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xii FOREWORD flying field at the point of destination, and is called radio beacon station. It was found that a radio beacon would bring an airplane to the immediate vicinity of the flying field but would not exactly locate it during periods of poor visibility when the ground could not be seen, due to the fact that there is apparently a blind spot around each regular transmitting station, the size increasing with the wavelength. The pilot would therefore lose his direction just when he needed it most. It was to fill this gap that the radio field localizer was developed. This device is a peculiar kind of radio transmitter which transmits verti- cally in the form of a cone which rapidly gains diameter with increased altitude. At the height of three thousand feet above the field, such a cone can be made to have a diameter of nearly a mile. The pilot flying through fog or rain enters this zone of sound before the radio beacon station has become ineffective. Radio thus lends a guiding hand to the pilot from the time he leaves the ground with the latest radio information gathered along the entire route to the time he lands his airplane at his destination many miles away. Radio changes what would otherwise be a trackless sea to well defined airways over which the commerce of the future will move with safety and at a speed which will bring San Francisco within daylight distance of New York. Radio has another important function in aiding the com- munications of every-day life. Few know that telegraphy and telephony over land wires have taken advantage of this friendly magician. It is indeed magic, for radio now makes it possible to transmit many telegraph and telephone messages simultane- ously over one wire. In this case the wire becomes a guide, and the many messages are carried on high frequency waves which move in the space adjacent to the wire without interference. This system is called "wired wireless" or "line radio." It i«