OSCILLOGRAPHS 



369 



on long strips such as motion picture film and will be called a "con- 

 tinuous-film oscillograph." ^ 



Features Common to Both Oscillographs 



Since both of these oscillographs were designed for recording the 

 same sort of phenomena and for operating under somewhat similar 

 conditions, they have a number of features in common. 



As in most oscillographs the optical system consists of a light source, 

 a mirror capable of being vibrated about an axis in its plane with an 

 amplitude proportional to the signal to be recorded, and a lens system, 

 including the mirror, to form an image of the light source on light 

 sensitive film moved in a direction perpendicular to the plane of vibra- 

 tion of the mirror. 



The light source consists of a concentrated filament flashlight lamp 

 placed as close as possible to a pinhole aperture in such a way that the 

 aperture, as viewed from the vibrator side, appears to be completely 



PERISCOPE 



PRISM 



REVOLVING i~3^!: - 



FILM HOLDER^ I^I^LENS AND SHUTTER 



CLOCK 



LIGHT TIGHT COVER 



LIGHT BEAM 



-^~-:^ 



LENS 



Fig. 1 — Essential elements of polar oscillograph. 



filled by the lighted filament. No condensing lens is used because of 

 the small size of the bulb which permits the filament to be brought 

 close to the aperture. The filament is brighter than its image and the 

 use of a condensing lens in this instance would waste light unnecessarily 

 by reflection and absorption, and would make the optical system 

 larger. 



The vibrator is of the moving-iron balanced armature type similar 

 to a driving element frequently used in loud speakers. The armature 

 is attached by means of a stiff rod to a mirror free to vibrate about an 

 axis in its plane in such a way that, as the armature of the element 

 vibrates, the mirror vibrates at a relatively large angular amplitude. 

 With this type of vibrator it has been possible to employ a mirror half 

 an inch in diameter and still retain a satisfactory frequency range and 



^ This instrument is frequently called a "Movie Oscillograph." 



