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BELL SYSTEM TECHNICAL JOURNAL 



one of the two or three strings provided is shown on the diagram. The 

 images of the strings are focussed by the projecting lens onto the 

 sensitized paper used for the record, where they appear as shadows on 

 a Hght background. An achromatic cylindrical lens in front of the 

 paper further focusses the light into a narrow band with a width of a 



Fig. 12 — Rapid record oscillograph. Front view with mechanism exposed. The 

 developing and fixing tanks may be dropped in a few seconds when required. 



few thousandths of an inch on which the shadows of the strings fall. 

 As the paper is drawn through the machine, the shadows of the vi- 

 brating strings thus photograph on it a trace of the motion of the 

 middle of the string. 



Between the lamp and the condensing lens is a timing wheel whose 

 rotation is controlled by an electrically driven tuning fork. Spokes 



