The Signal Service in the European War of 1914 to 1918 – Chapter 6

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«-- The Signal Service in the European War of 1914 to 1918
Chapter VI
Chapter V Chapter VII

Chapter VI.


Enemy Overhearing. — Early Experiments and Consequent Precautions. — The First French Listening Post. — The Vibrator and Buzzer Fall into Disuse. — Metallic Circuits. — Cable Twisting. — Policing of Circuits. — Code Calls and Position Calls. — Our Own Listening Set Organization. — Description of a Listening Post and its Work. — Fullerphones and Screening Buzzers.

An outstanding feature of all great catastrophes which shake the fabric of society to its deepest foundations and by which the faith of mankind in the durability of the established order of things is rudely disturbed, is the fear of the unknown. This invariably shows itself in one manner or another.

In the particular instance of a great war — perhaps the most earth-shaking of all social phenomena — this is usually manifested in the ever-recurring "spy" mania. The present war was no exception to the rule. From the day of its outbreak the wildest rumours were bandied about from mouth to mouth.

Certainly, very clever espionage was taking place, and — to deal only with instances affecting the British Signal Service — numerous cases both of malicious cutting and tapping of our lines, were reported during the early months of the war. Officers returning home at dusk would report strange wires joined on to our system at points where no headquarters existed. Unfortunately, it seldom appeared to the original observer to be within the scope of his duty to follow up the wire at the time of his discovery. On report being made to the local signal officer, a party would be sent out, usually only to discover that the supposed line was non-existent or that it had been laid for some legitimate purpose by our own signallers. In every surge backwards and forwards of the tide of war cases undoubtedly did occur of German agents being left behind with orders to secure information and transmit it by any means possible to the new German line. By far the greater number of cases, however, existed only in the imagination of the officers and men concerned; the cutting of lines by unfriendly peasants and by straggling parties of the enemy only needing exception from this statement.

As the situation settled down and everyone became more used to service conditions, so the rumours decreased in volume, until in the summer of 1915 the enemy did suddenly appear to be extra-ordinarily well informed of all that was going on behind our lines. This was manifested in many ways. Carefully planned raids and minor attacks were met by hostile fire, exactly directed, and timed to the minute of the attack. Trenches where a relief was taking place were heavily shelled at the very time of the relief, when they were naturally filled with double their complement of men. This occurred too often to be a mere coincidence. The arrival of a new gun, the establishment of a new machine-gun post, the installation of a new trench mortar — were all signalized by immediate and accurate fire before ever the new weapon had advertised its presence by its own activity. Lastly, when relief was taking place in a quiet sector, the relieving troops on the following morning would be greeted by shouts of welcome from the opposing trenches. The greeting was usually more pointed than refined, but was intimately and accurately associated with some salient feature, perhaps of the history of the battalion or regiment, perhaps of its nationality. On some occasions boards were displayed with suitable inscriptions; sometimes with the name of the regiment on them; sometimes the number of the battalion, and usually with a cordial invitation to "come over" and a promise of a hot reception. One day, even, a well-known Scotch battalion took over its new front to the strains of its regimental march, exceedingly well played upon a German cornet.

That leakage of information was taking place was sufficiently clear, and at first this was put down to the activities of spies within our lines. The search for and precautions against these unbidden and unwelcome guests were redoubled. In spite of the greatest efforts, however, no signs of espionage sufficient to account for even a small proportion of the leakage could be detected. Innumerable false scents were taken up and abandoned after thorough investigation.

