THE FUTURE OF TRANSOCEANIC TELEPHONY 3 



than one cycle per second. Lord Kelvin with his mirror galvanometer and 

 later with his siphon recorder and other improvements increased the eflfec- 

 tive band width and raised the speed to three words per minute. He 

 showed also how to design cables to raise the frequency limits further, 

 with the result that ocean cables were soon made to work at much higher 

 speeds. 



Other important advances in terminal apparatus and in methods of 

 operation followed. The application of the duplex principle, permitting 

 simultaneous operation of the cable in the two directions, practically 

 doubled its traffic capacity. Improvements in methods of correcting the 

 signals for the distortion introduced by the cable, and the introduction of 

 mechanical means of sending resulted in a further increase of a third in 

 traffic capacity. Another increase of about the same amount was realized 

 when the cable magnifier was introduced in the early part of the present 

 century. 



A major advance ensued with the introduction of the permalloy loaded 

 cable in 1924. The advantages of inductive loading for reducing the 

 attenuation in long circuits had been known for some time and some 

 applications of the Krarup or continuous method of loading had been 

 made to short submarine telephone cables. No practical means, however, 

 of applying this principle to ocean cables was available, since for telegraph 

 frequencies loading with iron wire was not advantageous because of its 

 low permeability. The discovery of permalloy, a material with very high 

 permeability at low flux densities, together with the invention of means 

 for protecting the loading material from the severe stresses that it would 

 otherwise encounter at the ocean bottom, made it possible to build a cable 

 with many times the band width of corresponding non-loaded cables. The 

 increase in traffic over the cable was, however, less than proportional to 

 the increase in frequency range, because duplexing the loaded cable involves 

 a greater sacrifice of one-way speed than is the case for the non-loaded 

 cable. The fastest loaded transatlantic cable has an effeiptive frequency 

 band of over 100 cycles per second and can carry four times as much traffic 

 as a non-loaded cable of the same size and length. 



Development of permalloy loading for telegraph cables naturally led to 

 consideration of the possibilities of a loaded telephone cable to span the 

 Atlantic. Whether viewed as an extension of frequency from 100 cycles 

 to the 3000 cycles needed for high grade telephony, or as an extension of 

 distance, the step was a formidable one. The longest deep-sea telephone 

 cable reached only 105 nautical miles from Key West to Havana, where 

 three cables continuously loaded with iron wire were laid in 1921. The 

 transatlantic span called for a minimum of 1350 miles via the Azores, or 

 1800 miles by the more direct route from Newfoundland to Ireland. It 



