SEVENTY-FIVE YEARS OF THE TELEPHONE 225 



cury-arc oscillator^^ was used as a source of currents for measurement with 

 a thermocouple operating a galvanometer. Telephone circuits were measured 

 by these cumbersome means but there were limitations to the range of fre- 

 quencies and levels. This oscillator was used also with various forms of 

 bridges for measuring the impedances of lines and apparatus. 



The vacuum tube amplifier and oscillator quickly opened up the electrical 

 measurement of telephone circuits at the levels of speech current and re- 

 placed the laborious computation of circuit performance. Also the amplifier 

 made it possible to use the oscillographs of the time to make photographic 

 records of speech currents and of single frequency currents of corresponding 

 levels.^'*' ^^ These measurements and records revealed a lot about telephone 

 transmission properties of lines and apparatus and put the design of trans- 

 formers^® and other circuit elements on a better basis. 



In 1915 a proposal was made by Dr. H. D. Arnold, the carrying out of 

 which had momentous effects. The proposal was that the vacuum tube 

 amplifier be associated with as nearly perfect devices as could be developed 

 to carry out the functions of transmitter and receiver and by these means 

 to create a practically perfect telephone transmission system which would 

 approach air transmission. With the large amplification available, it would 

 be possible to utilize transmitters and receivers in which efficiency of con- 

 version could be sacrificed to the extent necessary to approach freedom from 

 distortion. 



Arnold also had the conception that, with this nearly perfect transmission 

 system, the effects of distortion on the intelligibility of reproduced sounds 

 could be studied in a controlled manner — that is, distortion could be in- 

 troduced into the electrical part of this transmission system by electrical 

 networks and therefore be specifiable and reproducible. 



In carrying out this proposal, Crandall and then Wente worked on the 

 development of the required transmitters and receivers and from this work 

 came the condenser transmitter^"^ ■ ^^ and later the high quaUty moving coil 

 receiver.^ ^ 



From this activity then came some more important concepts and results. 

 A transmitter of the condenser type which was stable and uniform in re- 

 sponse over a wide range of frequencies was used with an amplifier operating 

 into a meter to give a direct-reading indication of the magnitude of sounds 

 even at quite low levels. With this there was developed^o- 21. 22 the theory 

 of the thermophone as a means of setting up, in a specified closed chamber 

 associated with the condenser microphone, an absolute level of sound, so 

 that the combination of the condenser transmitter and amplifier could then 

 be used to give an absolute measurement of the intensity of a sound field 

 at a point. This made possible the absolute measurement of sound over the 

 range of intensities and frequencies involved in speech and hearing. By 



