TOLL TRANSMISSION IN THE UNITED STATES 129 



depends on how completely the transmission system is free from 

 external interference. 



Two methods are commonly employed to minimize external inter- 

 ference: shielding, and a geometrical arrangement of the conductors 

 of the circuit so as to balance out certain forms of interference. The 

 open-wire line, which has no shielding, depends wholly on the symmetry 

 of its conductors and the transpositions to balance out interference. 

 Conductors surrounded by metal sheath, such as cable pairs, are less 

 subject to interference than open-wire lines. The conductors of a pair 

 are close together, are well transposed by twisting, and are shielded 

 to a certain extent by the outside lead covering. As a result of this, 

 the noise due to outside sources has a low level, and the telephone 

 speech currents can be permitted to become attenuated to a relatively 

 low value before reaching an amplifier where they are stepped up to 

 their original value. It is evident, of course, that such cable pairs, 

 being made up of small conductors close together and having paper 

 dielectric, have correspondingly high attenuations. 



Fig. 8 — Diagrammatic representation of the coaxial structure. 



The ideal conductor would be one for which the attenuation over 

 the whole operative frequency range was not too great, and at the same 

 time one completely shielded from the influence of external electric or 

 magnetic fields. The so-called coaxial conductor approximates to 

 these requirements. This conductor transmits efficiently over a wide 

 frequency band, and at the same time is well shielded from external 

 influences, the degree of shielding being higher at the higher fre- 

 quencies where greater amplification is needed to overcome the 

 greater attenuation. 



The coaxial conductor upon which we are experimenting consists of 

 an inner wire and outer tube separated by spacing insulators. It is 

 desirable to separate the two conductors by a minimum amount of 

 solid insulation to the end that the dielectric will be largely air, and 

 the losses at high frequencies be at a minimum. Figure 8 shows 

 diagrammatically a coaxial structure. At high frequencies the current 

 travels chiefly on the outside of the inner conductor, and on the 

 inside of the outer conductor. It will be obvious to you that there is 

 a wide latitude of choice in the dimensions of coaxial structures- 



