ox LOADED TELEPHONE CABLES. 



By I)K. J. A. FL1:MI\G. F.K.S. 

 Professor of Electrica! Eiiiiiiccrun; in L'liivjrsity College. London. 



W HEN the telephone was first invented and bu.L;;in 

 to be used, more than thirty years ago, anticipations 

 were indulged that it would be possible to transmit 

 the songs of an operatic prima donna or the speeches 

 of a public orator by submarine cable between 

 Europe and America. But a very little experience 

 showed that severe limitations existed to the trans- 

 mission of telephonic speech through a cable. The 

 reason for this is the electrostatic capacitv of the 

 cable. A copper wire surrounded by gutta-percha or 

 indiarubber, and buried in the sea or soil, forms 

 a virtual Leaden jar of large capacity The electri- 

 cal capacity of an ordinary Leyden jar. such as is 

 used in wireless telegraphy. ma\- be about one-five- 

 hundredth of a microfarad. The capacit\- of one 

 mile of submarine cable ma\- be one-third of a 

 microfarad, or nearly one hundred and se\ent\- times 

 as much. Hence e\'en twenty or thirt\- miles of 

 submarine cable has a \-ery considerable capacity : 

 and the capacity of an Atlantic cable is about eight 

 hundred microfarads, or about the same as the 

 capacity of the whole earth considered as a sphere 

 free in space. The effect of this capacitv is tliat if 

 we attempt to send an electric current through the 

 cable it has (so to speak) to be filled up with 

 electricity- before an\- current begins to flow out 

 at the distant end. Moreover, if we make sudden 

 changes in the strength of the current at the sending 

 end, these changes are not reproduced instantlv at 

 the other end, or in the same degree. The mathe- 

 matical theor\- shows that the speed with which 

 these current changes travel along the cable depends 

 upon their frequency. Also, the degree to which the 

 amplitude of these changes decreases, or attenuates. 

 depends upon their frequency, and upon the constants 

 or structure of the cable. Current changes of high 

 frequency attenuate luore rapidly and travel faster 

 than those of low frequence 



Now when an articulate word is spoken to the 

 diaphragm of a telephone transmitter, the rapid 

 changes of air pressure which constitute' sound, 

 compress the diaphragm and produce corresponding 

 changes of resistance in the carbon granules of the 

 microphone. These again cause variations of like 

 nature in the electric current entering the cable. 

 These current variations are a more or less complete 

 copy of the air pressure changes. If the cable could 

 transmit these current changes unaltered to the 

 telephone receiver at the other end, speech would be 

 perfectly reproduced. The wave-form, or mode of 

 current variation, which corresponds to articulate 

 speech, is very complicated, but it ma\- be resolved 

 into the sum of a number of vibrations of different 

 frequency and amplitude. The effect of the electrical 

 capacity of the cable, as alreadv mentioned, is to 



cause an attenuation, or weakening, in the amplitude 

 of the current vibrations as they are transmitted 

 along the cable, and this attenuation affects the 

 higher or shrill notes more than the lower or deep 

 notes. Also the higher notes travel faster than the 

 lower ones. It w ill easih' be seen that the result of 

 this inequalitv is that the wave-form of the current is 

 distorted by transmission. The different constituent 

 notes or harmonic vibrations arrive at the far end of 

 the cable unequalh" degraded, or attenuated, and 

 shifted in phase relativelv with each other, the high 

 \ibrations having outrun the lower ones. 



If this distortion has not proceeded be}-ond a 

 certain limit, the ear of the listener is able to guess, 

 from the sound heard, the meaning of the word, just 

 as in the case of bad or ordinar\' handwriting we are 

 able to guess from the general shape of the written 

 word what it means although the individual letters 

 are badly formed or distorted. If. however, the 

 distortion has proceeded bevond a certain point, then 

 the ear is unable to attach a meaning to the sound 

 heard, .\part. therefore, from anv imperfection in 

 the actual telephonic instruments, or in the speech 

 oi hearnig of the two comruunicants, we have a limit 

 to the telephonic transmission of speech, imposed 

 bv the distortional qualities of the cable itself. 

 •Vccordinglv, it was soon found that the limiting 

 distance of speech through an ordinary submarine 

 telegraph cable might be taken as twenty miles or 

 so. depending on the size of the core. In the case 

 of land, or overhead, lines, this limiting distance is 

 verv luuch larger. The cajiacitv of an overhead line 

 per mile is not a one-hundredth or one two-hundredth 

 of that of a submarine cable, and therefore 

 telephonic speech is possible through several 

 hundred miles of ordinarv overhead wire. 



The question of the improvement of telephony b_\- 

 underground and underwater cables soon began to be 

 discussed, and foremost amongst those \vhose writings 

 assisted in laying the true scientific foundation was 

 Mr. Oliver Heaviside. He showed that the true 

 antidote to the capacity effects of the cable was to 

 add to it inductance. This term may be defined, for 

 the ordinarv reader, as follows: — A coil of wire, 

 especiallv a coil of manv turns, possesses a property 

 in virtue of w liich a current started in the wire tends 

 to run on, anil also the starting of a current takes 

 time. Inductance, electrically speaking, corresponds 

 to inertia in the case of ordinary matter. Generally 

 speaking, we mav say that the presence of inductance 

 hinders rapid changes of currents in a line, just as 

 inertia in machinery hinders ver\- rapid changes of 

 speed in moving parts. For this reason non-mathe- 

 matical electricians of the old school had arrived at 

 the idea that inductance in a telephone line should be 



266 



