea of substituting for the alternating line current sent out by the buzzer and vibrator an infinitesimally small direct current which should be broken up by the receiving instrument itself in order to make it audible. The idea was a far reaching one. Once it was adopted , the evolution of the fullerphone was a matter of mechanical and electrical detail and was soon accomplished. The direct current in the line caused no induction worth speaking of, and the danger of overhearing was reduced at once beyond the limit of practicability under service conditions. The current in the receiver of the fullerphone was broken several hundred times a second by an electrically-driven interruptor. By means of this, the current  passing from line to instrument was alternatively driven into the condensers and through the telephone receivers, the alternations taking place at an audible frequency. The dots and dashes of the Morse code sent on the key at the sending end were, therefore, reproduced in the receiver without varying the steady line current. Induction was almost eliminated and with it the possibility of overhearing.

The general adoption of the fullerphone was delayed for some months by the exhaustive tests carried out with the earlier instruments, and by the inevitable delays of manufacture. The first instruments were tried in France towards the end of 1915 on a line run from Reninghelst via Goldfish Chateau to Ypres and back to Reninghelst by another route. This trail was successful and their adoption was recommended. In 1916, supply commenced on a large scale and signal officers were delighted with the new instrument which was, however, at first to add considerably to their work. The earlier patterns required delicate adjustment; in particular the earlier type of interruptor was not very good. Battalion signallers, while quite able to operate the instrument once it was adjusted, were quite incapable of adjusting it themselves. Compared with the robust telephone, D3, the fullerphone was a delicate mechanism and at every move the Brigade signal officer had to go round to all the offices and personally attend to its adjustment. Classes in fullerphone operating were at once started, and Divisional signal schools did good work both in popularizing the instrument and in familiarizing the operators with its vagaries. Towards the end of 1916, the fullerphone was firmly established; in 1917 it was in use at Brigade headquarters also, and, in 1918, many Divisions had adopted it for all their forward circuits. As time went on special types of trench fullerphone were devised which were better suited to the needs of position warfare. The scale of issue steadily increased until in June 1918. When semi-mobile warfare again set in, the fullerphone was no longer required for its original purpose. It could, however, be  superimposed easily on a telephone circuit and never lost its popularity in some Divisions.

By means of this instrument messages could be sent right up to company headquarters with immunity. Always, however, one thing had to be borne in mind, and prevented the new instrument from being absolutely safe. Most types of fullerphone were fitted for emergency use with a telephone hand-set, and speech on this was, of course, still liable to be overheard.  The "call" on the fullerphone was also an ordinary buzzer call. It was therefore , necessary to prohibit the use of this call, under threat of heavy penalties, for any purpose but that for which it was intended. The danger of the hand-set being used indiscreetly was overcome in some types of instrument by making it detachable. It was then carried in the pocket of a responsible officer and could not be used without his knowledge and express permission.

Enfield produced Fullerphone Mk III*   No 14257

Enemy overhearing had first been countered with definite success in the winter of 1916-1917. The danger of the indiscriminate use of telephones and buzzers within 3,000 yards of the front line had by now been driven home to all ranks both within the Signal Service and without. Telephones were installed, in many cases as far forward as company headquarters, in some cases in the front line, but except for artillery observation their use forward of battalion headquarters was forbidden in normal trench warfare. Many officers were in favour of entirely forbidding their use forward of Brigade Headquarters.  The buzzer had by this time been replaced to a great extent by the fullerphone and definite orders had been given that no messages were to be sent by buzzer or vibrator in advance of Brigade headquarters. This was tantamount to the abolition of these instruments during periods of stationary warfare, for sounders were now installed at most Brigade headquarters. The latter instrument was universally preferred to the more clumsy vibrator with its raucous, nerve-wearing note. The early troubles with the fullerphone were by now largely overcome, and improved instruments and a well-directed educational campaign were rapidly popularizing these substitutes for the "D3" buzzer. The summer sun of 1917 shone on a front which approached much more nearly than ever before to the silence which was the ideal of the Intelligence Officer and which it was the aim of the signal officer to achieve. How was such a state of affairs going to suit offensive warfare? This was a question of paramount importance, for an Army compelled to advance in telegraphic and telephonic "silence" and hedged about with cipher and code restrictions, was going to be deprived of a very large proportion of its available means of communication.

It was admitted by the Intelligence Staff, however, that the enemy would be too much occupied when our attacks had commenced, to be able to pay much attention to individual remarks made over the British telephone system. In addition a successful British advance would involve the destruction, capture,  or withdrawal of his listening posts. Also, even if news of importance was overheard,   it would be practically impossible to convey it to the responsible Staff in time for it to be of use before the situation had radically changed. All these considerations tended to revolutionize the whole situation immediately the advance commenced. Particular care was, therefore, taken that nothing was given away before the date fixed for an offensive, but, once the attack was under way, all restrictions on the use of the telephone and  telegraph were either swept away or allowed to go by default. Buzzers, forward telephones, even earthed lines made their appearance whenever and wherever the situation demanded. Wireless code and cipher regulations were not entirely relaxed, but much more latitude was allowed to the individual officers who framed the messages transmitted by this means. It was not until the situation hardened once more and position warfare conditions again set in, that precautions were resumed and restrictions re-imposed. In the meantime, it is likely that that little harm was done and there is no doubt that persistence in restrictive legislation would have so crippled the signal system that an infinitely greater amount of damage would have resulted than can ever have been the effect of the uncontrolled use of the telephone and buzzer and the temporary suppression of code and cipher regulations.

Source: The Signal Service (France) by R.E. Priestly, M.C. , B.A. (Late Major, R.E.) Chatham: W. & J. Mackay & Co., Limited.     1921

 

The Fullerphone Mk V (cc 1944)

Link to  excellent Fullerphone website: http://home.hccnet.nl/l.meulstee/fullerphone/fullerphone.html

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