OSCILLOGRAPHS 



373 



apparatus in operation. The time elapsed between the arrival of a 

 pulse and the closing of the operating contact is less than 0.01 second. 

 A schematic diagram of the line relay is shown in Fig. 3. 



A polar oscillograph is shown in Fig. 4 with the light-tight cover 

 removed to show the optical system. The film is held in a standard 

 film holder in a rotating member at the extreme right of the picture. 

 The use of standard film holders facilitates loading in daylight as in 

 an ordinary camera. The film is rotated by a small motor geared to 

 the rotating member. The rotating member is separated from the 

 remainder of the oscillograph by a circular light trap which permits 

 free rotation while shielding the film from external light. The circular 

 light trap used is illustrated in Fig. 1 and in Fig. 5. With this arrange- 

 ment films may be exposed for days at a time under ordinary light 

 conditions without appreciable fogging. 



Fig. 6 — Oscillograms illustrating the use and omission of a light shield over the 



zero line. 



The flashlight lamp is housed in the small light-tight box on the 

 base near the film rotor. Excessive scattering of the light is prevented 

 by a small tube, with a diaphragm near the end, directed toward the 

 vibrator mirror. The vibrator and mirror and the lens of the optical 

 system are mounted on the base near the other end. 



The chief value of this oscillograph lies in its ability to record with 

 good resolution from the very beginning of a transient, regardless of 

 the time at which it occurs, and regardless of the angular position of 

 the film at which it begins. To accomplish this, the lamp is lighted 

 continuously during the time a transient is expected and a narrow 

 shield is placed in the light path of just sufficient width to prevent 