One such trail which led to nothing but some amusement, and a certain amount of suspicion of the bona fides of certain civilians, was connected with an estaminet just within the British lines. Listening in at Divisional Headquarters on a line which had already been reported as subject to strange voices breaking in upon conversation, the O.C. Signal Company heard on the line what was obviously the noise from an estaminet. It was just what one might have heard when passing the door of the estaminet. A melodion was playing; men were singing "Keep the Home Fires Burning;" a woman was chaffering with the men about money for drinks, etc. The noise was traced down stage by stage through various exchange operators who had heard it and who, by pulling out their plugs, ascertained on which line it was heard, down to the line from a battery exchange to the battery horse lines. Investigation followed and it was ascertained that in all likelihood the disturbance proceeded from a small estaminet which this line passed. In this estaminet were pigeons confined in a basket and other suspicious signs. Nothing tangible could be proved, however. The noise may possibly have been due to a spy working in a room adjoining the estaminet and accidentally pressing the speaking switch of his telephone. Whether this is so or not, the story has more interest in the present connection as an illustration of the extent to which overhearing may inadvertently take place. Here was a circuit on which, undoubtedly, conversation between Division and Brigade headquarters must have been plainly audible at least to within a few hundred yards of the front line.

At the time that the leakage of information appeared to be most widespread, that is about June, 1915, the trouble with induction and overhearing over the jumble of lines immediately behind our front was causing serious anxiety and demanding the closest attention both of the Signal Service and the Staff. It is no exaggeration to say that at this time it was possible to speak from any given signal office on the British front to almost any other. In fact no "one knew when he picked up a telephone, to whom he would be speaking or whether he would be able to pick out from the melee of sounds the words of his own vis-a-vis.

It was a step in a logical argument to the further conclusion that the universal overhearing which was rapidly ageing both Staff and signal officers might be affording interest from a different point of view to the enemy and in the same month orders were issued that experiments should be carried out to test this theory. It was pointed out that, even if overhearing could not take place without some friendly conductor, yet there were railways, pipelines, and water channels, leading straight from our own to the enemy's lines, which might be affording him an opportunity of overhearing conversations and messages not meant for his ears. Certain experiments at home as to the possibilities of earth induction telegraphy also made everyone concerned feel very certain that overhearing might take place even without the presence of a better conductor than the earth itself.

At about this time, the French Signal Service reported a deliberate attempt on the part of the Germans to tap French artillery wires. The enemy had run a wire along the bed of a stream across the front of a French post and through the French barbed wire entanglement. The attempt was discovered, almost by accident, and amongst other things a pole was found sawn through lengthwise and with a length of telephone wire inserted, evidently with the intention of substituting it for one of the French poles at a favourable opportunity. Through this and through the frustration of other similar attempts, the attention of the Allied signal services was kept on the alert. The result of the experiments was to direct their thoughts into slightly different channels, for they confirmed the suspicion that the presence of specific good conductors was not essential to regional overhearing.

In the first primitive experiments on overhearing in First Army area in June and July, 1915, very simple apparatus was used. A wireless receiving set with repeating coils in a ratio of 1 to 16 was employed, but even with this comparatively insensitive apparatus speech could be overheard at a distance of 100 yards and buzzer signals up to at least treble that distance. It was an epoch-making discovery from either of two points of view. Its most interesting possibilities seemed at first sight to lie in the direction of overhearing enemy conversation and messages. Far more important, really, was its relation to the leakage of information that was already taking place from the British lines. Here was an explanation sufficient to account for all the knowledge the enemy had obtained in the past; equally staggering was it to contemplate what he was overhearing every moment in the present, and what he might overhear in the future unless precautions were promptly taken.

No wonder he was well-advised as to our movements, our plans and our dispositions. With a Staff who trusted almost entirely to messages buzzed over the forward lines, often to the most advanced posts; with an inexperienced soldiery, unused to the exercise of discretion in their every day conversation over the telephone, there was no end to the devastating possibilities presented by this newly-exposed factor in the situation.

The consequences of overhearing were so far-reaching that it seemed impossible to devise adequate means of protection against them, and meanwhile, day by day, fresh and more convincing evidence came to light. At the beginning of August, 1915, information was received through a liaison officer that our Allies, whose attention had been early directed to the same problem, had a station established well forward in their own lines and were overhearing scraps of German conversation. Nothing useful had yet been obtained, but on the 7th of August the O.C. Signals, First Army, visited the French post and examined the installation. The product of the fertile brain of a French infantry private who had been an electrician by profession, this forerunner of the listening set is of great interest and a general idea of the installation is given in Plate XI.

