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BELL SYSTEM TECHNICAL JOURNAL 



rotating mirror, and if the film speed is doubled, one wave in every 60 

 will be recorded. 



Individual facets on the revolving mirror may be inclined to the 

 axis of rotation in order that successive exposures may be made from 

 different vibrator elements. For example, three facets suitably 

 mounted could be employed to show in succession sample waves of 



Fig. 20 — Samples of polar oscillograms. 



current from the three circuits in a three-phase line, the motor driving 

 the mirror being operated synchronously in phase with the three-phase 

 voltage. 



The advantage in this recording method lies in the ability to obtain 

 a good record of slow changes with good resolution and without the 

 use of a large amount of film. 



Another method of sampling which gives a somewhat dififerent kind 



