224 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



While much of this theoretical material had very little effect at the time 

 upon the development of telephone instruments, it provided a storehouse 

 to be drawn upon later for that purpose by the brilliant concept of an 

 analogy. 



A further pubUcation to be noted in that first decade of this century is 

 another by Campbell on the use of syllables to measure the efficiency of 

 telephone circuits in reproducing intelUgible speech.^^ 



Measurement 



With this sketch of the roles of invention, experiment and theory, the 

 stage is set for the great part to be played by measurement and what it 

 fostered. The major theme of this part may be briefed as the role of measure- 

 ment in promoting and implementing the application of theory to design. 



In communication by telephone, the performance of the telephone system 

 is inextricably combined with the performances of its users. This relation- 

 ship is close for all the devices of the telephone set which directly involve 

 the user, but is especially so for the instruments. This means that not only 

 are physical measurements needed of instrument performance — input and 

 output sounds and corresponding electrical counterparts — but also subjec- 

 tive measurements of performance involving the talkers and Hsteners — the 

 generation and understanding by them of speech sounds and their reactions 

 to the conditions of telephony. 



Until the early part of the 20th century there was no means of measuring 

 electrical currents or voltages of the magnitudes and frequencies involved 

 in telephony. Progress in the field of acoustics also had been small because 

 the means for quantitative measurement there were Hmited. For the design 

 of telephone instruments there was little quantitative information as to the 

 relations which should be maintained between the original sounds and the 

 reproduced sounds to provide for their recognition. This situation tempers 

 any criticism against the lack of great progress in the period which was 

 necessarily Hmited to development by "cut and try" and crude qualitative 

 judgments of performance. 



Physical Measurements 



The electronic vacuum tube — the epochal invention of Lee DeForest — 

 was first welcomed into telephony as the long-sought means of stretching 

 the toll fines across the country and thus making Bell's ''grand system" 

 cover the nation. Soon after this accomplishment, however, it was recog- 

 nized that the vacuum tube had other important appUcations — as an ampli- 

 fier for measuring the currents and voltages^- of telephony and as an oscil- 

 lator in generating currents of the frequencies in the voice range. For a 

 short time prior to the availability of the vacuum tube, the Vreeland mer- 