Advantage had been taken of a series of saps and mine galleries that had been tunnelled towards the German lines. Wires had been run along these saps and tunnels, and to the end of each cable a number of 75 mm. shell-cases were attached as earths. These were buried in charcoal to prevent oxidation, and the earth was rammed down over them to make a good earth connection. A width of about 350 yards of front was covered, and in this area, as can be seen from the diagram, there were no less than four separate series of earths, the furthest advanced of which were believed to be within a few metres of the German trenches. Indeed, the installation compared favourably both in thoroughness of conception and of execution with many which were devised much later in the war.

The only weak point was the overhearing apparatus itself. It was before the days of the application of the three-electrode valve to this work, and the only instruments available were a pair of ordinary low resistance telephone receivers of the French Post Office pattern. In charge of the listening post was the French private to whose genius the idea was due and two interpreters patiently awaited opportunities of sorting out intelligible German conversation from the disconnected phrases which were overheard from time to time. Such was the first primitive listening post from which was destined to grow a large and complicated intelligence organization.

Important as it was to make use of the new discovery to overhear the intentions of the enemy, yet the policing and improving of the British telephone system was a still more urgent matter. The measures taken were of two kinds, technical and general. All signal officers were at once instructed to improve the insulation of their circuits in every way possible. Earths were to be brought back at least 100 yards from the front trenches. At the same time memoranda, detailing subjects that were in future not to be referred to forward of battalion headquarters, were circulated to all telephone subscribers. The principal of these tabooed subjects were: —

(a) Names of units or unit calls.
(b) Times of relief.
(c) Movements of units.
(d) Information regarding result of artillery fire.
(e) Location of guns.

A glance at the list will give a very good idea of the value of the information the enemy must have been receiving prior to the issue of the order. Indeed, for a year or more after the first discovery of the fact of enemy overhearing, it can be seen from the reports of our own listening sets that an almost incredible ignorance or obstinacy pervaded all the units concerned. As late as October, 1916, the main obstacle in the path of the men who were endeavouring to overhear German conversation was the never-ceasing conversation on our own forward lines. Officers could not be made to understand that half their own worries and a considerable proportion of the casualties suffered by their units were due to their own indiscreet use of the forward telephones. Order after order was issued; precaution after precaution was insisted upon; still the leakage continued in slightly less degree only. It was not until disciplinary action was taken and carelessness made the subject of a court-martial charge, that forward telephones were used with any degree of care. Probably more was given away in 1916 than even in 1915; it was certainly to the later year that the classic example of stupidity in this respect belongs.

When discretion did at last set in, it took the form that might have been expected. In all forward units there sprang up a dislike for and distrust in the telephone. The system was still installed, but was less used, and this gradual atrophy of the telephone habit was of great importance in view of the change in the conditions of warfare which marked the closing phases of the war. Overhearing, which was the direct cause of thousands of casualties in 1915 and 1916, had dwindled to small proportions in 1917. By 1918 its result had been to wean the forward officer from his dependence on the telephone which could no longer be provided for him.

There were several main lines along which improvement might be hoped for. The retirement of the earths of earthed circuits was the first precaution taken. (Plate XII). As experiments continued and apparatus improved, the distance to which these earths were carried back increased. From the first trifling distance of 100 yards, the figure leapt rapidly in 1915 to 250, 500, and 1000 yards, and at the beginning of 1916, at the time of the introduction of the first valve listening set, to 1,500 yards, at which figure it remained for some time. At the same time, experiment proved that it was not sufficient to duplicate the line anyhow, but that the second cable must be carried back close alongside the original circuit. Careful test next demonstrated that to obtain a good metallic circuit the two cables were best twisted together.

