374 BELL SYSTEM TECHNICAL JOURNAL 



light from reaching the film when the vibrator is at rest. In this way 

 fogging of the film is prevented during the time when no current is 

 flowing into the vibrator but a record is made of any disturbance of 

 sufficient magnitude to move the spot off the shield. A record made 

 in this way appears like an ordinary oscillogram except that a narrow, 



Fig. 7 — Shields to prevent fogging on polar oscillograph. 



clear space is left where a zero line is usually obtained. This is 

 illustrated in Fig. 6-A. 



In the absence of a shield the film soon becomes so badly fogged 

 that an oscillogram made upon it is useless. Fig. 6-B shows the 

 fogging obtained with an exposure of one minute on the zero line with- 

 out a shield. 



If it is desired to record only disturbances in excess of a certain magni- 

 tude, the width of the shield may be increased so that no exposure 



