Telegraph

Among the printing systems—exclusive of multiplex—employ ing the five-unit code (Plate I. fig. 6) may be mentioned the Murray, Morkrum, Kleinschmidt, the Siemens and Halske and Creed systems. Other printing systems have been developed by Buckingham, Creed and Barclay, which Use the dot-dash code.

Multiplex Methods.—It is possible, with the printing systems employing synchronous distributors, to provide additional trans missions or channels of communication. This is accomplished by dividing the use of the line between the several channels, the distributors being made use of for this purpose. The multiplex principle may be illustrated by fig. 19, which shows a sending and receiving distributor, the brushes of which are rotated in constant synchronism and phase. The sending distributor is provided with five segments to which are connected five keys, which may be operated manually or by automatic means, to set up the five-unit combinations of the Baudot code. Five corresponding segments on the receiving distributor are connected to the five selecting magnets of a printer. Assuming that a combination corresponding to the desired character has been set up on the transmitting keys, signals forming this combination will be transmitted to the line as the sending brushes pass over the sending segments. At the instant when the sending brush is in contact with the first send ing segment the receiving brush will be in contact with the first receiving segment and if No. i key is in an operated position the circuit will be completed and the first selecting magnet will be energized. As the sending brushes pass to the succeeding seg ments the remaining selecting magnets will be operated or remain inoperative, depending upon the position of the corresponding key at the sending station; thus in each revolution an entire char acter is transmitted by the transmitter and received by the printer. While the diagram illustrates only one channel of communication it is obvious that the section of the rings A and A' between seg ments 5 and I may be provided with additional groups of five segments which can be connected to other sets of keys and selecting magnets to provide additional channels of communica tion. The character of transmission is such that duplex methods may be applied to permit simultaneous operation in both direc tions. This principle is widely employed and the modern multi plex systems ordinarily provide from two to five separate trans missions in each direction simultaneously. These systems are

usually designed for a maximum speed of about 36o letters per minute per channel, that being considered as the maximum speed at which operators can economically work for extended periods. Multiplex methods were first introduced by Farmer in America. He suggested the division of line time between two or more opera tors. Myer and Delaney endeavoured to improve the system, but it was Baudot who developed the first practical multiplex print ing system using the five-unit code. This system has become the basis of the other modern multiplex systems, such as the Murray, American and Morkrum multiplex systems. Multiplex systems developed by Rowland and Potts employ eleven and eight unit codes, respectively.

The growth of automatic telegraphy has been particularly rapid during the past 20 years. Practically all ocean cables are operated by automatic methods and approximately 70% of land line traffic in America is handled by printing systems.

Repeaters.

All telegraph circuits are subject to certain trans mission difficulties which limit the length of line over which direct working is practicable. The resistance of the line conductor and its imperfect insulation tend to reduce the strength of the current available to operate the receiving instruments. In addi tion, and what is frequently of greater importance, the current pulses representing signals are distorted from their original form by the electrostatic capacity of the line conductor and any induc tance, such as that of the electro-magnets of instruments in the ing distributors must rotate in synchronism. This is accomplished in several different ways with different systems. In some, extra impulses are transmitted for this purpose, while in others the signals themselves are used. The printers used in connection with these systems are arranged to record the message on a page in final form for delivery or to record on a gummed paper tape which is pasted on a blank by the receiving operator. (See Plate I., fig. 8.) Various methods of transmission are used with these systems: circuit (see Theory of Transmission below). Because of these difficulties it is the practice to break up long telegraph circuits into two or more sections, each of which is relatively easy to operate. These sections are then connected together by repeaters which receive the signals from one section and automatically retransmit them into the next section.