1914] on Improvements in Long-Distance Telephony 99' 



Office, read to the Institution of Electrical Engineers about three 

 years ago.""" 



This cable has a length of 21 nautical miles, and contains four 

 copper conductors forming two separate circuits, these being so 

 arranged that no mutual inductance, and therefore no " cross talk,"" 

 exists between them. The copper is insulated with gutta-percha and 

 armoured, as usual in the case of deep-sea cables. At intervals of 

 1 nautical mile iron-cored inductance coils are inserted in each line, 

 these coils each having a resistance of 6 ohms and the inductance of 

 O'l henry at 750 periods per second. Each iron-wire core has on it 

 two windings which are placed respectively in the circuit of the two 

 wires forming the lead and return of each circuit. 



These coils are of such form that when insulated over they merely 

 present a certain extra thickening of the cable for a length of about 

 80 inches, and with some extra care presents no insuperable difficulty 

 in either laying the cable or picking it up. 



The construction of the core of this 1910 Anglo-French tele- 

 phone cable is such that if no loading coils were introduced, the 

 attenuation constant at a frequency of 750 would be nearly ^^ or 

 0*0520. This means that in a run of twenty miles the amplitude of 

 the current would be reduced to 36*7 per cent of that at the 

 sending end. .As loaded with the coils of 100 millihenrys induct- 

 ance placed one nautical mile apart, the attenuation constant is 

 actually only -^q or 0*0166, which means that the current would be: 

 reduced in the above ratio by travelling sixty miles along the cable. 



In the case of this 1910 Anglo-French telephone cable, special 

 measurements showed that the attenuation constant for different 

 frequencies between 700 and 1700, varied almost proportionately to 

 the frequency. This shows that, although Heaviside's condition for 

 complete non-distortion is not fulfilled, yet the diminution in the 

 attenuation which is obtained by loading resulting in greater loudness 

 in the sound, is a very great improvement over the unloaded cable. 



A second loaded cable was laid in 1911 by the British Post Office 

 across the Channel between England and Belgium, running from 

 St. Margaret's Bay near Dover, to La Panne in Belgium. This- 

 cable, which has a length of forty-eight nautical miles, was also 

 manufactured by Messrs. Siemens Bros., and is of the same general 

 construction as the Anglo-French (1910) cable in that it contains 

 two complete metallic circuits each separately loaded. It is note- 

 worthy, however, that the guttapercha used in the insulation of the 

 Anglo-Belgian cable is telephonically much superior to that in the 

 Anglo-French cable. 



TVip pxplanation of this matter is as follows : — Every dielectric 



* See Journal of the ProceediiiL,s cf the Inst. Elect. Eng. vol. xlvi. The 

 lecturer was indebted to Major O'Meara for the loan of slides for this discourse^ 

 illustrating the structure and laying of this 1910 Anglo-French loaded tele- 

 phone cabl'^ 



H 2 