The next step was the adoption of twisted cable for all circuits in the forward area, but this reform was long delayed by difficulty of supply. Not only was twisted "D5" cable not an article of normal supply, but single "D5" cable sufficient to duplicate all circuits in the forward area was not available in France. It was calculated that no less than 50 miles of cable would be required to convert the forward lines of a single Division into metallic circuits. Armies and G.H.Q. were faced with another great problem, but steps were at once taken to solve the difficulty. A supply of twisted cable was placed on order, and in the meantime Army and Corps Signal Companies, with their usual fertility of resource, devised rough and ready machines for twisting cable for issue to the Divisions in their areas. Thousands of miles of cable were twisted, the machines working day and night for weeks at a time during periods of stress. Perhaps no single incident better displays the inventive genius shown by personnel of the Signal Service than the variety of the cable-twisting machines which sprang into existence during 1916. Twisted metallic circuits were not unknown in 1915, indeed they were much used in at least one Army for local telephone circuits early in the summer of that year. Their use was, however, in those days, the exception rather than the rule. The majority of the metallic circuits in the early buries were made of two lengths of "D5" cable laid alongside each other and seized together at frequent intervals with spun yarn. Now, with the new universal demand, suitable machines were designed everywhere, the principal ingredients used in their construction being lorries, wheels of limbered wagons, airline barrows, wire stays, and odds and ends of ropes. Nor was the work carried out in a slipshod manner. The early twisted cables produced with these cable twisters would bear comparison with the products of the best-fitted workshops and they were subjected to severe test before they were accepted as satisfactory. In the Fourth Army, for example, lengths of a mile long were reeled on drums; laid at a canter from an ordinary cable wagon, and reeled up at the same pace. Had there been any sign of wear and tear greater than the normal, or had there been any kinking or unevenness of winding, the cable would have been rejected. The machine was, however, well-designed and carefully tended and the cable produced was satisfactory in every way.

The issue of twisted cable from England did not take place in large quantities until the end of 1916, but long before that time practically all circuits in the forward area were made metallic, and while the likelihood of enemy overhearing was diminished it was found that other difficulties had been surmounted at the same time. The great problem which required solution before the concentration of forward circuits into certain main buried routes could be thoroughly carried out was the induction between the various lines which rendered all of no value if used simultaneously. This difficulty disappeared entirely with the general adoption of twisted cable, and as many circuits as were required could be laid in the forward buries practically without danger of mutual interference. Once again two great obstacles to the efficiency of the Service had been overcome by the same remedy and a great step forward in signal practice had been achieved.

Further precautions against enemy overhearing were particularly necessary during the transition period when the duplication of circuits was taking place surely but very slowly. Throughout the early part of 1916, although earthed circuits in the forward area were gradually disappearing, the number in existence was still considerable. Even when the metallic system was theoretically complete, information might still be given away through badly insulated and otherwise defective circuits. In particular, it was soon found that the earth plate in the "D3" telephone was a source of leakage, and orders were issued that "D3s" were to be kept in their cases while in use and that future issues should have the earth-plate disconnected as a precaution. Another danger always present was the possibility of leakage to earth through the operators themselves. Yet another source of considerable anxiety were the large numbers of disused cables, half or wholly forgotten, which were lying about all over the forward area.

Attempts had already been made to clear up the mess in order to simplify the forward system and reduce induction troubles. These attempts were now redoubled and Corps and Divisional Signal Companies and forward signallers of artillery and infantry, alike, worked day and night on this essential duty. Trenches leading forward towards the enemy lines were deepened for at least two feet in the endeavour to trace and cut lost cables buried underfoot in the mud; every effort in fact was made to clear the ground. All this work could not fail to have good results and by degrees the congestion of wires in the forward area was relieved and quantities of salved cable were sent back to be destroyed or to be renovated at the Signal Depot where a plant had been erected for this purpose.

While all this was going on, however, it was necessary to make sure that the quantity of information of value to the enemy which was passing over the forward wires should be reduced to the greatest possible extent. Carefully-framed orders and circulars pointing out the dangers of overhearing were printed and these were issued to everyone in possession of a telephone. Disciplinary action was threatened if orders were ignored or disobeyed, and more rigid restrictions as to the use of the telephone and buzzer were promulgated and enforced.

Once again considerable difficulty was experienced in overcoming the inertia which opposed all change and improvement of method throughout the Army. Obsessed with matters of life and death on their own account, officers in the forward area had no inclination to be careful of what appeared to them unimportant details of the particular form of their conversation over the telephone. Immediate necessity frequently overbore the general instructions for discretion, and the most momentous affairs were still discussed in detail with a candour which was as naive as it was dangerous.

