Submarine Telegraph Cable Service

In illustrating the advances that have been made, one writer⁴⁸ compared the output of a cable, laid 50 years ago, with the various methods of operation applied. With the galvanometer, the output was about 70 letters per minute; with the siphon recorder, about 80 letters per minute; when duplexed, about 160 letters per minute; with automatic transmission, about 220 letters per minute; and with printing equipment, about 375 letters per minute.

Originally, most cables were operated in sections;⁴⁹ for example, between New York and London the messages were received and re-sent at Nova Scotia, Newfoundland, and Ireland. About 1918 the first successful through operation was made by connecting the sections through equipment which reshaped and amplified the signals.

Two types of amplifiers^46,50 (magnifiers or repeaters, as they are often called) have been used. The first type consists of mechanical regenerative repeaters, including the rotary type, and the synchronized tuning-fork type. Vacuum-tube amplifiers (the second type) are superior to the mechanical devices, especially for high-speed work. The development and installation of loaded cables for submarine telegraphy was a great advancement.

The first high-speed loaded submarine telegraph cable was laid by Western Union in 1924. The capacity of this cable was 1500 letters per minute. Another similar cable was laid in 1926, having a speed of 2500 letters per minute. In 1928 a cable was laid which had a speed of 2100 letters per minute in one direction, but which could be duplexed, giving over 2800 letters per minute. Pictures are transmitted over transoceanic cables.⁵¹

Submarine cables are subject to interference,⁵² and this is one of the factors limiting cable-transmission speed. The interference comes from artificial sources, such as electrical power plants and railway systems in the vicinity of cable terminations, and from natural sources, for example, electrical and magnetic storms accompanying the aurora borealis. At certain times these natural disturbances may render the submarine cables inoperative.