SEVENTY-FIVE YEARS OF THE TELEPHONE 223 



in telephony strove to be quantitative. In their judgments of the relative 

 talking performance of two instruments or circuits, percentage figures were 

 used to show degree of difference. Later the length of trunk in one of the 

 circuits was adjusted to get judged equality of performance and, at the be- 

 ginning of the century, there was adopted the Standard Cable Reference 

 System, with an adjustable network representing a 19-gauge cable pair in 

 the trunk connecting commercial type common battery station sets.* By 

 comparisons and substitutions, numbers of "miles of cable" were associated 

 with relative performances on the basis of effect on the loudness of the re- 

 produced sounds. 



In 1912, a bulletin was issued for the use of the Bell System Operating 

 Companies — largely the work of O. B. Blackwell — in which quantitative 

 ratings in terms of the cable reference system were placed on the perform- 

 ances of various instruments and sets, and loops and trunks of different 

 gauges of conductors. 



Theory 



Around the beginning of the 20th century, the "theory" factor began to 

 Increase significantly. Prior to that, the theoretical material appHcable to 

 telephony was very limited — such as that produced in Europe by Helm- 

 holtz. Hertz, Rayleigh, Poincare and Heaviside. Within a decade, there was 

 produced a wealth of theoretical material dealing specifically with the prob- 

 lems of telephone transmission. This is exemplified outstandingly by the 

 work of G. A. Campbell— his theory of loading,^ from which he developed 

 the theory of the electric wave filter,^ theories of electrical networks as 

 pubHshed in his article "Cisoidal Oscillations"'^ and his exposition of maxi- 

 mum output circuits^ covering aU ways of achieving, with one transformer 

 and one balancing impedance, what has come to be called the "anti-side- 

 tone" station circuit. While some of this work of Campbell's was not pub- 

 lished until later, it was available to his colleagues. 



The results of these theoretical studies of Campbell and those of his con- 

 temporaries of that time, notably Blackwell, were utilized to explore by 

 computation the transmission of telephonic currents over lines and through 

 the various circuits associated with these lines in central oiOSces and at tele- 

 phone stations. 



Because of the complexity of many of these circuits and the need for ex- 

 ploring them for the range of frequencies involved in telephonic transmis- 

 sion, use developed of the equivalent network for computing the transmission 

 effects of circuits consisting of pieces of open wire and cable, with the trans- 

 formers, relays and networks associated with them at terminal or switching 

 points. The application of these theories to the solution of telephone net- 

 work problems was presented in books by K. S. Johnson^ and T. E. Shea.^® 