Our earliest authenticated positive information that the enemy really were overhearing matters of importance had come from a British civilian who had been interned at Ruhleben Camp. He had heard the medical orderlies at the camp hospital discussing with German visitors and sightseers the fact of their possession of an apparatus which was securing them valuable information in this manner. "They were so pleased with the new invention that they could not keep quiet about it," and it was obvious that the matter was of the very highest importance. In this respect, as in many others at this period of the war, the Germans were well ahead of us. It is impossible to overestimate the importance of what they must have overheard in 1915 before the necessity for precaution had been realized. As the months went on, prisoners captured from the German Signal Service admitted under examination that results of tremendous importance were still being obtained in the same way.

The classical example of obstinate stupidity was brought to light, when, on the Somme in the autumn of 1916, after thousands of casualties had been suffered in earlier attempts to capture the village, Ovillers-la-Boisselle was taken from the Germans and our troops, swarming over the German defences, billetted themselves in enemy dug-outs and converted cellars. In one of these was found a complete copy of a former operation order issued by one of the British Corps for a previous operation. This had been overheard by enemy interpreters, taken down from beginning to end, and issued to the German commanders concerned in good time to enable them to take measures to defeat the attack. Who can wonder that the battalions, debouching from their trenches at zero hour, had been decimated before ever they had crossed "No Man's Land and reached the enemy trenches. Inquiries were made and it transpired that the order had been repeated in full over the telephone by a Brigade Major to one of the battalions of his Brigade. He, knowing the danger, had protested, but had been over-ruled by his Brigadier. Hundreds of brave men perished, hundreds more were maimed for life as the result of this one act of incredible foolishness, persisted in in the face of informed opposition. The occurrence is a stigma on the fair fame of the British Army, but it should be recorded as an example of the appalling results which may follow from a single failure to adopt precautions ordered by a specialist branch of the Service. It was not until this tale and many others had gone the rounds that officers commenced to realize that discretion was not only desirable but was a duty second in importance to no other. As the year wore on, this fact was gradually driven home and it was assisted by many reforms in procedure and by the organization of an efficient system of policing, by which indiscretions could be brought home to the offenders and bad cases punished with the severity they deserved.

Perhaps the most efficient of these reforms was the introduction of code names and position calls for units and for signal offices respectively. By the use of code names instead of the stereotyped unit calls, the identity of units was hidden until some indiscretion or the piecing together of odd scraps of information enabled the enemy intelligence personnel to pierce the veil thrown over the whole front by the general adoption of the system. This temporary security from identification was made almost permanent by a constant change of names. Every few days fresh lists were issued and their use was carefully insisted upon. Until officers and signallers grew accustomed to the use of the new pseudonyms, there was always the risk of the indiscriminate use of the old and new code name and the unit's permanent call on one and the same day. Familiarity soon decreased the danger, however, and after 1916 cases of the use of unit names or permanent calls seldom occurred.

At the same time the adoption of the position call for signal offices further assisted to prevent the detection of reliefs. No longer were the call signs of the various signal offices associated definitely with the particular unit which manned them. The whole of the front was divided into numerous areas. These again were sub-divided into sub-areas. Each area and sub-area was given a letter. The combination of the two letters was the position call of the signal office in any particular sub-area. If more than one such office existed they could be distinguished by a number. The position call remained the same, irrespective of the formation which occupied the area, until it was considered advisable to change the whole series. Orders were then issued from Army Headquarters and the change in denomination took place simultaneously over the whole Army front, care being taken that the date of the change did not by chance synchronize with any alteration in the tactical disposition of the major formations of the Army.

