Wireless Telephony in Aerial Service.— The exigencies of the Great War led to rapid development in wireless telephone apparatus and methods for army and navy needs in this country and abroad. There was especial need for wireless telephony in the aerial service and great progress was made in meeting these de mands. For the latter purpose apparatus of minimum weight was necessary. The dimen sions, for example, of a complete outfit for the use of the United States Signal Crops, con taining vacuum tubes, transformers and plate circuit dry batteries are nine inches by seven inches by five inches, and it weighs eight pounds. The input transformer used in this set is of the shell type, wound with 6,000 feet of No. 40 enameled wire with paper insulation be tween the layers of wire, and weighs two pounds.
In Figs. 7, 8, respectively, are given illus trations of transmitting and receiving types of three electrode vacuum tubes used by the United States Army and Navy signaling de partments in the late war. In Figs. 9, 10 are illustrations of airplane transmitting and re ceiving wireless telephone sets utilized by the United States Signal Corps. Wireless tele phone communication between flying machines in the air and between the planes and ground stations; also by means of wire telephony be tween the men in the airplane itself, was daily becoming of more importance toward the close of the war, and many valuable improvements were developed to facilitate this communica tion. Consult a paper on "Radio Telephony' by Craft and Colpitts (American Institute Electrical Engineers, 21 Feb. 1919).
Multiple Wireless Telephony.— In wire telegraphy. the term duplex telegraphy signifies a system in which two messages are sent over one wire simultaneously in opposite directions. By diplex telegraphy is meant the sending of two messages in the same direction over one wire simultaneously. The term multiplex teleg raphy usually covers telegraph systems by which more than two messages are transmitted over one circuit at the same time in either di rection. In wireless telegraphy the term "du plex telegraph? is applied to means by which two wireless messages are sent in opposite directions at once from two wires on one sup porting tower, or preferably from two wires on separate towers, and again preferably with the respective transmitting and receiving aerials By means of this system it is possible to trans mit over one wire simultaneously four or more messages by means of electrical vibrations.
In this system four tuning forks of different rates of vibration, say, 264, 320, etc., per second, are each given control of a battery by contact points suitably fixed on the forks. These bat teries are arranged in series in a main line circuit. The tuning forks are kept in constant vibration by an electromagnetic device similar to that of the ordinary electric door bell, the forks being used as the armature of the mag net. When in operation these forks open and close the circuits of their respective batteries at a rate corresponding to the fundamental vibration of the forks, and thereby set up four different trains of electric impulses in separated a distance of 25 or 50 miles, in which latter case the transmitting and receiv ing apparatus are connected by land wires, thus making it possible to localize the trans mitting and receiving operators in one room. This is termed remote or distant control. The actual adjustments of the wireless apparatus are made by attendants at the transmitting and receiving antenna. Utilizing remote control methods during the late war, the high-power wireless transmitter stations at Annapolis, Md., Tuckerton, N. J., New Brunswick, N. J., 'and Sayville, L. were operated by four operators in one room in the Navy Departthent Washington, D. C.
The term multiplex wireless telephony may also be utilized as mentioned in the case of duplex wireless telephony, but preferably it refers to the sending of two or more wireless messages simultaneously from different antenna at nearby stations, say, ships of a fleet, and their reception on the antenna of different ships of a fleet at a distance. One of the best known multiplex telephone systems for this purpose is due to R. A. Heising. The description of this system will perhaps be simplified by the inter polation of a brief allusion to the Gray Har monic Multiplex Telegraph system, formerly in use on land lines.
The Gray Harmonic Multiplex Telegraph.— the main line circuit,. of frequencies equal to those of the respective vibrating forks. A Morse telegraph key is arranged to control the impulses set up by each fork in such a manner that a series of short and long electric pulsa tions eorrespomling to the dots and dashes of the Morse telegraph alphabet may he trans ' mined over the main lines by each key.