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responsible for the organization and practice of wireless within the limits of the army.

Yet another direction in which wireless personnel found em- ployment was in the detection and prevention of the indiscre- tions which, in 1916 particularly, enabled the enemy to glean important information by listening to the traffic over the British telephone system. It was in 1915 that this menace first became important and in the following year " overhearing " became so serious that the forward telephone service was stultified. Many important results followed, directly, or incidentally. Of these may be mentioned:

(1) The general adoption of closed metallic circuits everywhere within 3,000 yd. of the front line.

(2) Alterations in the system of identification calls.

(3) The replacement of the buzzer telephone by the fullerphone in the forward area.

(4) The invention of the screening buzzer, a powerful vibrator used for drowning all sounds carried forward by induction from the front line.

(5) The invention and perfection of the 3-valve listening sets and the formation of detachments of the army wireless companies to work them. 1

(6) The growth of an organization for the interception of speech on enemy lines and the policing of our own telephone system.

(7) The application of earth induction telegraphy to signalling which resulted in the invention and evolution of the power buzzer.

(8) The increased employment of alternative methods of sig- nalling (visual, wireless, etc.) so obviously liable to overhearing or overseeing that they were used with caution.

It is difficult to decide which of the many results was the most important, but perhaps the most interesting from the present point of view was the evolution of the power buzzer. This was a powerful vibrator worked by the current from a lo-volt accumu- lator, and connected to inconspicuous earths of insulated wire which could, if necessary, be buried 6 ft. deep with little labour. It occupied a place in position-warfare signals for which no other instrument, except perhaps the loop sets which lately more or less superseded it, was suitable. Detachments of troops isolated by the enemy could send out code signals which could be picked up by listening sets, themselves inconspicuous, at ranges up to

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Earth

Earth Co Va/n Interval Intervatve. Valve to Phone

Transformer Transformer

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FIG. i.

3,000 yd. On several occasions of importance these sets remained the only means of communication with and from units that had advanced rapidly in attack, or become isolated in defence.

The diagrams in fig. I show the principles of the power buzzer amplifier system. The transmitter (a) is a powerful buzzer taking its current from a lo-volt accumulator. When the Morse key is pressed,

1 The early overhearing experiments were made with ordinary telephone receivers and results, while they pointed out the danger, were not very satisfactory. In the German, French and British armies, it was the discovery of the possibility of using the new 3-electrode valves] for magnifying extremely small changes in electric potential which at the same time raised the " overhear- ing " menace to its greatest pitch and caused the development of large branches of " Intelligence " and Signals to deal with this new branch of scientific warfare. The valves were used in receiving cir- cuits both as detectors and amplifiers and revolutionized both tele- phony and wireless telegraphy.

a current from the lo-volt battery flows through the key to the upper contact, across to the lower contact, along the armature, thence along the primary coil, and back to the battery. The cur- rent magnetizes the coil which attracts the armature, thus break- ing the contact, and allowing the armature to fly back and remake contact, etc. Each time the primary current is thus completed and broken, currents in the opposite direction are induced in the second- ary coil and are passed to earth through shut lengths of cable and earthpins. Condensers, as shown, are connected across the break to reduce the sparking to a minimum.

To obtain the best results in two-way working a three-valve amplifier (6) is employed. The currents, received on similar earths, pass through the primary circuit, are induced into the secondary of the earth-to-valve transformer which is connected to the grid and, through a single dry cell, to the filament of the first valve. The amplified signal from the first valve passes through the second and third valves and, finally, the three-amplified signal passes through a valve to telephone transformer with ordinary wireless receivers in series with the primary winding.

With all these developments, and especially with the rapid increase in the number of listening sets, the wireless service, as a separate entity, was becoming unwieldy, and its absorption into the general signal service organization was essential to its most efficient administration. In 1917 and 1918, therefore, the army wireless companies were broken up, the section which had com- posed them being allotted to the divisional, corps, and army sig- nal companies, according as they were equipped with portable trench sets, Wilson and listening sets, or the larger and more powerful Crossley motor sets used for supervisory and tactical work at army headquarters. In this form wireless organization survived the war.

The only change of moment in army wireless after this time was the application of the continuous wave system to army use. The early experimental sets made their appearance in 1917, but for some months they proved to be too delicate and untrustwor- thy for the work under the hard conditions of active service. Gradually, however, technical difficulties were overcome and more robust types of instrument devised. Before the end of position warfare, portable 3o-watt continuous wave sets of about the size and portability of the so-watt spark sets, but with forward aerials only 4 ft. high and a normal range of 1 2 m. were doing good work with heavy artillery observation stations. The Armistice in Nov. 1918 found continuous wave wireless still chiefly confined to the artillery, but new and more powerful sets had already been devised and tested. Since that date, spark wireless has been entirely ousted from its former position except for the short- range loop sets the successors of the power buzzer which are retained for work within the battalion and similar small units working in the immediate vicinity of the front line. There seems little doubt that in the future development of army signalling, continuous wave wireless is likely to play an all-important part.

While the chief characteristic of the earlier position-warfare period was the evolution of signal implements and the adapta- tion of signal organization to stationary tactics, it was in the great battles of 1916 and 1917 that signal policy began to crystal- lize in very definite shape. The first result of the stabilization of the situation was the running forward of lines in all directions to serve the multifarious units which now for the first time made good their claims to telephone communication. Magneto and buzzer telephones and magneto, buzzer, and combined exchanges made their appearance in all formations from brigade rearwards, and buzzer telephones and exchanges were issued to battalions and batteries. The lines to serve these telephones and exchanges had in many cases to be duplicated and even triplicated, and a festoon of lines, converging from front to rear, or stretched transversely and at all angles across the front, hampered move- ment and defied the utmost efforts of the signal personnel whose business it was to maintain them. The necessity for economiz- ing signal personnel and for the protection of lines alike tended to bring about two reforms. On the one hand, control was vested in the signal officers of superior formations; on the other hand, by their orders, all circuits were concentrated into a certain Limited number of well-defined main routes.

The first of these reforms in point both of importance and of time was the rearward movement of the centre of gravity of the