It is now necessary to give some account of the growth of our own listening set organization and its use both for policing the telephone system and for the overhearing of enemy conversation and messages. Little progress was made in this direction until, in February 1916, the French Signal Service introduced a new listening set involving the use of three three-electrode valves used in cascade, for the purpose of amplifying the alternating currents which were induced in the loops or leads laid out as part of the listening installation. The three-electrode valve had already been introduced into France in the first wireless compass. The development of this type of valve and its application to Army work is a subject in itself, but it has been fully treated in the various technical manuals and is far too involved to be dealt with in detail here. At the request of the Army wireless officers, experiments had been carried out by the wireless section of the Officers' Training Corps at Cambridge, who were also responsible for the production of a small portable wireless set carried on bicycles and capable of operating over a distance of 6 to 10 miles with a mast 20 feet high. This work, begun in peace time, was continued at Cambridge after the war had broken out, and resulted in the design of the "White" valve which owes its main interest — not to its lasting success, for it was early superseded by the more robust "French" type — but to the fact that it was the first wireless valve designed by and for the Army and used with Army sets.

The French three-valve listening set — better known to the British Army as the I.T. set — at once revolutionized the current ideas of overhearing.[1] Its possibilities were so enormous that in a few months the depth of the danger zone[2] was increased, in spite of the adoption of metallic circuits, from 1000 to 3000 yards. The telephone, as a normal means of unrestricted conversation, was forced back to Brigade headquarters and the need of some safe method of replacing the buzzer and vibrator was rendered of greater importance than ever. No circuit appeared to be safe from the eavesdropping capabilities of the new apparatus unless it was twisted and "perfectly" insulated. The perfect circuit could not, of course, exist under the conditions prevailing in the fighting zone. As the range of enemy guns increased, circuits forward of Brigade headquarters could under no circumstances be considered safe. Once broken or even grazed, and the possibilities of earth leakage were greatly increased. More and more did it become evident that freedom from enemy overhearing must be dependent more on discipline than on technical improvements to lines or instruments.

With the introduction of the metallic circuit and the deep bury, these latter methods had been carried as far as possible. It now remained for the listening set organization which was created in 1916 to ensure that all indiscretions should be reported and visited on the heads of the people responsible. Naturally the service was unpopular. The appearance of an I.T. set was always viewed with suspicion. Definite orders backed by high authority were necessary to secure accommodation and rations for instrument and personnel. The life led by the unfortunate operators, linemen, and interpreters who manned these sets, was never an enviable one. In the early days it was made as uncomfortable as possible both by the enemy and by our own forward troops.

These detachments were intended for two purposes: one, the policing of our own telephone system; the other, the overhearing of enemy conversation. Both reasons for their existence demanded that they should be as far forward as possible and occasionally sets were installed in disused trenches or saps in "No Man's Land" or in our advanced posts in a most uncomfortable position.

For the efficient use of these listening sets some organization was necessary. This was gradually made more efficient, until in the winter of 1916 (the period of their maximum usefulness) a service had been built up consisting of from 20 to 30 sets controlled by an officer at general headquarters with the title of Inspector of Listening Sets. Some of these sets were used for policing purposes only, and some both for this and for overhearing enemy conversation. Success in the former role was never in doubt. Month by month reports were sent in giving detailed conversations, some indiscreet, some frivolous, some only superfluous. All bad cases were taken up, and little by little the amount of dangerous matter passing over our wires decreased. The improvement was very slow, however. The front of all Divisions could not be policed at once. Once the removal of a listening set was generally known, indiscretions were liable to recur. Typical fragments of conversation overheard as late as October, 1916, are given below: —

"... went up there but there was nobody there except about 25, majors, 17 colonels, one 2nd lieutenant and a lance-corporal. Tell old Cooper not to laugh so loud, the Germans will hear him."
" You know that at 'C' Co. headquarters in the support trenches two new 18-pounder guns have been placed behind the hedge— they are to fire at Gommecourt during the attack."

