26S 



KNOWLEDGE. 



JlLV. 1911. 



easily 

 underground 



turns of silk-covered copper wire like a trausfurnier 

 (see Figure 2). These loading coils are enclosed in 

 iron cases and fixed on the 

 posts. It is easy to construct 

 such a coil to have an induct- 

 ance of about (I'l heiirw and a 

 resistance of onh- ten to fifteen 

 ohms. 



Loading coils can also be 

 inserted in the run of 

 telephone cables, 

 and such loaded circuits have 

 been much employed of late 

 years In' the National Telephone 

 Companx' (see Figure 3). A 

 more interesting and recent ap- 

 plication is in the case of sub- 

 marine telephone cables. So 

 inferior is even a length of 

 twenty miles or so of submarine 

 cable to an equal or even much 

 greater length of aerial line in 

 the transmission of telephonic 

 speech, that the great obstacle 

 in conducting telephonic com- 

 munication with Ireland oi the Continent has al\\a\s 

 been the interposition of the t\\cnt\- to eight\" miles 

 of submarine cable necessar\- to cross the English or 

 Irish Channel. The problem of loading a submarine 

 cable was. however, one of peculiar difficult^ In an\- 

 case the la\ing of an unloaded submarine cable 



rather reluctant to embark on the enterprise of la\ing a 



suhmarine cable 



having heavy protuberances in it 

 every mile or so. Hence as a 

 first step attempts were made 

 at continuous loadin 

 inductance of 

 increased hv 



6- The 

 a cable can be 

 windine o\'er it 



\ brick pit for coiitaiuint,' the loadin,;^ 

 coils inserted in telephonic cables. 



many lavers of fine iron wire 

 separated, however, from the 

 copper b\' an insulation. The 

 effect of this is to increase the 

 magnetic field surrounding the 

 conductor when a current flows 

 throui^h it. .Such a continuousU' 

 loaded cablf will be thicker 

 and heavier than an ordinary 

 unloaded cable, but the risks 

 in laying it are not thereby 

 seriously increased. Several 

 continuoush' - loaded telephone 

 cables were laid in Denmark. 

 There are howe\'er. difficulties 

 in the wa\- of foretelling the 

 actual performance of such a 

 class of cable, which do not 

 exist in the case of cables loaded in the Pupin 

 manner. The first cable of this last type laid under 

 water was that put down a few \'ears ago in Lake 

 Constance. The length of this cable is nine miles 

 and it consists o( a paper-insulateci lead-co\ered cable, 

 having loading coils inserted in it at intervals. The 



FiGliRB 4. — Section of cable containing loadin;,; coils, complete with sheathing wires 



Figure 5.- — .Arrangement of coils in cable. 



JUTE SERVlNGlZ UAVERS) JUTE PACKING 



S'e'fabout) ■*- 



Figure 6. — Method of sheathing over coils. 



25 - 



involvesengineeringdifficultiesof a considerable kind, performance of this cable was such as to encourage 

 Even when the advantages of inserting the loading further developments, and a year or so back, when the 

 coils had been well ascertained, cable engineers were authorities of the British Postal Telegraph service 



