During the war the offices of the Chief Signal Officer were literally besieged by persons who claimed to have invented various systems of message transmission which could not be tapped, or which, if they were tapped, could not be understood. It was perfectly well known to us, of course, that the German Listening-In Service, particularly in the front-line trenches, was well organized and extremely efficient, and that telephone and buzzer conversations held over our wires were frequently intercepted. It was known, moreover, that Germany had spies, both in France and the United States, whose sole duty it was to tap the governmental telephone and telegraph systems for the purpose of obtaining military information. Scores of devices designed to secure the inviolability of the vitally important messages which were constantly passing over the wires were submitted to the Signal Corps. Anxious as they were to obtain a system of message transmission which could jeer at the efforts of the enemy’s spies, the experts of the Signal Corps steadily maintained that such a thing did not exist, for, as they said with truth, if an instrument could be devised which could transmit and decode a message, there was no reason why the Germans could not in time manufacture one like it, put it on the line, and thus obtain the information desired. One of the inventors who approached the Signal Corps asserted that, though he did not claim to have a device which would render a message indecipherable, he had a system which made it impossible for an enemy agent to tap the wire over which messages were being transmitted without the sender and receiver being instantly notified that some one was eavesdropping upon them, whereupon their conversation would, of course, cease. “Prove it to us,” said the Signal Corps, and provided the inventor with an opportunity to demonstrate his system over a miniature line. Without the slightest difficulty the military experts tapped the line and, with the aid of a stenographer, recorded every message which was sent over it, the quantity of energy which they withdrew for the purpose being so minute that the delicate detectors failed to record the fact that the line had been tampered with.