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BELL SYSTEM TECHNICAL JOURNAL 



sensitivity. The use of a large mirror makes it possible to use either 

 less sensitive film or a less intense light source than ordinarily would be 

 required for a given recording frequency and film speed. With a half 

 inch mirror it has been found practicable to use a lamp requiring only 

 about two watts for recording on par-speed film. 



Fig. 2 — Calendar clock used with oscillographs. Door is open to show lamp sockets. 



A plane mirror is used on the vibrator, and a single meniscus lens 

 mounted in front of it serves, in virtue of the reflection, as a symmet- 

 rical lens in forming an image of the pinhole on the film. When greater 

 resolution along the time axis is required than can be obtained with 

 this simple system, a cylindrical lens with short focus is placed in the 

 light path just in front of the film. 



Each oscillograph is equipped with a camera for the purpose of 

 photographing a clock on the oscillogram to indicate the exact time 

 of occurrence of the disturbance recorded. Any other information it 

 is desired to associate with the records made by a particular oscillo- 

 graph may be recorded photographically along with the clock. In 

 several cases calendar clocks have been employed indicating the day 

 and the month, and indicating whether the time is A.M. or P.M. 



