TRENDS IN PRACTISE AS AFFECTING COORDINATION 171 



with the previous standard of 12 in. Often transpositions are made 

 as frequently as every second pole and are of an improved type givin.g: 

 better balance between circuits; also on the circuits on which carrier 

 telephone is used the i:>hantoms are abandoned. The relative suscep- 

 tiveness to noise frequency induction of the various types of aerial 

 wire construction has been tested for various typical conditions. The 

 results of these investigations are summarized in Table II. 



TABLE II 



Type of .Approximate Relative 



F'lcility Transposition* Susceptiveness t 



1 2 in. phantom Voice (brackets) 1.00 



12 in. side Voice (brackets) 0.50 



8 in. pair Carrier (break irons) 0.25 or less 



* Voice circuits are not so frequently transposed as carrier circuits. Bracket tvpe 

 transpositions require two spans to complete the transposing whereas the break iron 

 type completes the transposing on a single crossarm. 



t Susceptiveness is used in the sense defined by the Joint General Committee, 

 namely, "Those characteristics of a signal circuit with its associated apparatus which 

 determine, so far as such characteristics can determine, the extent to which it is 

 capable of being adversely affected in giving service, by a given inductive field." 



Subscribers' Station Apparatus. — To a large extent the trend of 

 development in subscribers' station apparatus is toward new arrange- 

 ments which provide greater convenience and more closely meet the 

 needs of the users and which have no material effect upon the coordina- 

 tion problem. An important group of developments, however, centers 

 about the improvement of the electrical performance of the station 

 apparatus by removing impairments caused by the earlier types of 

 apparatus. These changes, by improving the quality of speech as 

 reproduced by the telephone system, tend to make more noticeable 

 the impairments caused by the effects of currents induced from external 

 sources. 



The tendency toward an increase in the range of voice frequencies 

 efficiently reproduced by the telephone system tends to increase the 

 range of frequencies of induced currents which may cause noise inter- 

 ference as discussed in the introductory section. An extreme illustra- 

 tion of this is the circuits designed to transmit programs for radio 

 broadcasting stations. The transmission characteristics of these cir- 

 cuits have been improved by including both higher and lower fre- 

 quencies, and in their most modern form these circuits efficiently trans- 

 mit currents of frequencies in the range between 35 cycles and 8000 

 cycles and are therefore capable of being affected by inductive noises 

 over this wide range. 



The room noise conditions at the subscribers' premises have an effect 

 on telephone transmission. This noise besides acting directly on the 



