236 BELL SYSTEM TECILXICAL JOURNAL 



given a slow uniform rotation about its vertical axis. With the 

 film on the drum, the record thus had a helical form. In this way 

 records of the requisite length were obtained. 



The condenser transmitter was of the type de\cl<)ped by E. C. 

 W'ente, its characteristics combining with those of the amplifier and 

 oscillograph \ibrator in such a wa\' tluil tlie combined amplitude 

 response for the whole system was fairly uniform up to 5000 cj'cles, 

 while the phase lag was approximately a linear function of frequency 

 over the same range. This apparatus was therefore well adapted to 

 the production of faithful records of the vowel sounds. The photo- 

 graphic equipment permitted the use of a time scale as great as six 

 meters per second on the record (i.e. 2 inches = 0.01 sec.) 



Transform..\tion of Records for Ax.vlvsis* 



The oscillograms taken with tiie abo\c apparatus were line recortis; 

 in order to analyze these wave forms by the photo-mechanical method 

 outlined below, it was necessary to transform the line record into a 

 black profile. This was accomplished in the following steps: 



(1) A positive print of the wa\e form on the original record was 

 made on motion picture film. 



(2) The emulsion of the positi\e print was then cut through to 

 the base along the line of the wave by means of a stylus. 



(3) The entire strip was blackened (on the emulsion side) with 

 printer's ink. 



(4) The emulsion on one sidt' of llu- wa\e was stripped from the 

 base, thus leaving the profile. 



(5) The beginning and end were joined to form an endless belt. 



Piioto-Mechankal .Analysis of the Prepared Records* 



The i^rinciple of the |)hoto-mechanical analysis is as follows: The 

 motion of the strip past the image of an illuminated slit causes fluc- 

 tuations in a beam of transmitted light which in turn, produce volt- 

 age fluctuations in the circuit containing a selenium or photo-electric 

 cell. This voltage is then anah'zed by means of a tuned circuit, 

 an amplifier and a rectifier. The frequency of any component 

 selected in this manner is determined by the tuning frequency' divided 

 by the ratio of speed transformation (analysis speed divided by the 

 original speed of recording). The measured amjilitude of the selected 



* Pliys. Rev. 23, 1924, p. 309 It is phinned to publish a more detailud descrip- 

 tion of this apixiratus later. 



