received by printing-telegraph apparatus. The introduction of machine telegraphy took place after 1910.
Progress can be divided into two general classes: (1) that relating to terminal equipment, such as printers; and (2) that relating to the methods of working lines.
Printing Telegraphy
(a) Multiplex System.—The multiplex system giving double-duplex, triple-duplex and quadruple-duplex service, as applied in America, was that developed jointly by the Western Electric Co. and the Western Union Telegraph Co. It uses the Baudot code and a system of speed correction for rotating distributors in which correcting impulses are generated from the character signals, thus saving line time.
Fig. 5.—Simplified Terminal Circuit of Multiplex Printer System.
The fundamental features of one arrangement for quadruple-duplex operation are shown in fig. 5. The sending, receiving and correcting rings are parts of a distributor driven by a La Cour or phonic-wheel motor. The common sending ring is connected to the midpoint of a differentially wound line relay and the common receiving ring to the armature of a printing relay included in the local circuit of the line relay. This local circuit also includes an impulse relay, so arranged that short-current impulses are sent to the common ring of the set of correcting rings when the line relay armature moves from one contact to the other. These impulses come at intervals determined by the signals transmitted by the distant station. There are twice as many correcting segments as sending segments, and these are alternately connected to the windings of a correcting relay. One distributor on a circuit sets the speed for the other and if the corrected distributor is running too fast, the operation of the correcting relay causes the driving fork for its motor to be retarded in its rate of vibration. If, on the other hand, the distributor is running too slowly, the fork is accelerated. It is essential to secure correcting impulses when all printing channels are idle. This is done by reversing the polarity of the marking signals of one or more channels. Various types of printer units have been successfully used with this system. Speeds as high as 50 to 60 six-character words per minute per channel are maintained by operators. The multiplex printer system has thus greatly increased operators' loads as well as the number of telegraph channels which can be obtained from each line circuit. Means have been developed for economically extending the single channels of a multiplex-printer system from the multiplex terminal station to branch offices.
Fig. 6.—Circuit of Start-Stop Printer System.
(b) Start-Stop Printer Systems.—Successful systems giving single-channel working, or two-channel working when operated duplex,