The first is merely frivolous and needed no action except a little satire. The second is a good example of dangerous conversation and led to the postponement of the attack for a month. If our own listening set picked this up the chances were that the Germans had also done so. In September, 1916, one single set heard between 30 and 40 units mentioned by name, including one Army and several Divisions. Movements of troops were referred to. Infantry operations were discussed. Whole operation orders were quoted. Positions behind the line were mentioned. One unit even reported 50 casualties from our own Stokes mortars, information likely to cheer the whole German Army. All this in one month, within a semicircle with a radius of 3000 yards. It would not have been surprising if the German Intelligence Service had been able to reconstruct from its listening set reports practically the whole constitution of the British Army as it existed at that time, and to anticipate the most jealously-guarded intentions of the Staff. If they were unable to do so, his Intelligence Corps must have been far less efficient than our own. The policing sets began to take effect in November, 1916, and from that time indiscreet use of the telephone decreased considerably. In March, 1917, the post of Inspector of Listening Sets was abolished and the detachments — already technically controlled by Corps — were entirely administered from Corps headquarters, the results obtained being co-ordinated and acted upon by "Intelligence" at headquarters of Armies. The stations continued in use for policing purposes until the close of stationary warfare. By this time their presence was well known to the whole of the British Army, and the cabalistic letters "I.T." represented to many of its victims the outward symbol of an almost uncanny power.

From the point of view just considered, the listening set had indeed proved its value. In its other function — the overhearing of the enemy, it had been almost equally useful, though perhaps less conspicuously successful. Many reasons prevented such outstanding results from the attempts to spy upon the German telephone system. The methods used by the German General Staff and commanders differed fundamentally from our own. These differences were probably accentuated by the fact that the very success which had attended the early efforts of this branch of the German Intelligence Service had placed the enemy on guard against similar indiscretion on the part of his own telephone subscribers.

As early as September, 1915, German prisoners had betrayed the fact that the enemy was very nervous, indeed, lest we should overhear his plans and dispositions. Stringent orders had been issued by the German General Staff long before reforms were initiated on the British side of the line. To this early-roused suspicion must be added the possession of a more highly-trained Staff, the stricter signal discipline of a conscript army, the better material available for the making of linemen and operators in a nation long trained in bulk for war, and, lastly, the more workmanlike system of trenches in which his lines were laid. It is not surprising, therefore, that, at the first attempts, little German conversation was overheard. His circuits were fewer and better controlled.[3] His signal discipline was good both in his Signal Service and the rank and file of other arms. An efficient trench system, organized in depth, enabled a good and comparatively safe line system to be carried well to the rear of the danger zone. One example of the trouble faced by the British Signal Service in overcoming the overhearing menace even when it was fully realized, is seen in the equivocal position of the Divisional signal officer. Armed though he was with a general authority over all lines within his area, if he desired to make his control effective, he was continually obliged to make complaints to senior officers. The latter, too often, only recognized him as a regimental officer and not as the representative of the Staff. Battalion commanders frequently took no notice whatever of his advice. If lines were bad he was expected to mend them or re-lay them himself. Backed up by an energetic General Staff he was all-powerful, but if left to his own resources as he sometimes was, he could do little to enforce his own regulations.

It was not until the French listening set appeared, with its great advance in efficiency over the earlier primitive circuits, that much German conversation was overheard. Even then the results were very patchy.[4] In one sector nothing at all would be heard. In another a listening set would continually record a good quantity of useful enemy conversation. Specific cases when such information proved of immediate use were many, and the information obtained was always of assistance to the Intelligence Service at Army and Corps headquarters.

The general use of the listening set for overhearing enemy conversation led to the creation of an entirely new Army trade — that of interpreter operator. With each listening post, special interpreters were employed to record the fragments of German conversation heard. At first these men were collected from here, there, and everywhere amongst the infantry, but after this source of supply gave out, specially qualified men were enlisted. In either case the first step in their training was to teach them enough knowledge of Morse to enable them to pick up enemy call signs and signals buzzed by the comparatively slow regimental signaller. A curious instance of the alteration in detail which was constantly exercising the minds of the Signal Service is afforded by the fact that, when the use of power buzzers by the Germans became general in 1917, these interpreter operators had to be withdrawn for further training or even replaced by men of better qualifications as Morse operators. They were quite incapable of reading the Morse messages sent by the experienced wireless operators who were manning the German power buzzers which had become at that time the main source of information obtainable by means of listening sets.

