OSCILLOGRAPHS 375 



occurs until the vibrator moves at more than a predetermined ampli- 

 tude. In Fig. 7 a shield for this purpose is shown which may be ad- 

 justed to cover a portion in the center of the record from the width of 

 the spot to about half an inch, or removed from the field entirely. 

 As may be seen from the illustration, this is accomplished by moving 

 the shield in guides about the axis of film rotation. 



Fig. 8 — Oscillogram illustrating the use of removable shield. 



There is a disadvantage in using a very wide shield of the type de- 

 scribed. If, for example, a disturbance occurs which is just great 

 enough to be recorded, the major portion of the wave is hidden by the 

 shield and all that can be deduced from the record are the peak ampli- 

 tude, frequency and time. This difficulty can be avoided readily, 

 however, by attaching a shield to the armature of an electromagnet so 

 that it can be removed from the light path when the magnet is ener- 

 gized. The magnet may be operated by the high-speed line-relay, 

 which is adjusted to operate when the disturbance exceeds a certain 

 amount, in this case the same amount that moves the light spot at an 

 amplitude greater than the width of the shield. The oscillogram 

 shown in Fig. 8 was made with a removable shield of this type. The 

 shadow of the shield is indicated in the first four cycles but does not 

 appear during the remainder of the oscillogram. For convenience in 

 interpreting the results, a zero line is automatically recorded im- 

 mediately after the recording of the oscillogram. 



