OSCILLOGRAPHS 377 



light to a small fraction of what otherwise would be obtained. The 

 vertical slit can be seen in Fig. 7 behind the variable shield. A slight 

 corona-like fogging on either side of the circular shadow of the shield, 

 as shown in Fig. 8, is obtained after a few hours of exposure. In 

 practice a film may be exposed for twenty-four hours or more without 

 the fogging becoming so serious as to obscure a record. 



To avoid confusion due to overlapping records a device is used to 

 stop recording after one complete revolution of the film.' It is put in 

 operation at the beginning of a transient by the high-speed line-relay 

 and allows recording to continue for one complete revolution, regardless 

 of the angular position of the film at which it begins. 



This device is shown on the oscillograph in Fig. 9. A vertical shaft 

 driven at half the speed of the film carries a magnetic clutch fastened 

 rigidly to it and a commutator which idles on the shaft except when 

 engaged by the clutch. This commutator has one insulating segment 

 and one conducting segment, each of angle about 180 degrees. Con- 

 tacts, controlling the current to a relay winding, are normally on the 

 insulating segment of this commutator, but when the high-speed 

 line-relay operates, the magnetic clutch is energized and the commuta- 

 tor is rotated until the contacts touch the conducting segment, thus 

 operating the relay. One contact on the relay automatically releases 

 the magnetic clutch, preventing further rotation of the commutator. 

 Another contact interrupts the current going to the light source. 

 Since the filament of the lamp is small the light is extinguished in a 

 very short time and, of course, recording is stopped immediately. 



After the exposed film has been replaced, and it is desired to put the 

 oscillograph in operation again, the clutch and commutator are restored 

 to their original condition by operating a key which energizes the clutch 

 magnet and releases it again automatically after one-half revolution 

 of the vertical shaft. 



If it is desired to make a record covering more or less than one 

 revolution it can be arranged simply by changing the gear ratio between 

 the film shaft and the vertical commutator shaft. The film speed 

 may be changed either by changing the gear ratio between the motor 

 and the film driving shaft or by varying the speed of the motor. 



The camera for photographing a clock is shown in Fig. 4. It 

 consists of a lens and shutter, shown at the top, and a periscope con- 

 sisting of two right-angled glass prisms mounted in the vertical tube. 

 The periscope places the image in the center of the film and since there 

 are two reflections the image will be the same as if none were used. 

 The shutter is equipped with an automatic release which is operated a 

 definite time interval after the beginning of an oscillogram, the time 

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