A special use of the listening set took place in the Fifth Army in December, 1916. A set was made as mobile as possible, and was taken forward immediately behind attacking troops. The object was to connect the instrument to the enemy's abandoned line system and endeavour to overhear his plans and his new dispositions. The idea had its possibilities, but was not successful in practice. Our attacks were so thoroughly prepared by the artillery that the German line system had been completely shattered by our shells before ever the assault took place. Fragments were sometimes overheard, but nothing of sufficient importance to justify the general adoption of the new scheme which was therefore abandoned.

Two important developments directly dependent on the question of overhearing were the invention and introduction of the fullerphone and the screening buzzer.[5] The power buzzer was another innovation attributable to the same cause, but the introduction of this instrument as a means of forward signalling belongs mainly to the 1917-1918 period of stationary warfare and is better left to a later chapter. The abolition of the vibrator and the restrictions placed upon the use of the buzzer in forward areas at once very much reduced the resources of forward signal officers. This was felt the more in 1916, because the introduction of the deep bury had just indicated a way by which line communication might be made very much more reliable and continuous, as least as far forward as battalion headquarters.

The properties possessed in common by the vibrator and the buzzer which were the cause of the 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[6] is interesting. As early as August, 1915, the Director of Signals asked the Signal Service Training Centre to experiment on methods of communication without wires. Earth induction telegraphy proved suitable for this purpose, and this was an 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 buzzer and vibrator an infinitesimally small direct current which should be broken up in 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 interrupter. By means of this, the current passing from line to instrument was alternately 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 trial 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 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. One type, which was not, however, the most successful, was made by adaptation of the ubiquitous D3 telephone. The scale of issue steadily increased until 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.

Of considerably less importance was the invention of the screening buzzer. It was natural — in view of the prevalence of wireless jamming — that one line along which solution was sought, should be the evolution of an instrument which should "drown" the noise of the induced currents in the overhearing circuits. The experiments already made in earth induction telegraphy pointed out a way in which this might be accomplished. Slightly more powerful buzzers and specially arranged earths were all that was necessary, and these were soon devised. Early in 1916 their general use along the entire front was suggested by the Director of Signals to the General Staff at General Headquarters, but the proposal was over-ruled on account of the importance of overhearing enemy conversation. In June of the same year the Assistant Director of Fortifications and Works at the War Office once more raised the same question, his proposal being based on experiments carried out at home. This time it was decided to give the new instrument a trial and, in August 1916, the "screening" buzzers were first provided on a scale of 50 sets per Army. These instruments were only intended to screen speech, the apparatus required to "drown" buzzer signals being too bulky for use in the field. In October 1916, it was definitely decided to screen the whole front during trench warfare, shutting off the screening buzzers in certain sectors when it was desired to make listening reconnaissances with our own sets. The system, however, proved too cumbersome and not sufficiently safe. It was given a good test on most of the front, but enemy prisoners stated that it was quite possible to hear speech through the uniform screaming produced by the "screeners." The installation was, therefore, only very partially successful and, in view of its comparative costliness and inefficiency, was not long persisted in.

An interesting example of a method by which enemy overhearing was even turned to advantage is afforded by the careful preservation on one Corps front of a single badly-insulated earthed circuit. This was isolated from the other circuits in the neighbourhood and was named by a pretty turn of wit the "Wolff" circuit, after the Press Bureau of that name. On this circuit a series of false orders, information, and conversation, was spoken or buzzed for the enemy's special benefit. No particulars of the results of this effort are available, but the scheme had its possibilities. It is mentioned here as an example of how good may be extracted from the worst circumstances and use made of a situation which from every other point of view was as bad as it could well be.


  1. The first two of these sets used by the British Signal Service were installed at Vermelles in January, 1916. Three loops were laid down opposite the Hohenzollern Redoubt.
  2. That is, the area in which conversation was first restricted and later banned altogether.
  3. The British telephone service was used to a much greater extent than the German. In consequence, British listening sets not only heard little German, but were prevented from reading what they heard. On the other hand, it should be recorded that the British telephones were of more use for the purpose for which they were installed, namely, facility of intercommunication.
  4. Results depended largely upon the nature of the soil.
  5. Suggested in 1915 but not adopted as a precautionary measure until two years later.
  6. Named after its inventor.

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