98 BELL SYSTEM TECHNICAL JOURNAL 



nary telephone receiver. Consequently, to avoid overloading the 

 telephone repeaters when they were used on the public address circuits, 

 a device was proposed which would give visual indication on an instru- 

 ment when the speech level was such as to cause the telephone repeaters 

 to overload. 



Further development of this idea led to the experimental device 

 which was used in the Armistice Day ceremonies and which later, 

 with no fundamental change, became the well-known 518 and 203 

 types of volume indicators. This device consisted of a triode vacuum 

 tube functioning as a detector, to the output of which was connected 

 a d.-c. milliammeter. Associated with the input was a potentiometer 

 for adjusting the sensitivity in 2 db steps. The method of using the 

 device was, to adjust the potentiometer so that the maximum move- 

 ment of the milliammeter needle reached the mid-scale point on an 

 average of about once every ten seconds, occasional greater deflections 

 being disregarded. The volume level was then read from the setting 

 of the potentiometer which was marked in decibels with respect to a 

 reference volume level. 



The reference level was chosen as that level of speech which, when 

 transmitted into the long telephone circuits, would cause the telephone 

 repeaters with which they were equipped to be just on the verge of 

 overloading as evidenced by audible distortion. The gains of the 

 telephone repeaters were normally adjusted so that the level at their 

 outputs was 10 db higher than at the sending end of the circuit. 

 Reference volume was therefore specifically defined as 10 db below the 

 maximum speech level which could be satisfactorily transmitted 

 through the particular amplifier and vacuum tube used in the telephone 

 repeaters. This level was determined experimentally and the po- 

 tentiometer steps of the volume indicator were marked accordingly. 

 The reference volume was also approximately the volume delivered 

 over a short loop by the then standard subset when spoken into with 

 a fairly loud voice. 



It is apparent that the volume indicator was born in response to a 

 definite need, and it has filled an important niche in the rapidly growing 

 radio broadcasting industry and in other communication fields. Large 

 numbers of volume indicators similar to this early type have continued 

 in service to the present time. 



It is a frequent characteristic of a rapidly expanding art that at 

 first standards multiply, and finally a point is reached where simplifica- 

 tion and agreement upon a single standard becomes imperative. This 

 has occurred in connection with volume indicators and since the de- 

 velopment of the first one, a variety of instruments have been produced 



