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BELL SYSTEM TECHNICAL JOURNAL 



It involves, in addition to the usual optical system and means for 

 advancing the film, an additional mirror in the light path between the 

 vibrator and the film, rotating synchronously with the current or 

 voltage to be recorded, about an axis perpendicular to both the direc- 

 tion of motion of the film and the axis of the vibrator mirror. The 

 function of the rotating mirror is to sweep the light beam along the 

 oscillograph film past an aperture A in such a way that the efifective 

 film speed during exposures is many times the actual film speed, and 

 to permit of exposure during only a small part of the total time. 



Fig. 18 — Sample of records made by polar and continuous-film oscillographs used 



together. 



As an example, suppose that the mirror makes one revolution in 

 two seconds and that the wave to be recorded has a frequency of 60 

 cycles per second. If the distance of the rotating mirror from the film 

 is 8.5 inches, one cycle of the wave recorded will be spread over ap- 

 proximately one inch of film. If a rotating mirror with a single facet 

 is used, and if the aperture is just one inch wide, the actual film speed 

 should be one inch in two seconds and every one hundred and twentieth 

 wave will be recorded. If two facets 180 degrees apart are used on the 



