556 BELL SYSTEM TECHNICAL JOURNAL 



The present situation in transmission, particularly toll transmission, 

 is characterized by the growing^ use of high frequencies, the trans- 

 mission of broader frequency bands and the use of the so-called 

 "carrier" method. This we find in the most advanced form in the 

 proposed very broad band cable circuits of the coaxial and other types. 

 Transoceanic telephony and broadcasting are other outstanding uses. 

 The electrical wave-filter in its many forms is one of the most important 

 elements in all such systems. The filter appears as a means of sharply 

 separating the currents of different continuous bands of frequencies in 

 carrier telephone and telegraph systems, for sharp selectivity in radio 

 systems and for various other uses in these and other forms of trans- 

 mission such as telephone repeaters, telephotography, composite sets 

 and testing apparatus. Indeed, the filter has, within the last few 

 years, become almost as ubiquitous as the vacuum tube. The funda- 

 mental conception of the electric wave-filter arose out of Campbell's 

 analysis of loaded lines. The patent was issued to him in 1917. It is 

 evident to one reading his famous paper on "Loaded Lines in Tele- 

 phonic Transmission," published in the Philosophical Magazine of 

 1903, that even at that time he had begun to envisage the high-pass 

 and low-pass wave-filters. 



Effective station sets are fundamental to all good transmission. In 

 a memorandum dated October 8, 1906, Campbell disclosed the single- 

 transformer anti-sidetone station circuit which is achieving almost 

 world-wide acceptance. Later, he carried out a comprehensive and 

 conclusive piece of work in revealing all of the possible circuit arrange- 

 ments for doubly conjugate branches and in setting down the im- 

 pedance relations of the line, network, transmitter and receiver of these 

 various branches. This systematic analysis of the problem greatly 

 facilitated a comprehensive survey, giving assurance that all types of 

 circuits would be considered and that those which best fitted the 

 available transmitters and receivers would be selected. The work 

 was summarized in an extensive paper entitled "Maximum Output 

 Networks for Telephone Substation and Repeater Circuits" by 

 Campbell and Foster in the A. I. E. E. Transactions of 1920. 



It would appear that Campbell also originated the articulation test 

 which now finds a use wherever telephone development work is in 

 progress. In a paper entitled "Telephonic Intelligibility," which ap- 

 peared in the Philosophical Magazine of January, 1910, he describes 

 how, in connection with tests he had been conducting, he made up 

 and employed successfully articulation lists consisting of meaningless 

 monosyllables. 



This brief note is intended only to enumerate without elaboration 



