Page:EB1922 - Volume 32.djvu/730

Rh ciphered by an additional key. In deciphering the cipher message tape is placed in transmitter A and the characters combined with the proper key to obtain the original message.

(a) Simultaneous Telephone and Telegraph Working.—Considerable advance has been made in the technical knowledge of simultaneous telephone and telegraph operation of line circuits. Hand telegraph systems and single-channel printer systems usually operate at dot speeds of 10 to 25 per second and it has been found that in order to secure satisfactory service it is necessary to design line systems to transmit a frequency band of about 100 cycles per second. Since telephone frequencies range from about 250 to about 2,500 cycles per second, it is possible to secure satisfactory telegraph operation from telephone circuits by using frequencies below the lowest telephone frequencies and frequencies above highest telephone frequencies. More than 600,000 m. of telegraph circuits are obtained from telephone circuits in the United States.



(b) Metallic Telegraph System.—The increase in knowledge of the fundamental requirements of simultaneous telephone and telegraph operation has enabled a telegraph system to be developed for operation over long small-gauge telephone cable circuits. This system is arranged for metallic circuit working using a relay operating with a current of approximately 2 milliamperes. The general circuit arrangements of this system are shown in fig. 8. The cable circuit is divided by a composite set or filter into two branches, one for the telephone and the other for the telegraph, the telegraph branch absorbing frequencies below the telephone interval. All metallic lines in a single office are supplied from a common battery. The type of polar relay selected for this circuit is shown in fig. 9 and the magnetic principle illustrated in fig. 10. The relay is provided with a Gulstad vibrating circuit and the armature, a reed, is the cross piece of a magnetic bridge. The line windings surround the armature, and a current in one direction causes the armature to move toward one pole while a current in the opposite direction causes it to reverse its motion. Chatter at the contacts is practically prevented by cushioning contact springs attached to the armature. This system has been designed for cables containing as many as 300 working circuits. A telegraph repeater is shown in fig. 11. The simultaneous operation of telephone and telegraph circuits has been carefully worked out in connexion with the design of long interurban cables and the equipment used in connexion with them.

(c) Carrier Current Multiplex System.—One of the most interesting telegraph developments is the so-called Carrier Current System in which multiplex operation is secured by the use of a number of alternating currents of different frequencies and of resonant circuits for selecting them at the line terminals. This system uses vacuum tubes for generating, amplifying and rectifying the alternating currents and represents a radical departure in telegraphy. The circuits are of high signal quality, very stable in operation and free from the duplex balance difficulties of direct current systems. This system has enabled the following communication facilities to be obtained 11.—Telegraph Repeater for Cable System. commercially from a single pair of open wires: Twenty 1-way carrier channels; four 1-way direct current channels; and one and one-half telephone circuits including the phantom. The carrier telegraph circuit is illustrated in fig. 12. The terminal apparatus for one 2-way channel, which repeats between the carrier circuit and the direct current extension circuit, is mounted upon a vertical panel similar in appearance to that of the metallic telegraph system. The same sensitive relays are used in both systems.

(d) Rotary Repeaters.—Success has been obtained with the use of rotary repeaters in connexion with telegraph circuits operated by the multiplex printer system. This type of repeater restores distorted line signals to their original form and has enabled printer circuits 3,000 m. in length to be operated successfully at high speeds.

(e) Fundamental Telegraph-Transmission Research.—Considerable attention has been given to the telegraph-transmission problem and improvements have been made in the methods and means for measuring distortion of telegraph signals. The fundamental transmission requirements for different classes of service have been more carefully enumerated and advances made in the design of artificial lines.

(f) Interference.—Advances have been made in minimizing interfering currents in telegraph circuits both from high-tension power lines and from neighbouring telegraph circuits. Means have been devised to overcome the effect of differences in ground potentials on grounded telegraph circuits. This arrangement introduces a counter-electromotive force which is automatically adjusted to neutralize the earth-potential difference between any two given points.

(g) Codes and Sending Machines.—Codes, abbreviations, typewriters and automatic sending machines are now widely used by operators to increase the capacity of manually-operated telegraph circuits. The automatic machine is merely a vibrating reed mounted in a convenient and portable manner, adjusted to vibrate at telegraph speeds and provided with contacts for controlling the telegraph circuit. A movement of the controlling lever in one direction causes the instrument to transmit a succession of dots, the number depending on the length of time the lever is thus held. A contrary movement sends a dash. This instrument permits higher speeds than are otherwise possible to be maintained with considerably less fatigue on the part of the operator. It may be readily connected with any ordinary telegraph circuit.

Codes and abbreviations for shortening messages are used especially in distributing news. The Phillips Code is one that has been generally adopted and an illustration of its use follows:—

Transmitted message:—