A factor which greatly complicated the supply of the front-line forces with wire was the necessity for maintaining two-way or twisted-pair lines in order to avoid giving information to the enemy, for the detectors used in the German listening-posts were so highly developed that a telegraph or telephone message sent over a “grounded” or single-conductor line was to all intents and purposes sent direct to Berlin. This necessity for a double-conductor line relegated the old field-wire of open warfare to the scrap-heap, a long series of experiments being required to produce a twisted-pair wire which was light enough to permit of easy portability and rapid laying, strong enough to stand the strain of heavy traffic and shell-shock, and withal so well insulated that “leaks” to the ground might not reveal to the enemy listeners-in facts intended to be strictly confidential. The enormous demands for all types of wire and cables which came both from the A. E. F. and from our allies necessitated the United States being combed for every foot of available material and the speeding up of production until every wire-mill in America was working twenty-four hours a day. Yet, in spite of labor troubles, housing problems, and the difficulties of obtaining material and transportation, the wire-makers at home filled every requirement of the soldiers overseas. Of all the varied activities of the Signal Corps, none was more fascinating or mysterious in its operation than the work of the Radio Intelligence Sections, particularly the so-called listening-stations, which, by means of supersensitive receiving and amplifying instruments electrically connected with ground-plates placed as close as possible to the enemy positions, were enabled to overhear the ground-telegraph operations of the Germans and the conversation leaking from defective or non-metallic telephone and telegraph circuits. This remarkable service, some of whose achievements would seem to the layman to verge on the miraculous, combined the discoveries of Ohm, Volta, and Galvani with the methods of LeCoq and Sherlock Holmes. These stations could, of course, operate successfully only under favorable conditions, the chief requisites being that the enemy’s trenches should not be too far away and that the intervening terrain should be free of creeks, gullies, or other features which might sidetrack the currents which it was desired to intercept. The listening-stations were usually situated in the second line of trenches, the ground-plates being placed about 300 yards apart. In order to obtain satisfactory results it was necessary that the ground-plates should be placed as close to the enemy as possible, the work of installing them, almost under the noses of the Huns, being one of the most hazardous duties which the signal troops were called upon to perform. The men operating the listening-stations had to remain on duty for a week at a time—a considerably longer tour of duty than