818 EXPLORATION GEOPHYSICS 



operates a small synchronous motor which rotates a thin slotted timing disc. 

 This disc rotates at 10 revolutions per second and has 9 narrow radial 

 slots which record as .01 seconds, and a tenth slot, wider than the others, 

 which records .10 seconds. Light from the timing bulb shines intermittently 

 through these slots as the disc rotates. This light is reflected from a mirror 

 to the cylindrical lens, where it is focused as a line on the record strip. 

 When the light shines through the wider slot, a heavier timing line is photo- 

 graphed, to indicate tenths of seconds. 



The timing linesf can be maintained to an accuracy of better than 1 

 part in 2000, when 100 cycles per second is the frequency of the controlling 

 tuning fork. For higher accuracies, temperature-compensated tuning forks 

 of 1000 cycles per second have been used. Lower-frequency synchronous 

 motors have been accused of hunting, thus placing a limit on the accuracy 

 for instantaneous measurements of time. High-frequency synchronous 

 motors probably also hunt, but the error due to this cause is less. 



Unexposed recording paper, in standard rolls of approximately 200 feet 

 and of any width for which the oscillograph is constructed, usually from 

 Zy^" to 11'', is placed in the supply magazine. From there it is threaded 

 through a paper guide and drawn into a reroll magazine. On the exit side 

 of the reroll magazine is a steel knife for cutting off the record. This knife 

 also closes the paper slot to make the magazine light-tight when it is 

 removed to develop the record. The spool in the reroll magazine is driven 

 by a constant speed, centrifugal contact governor motor. A small red glass 

 window allows inspection of the quantity of paper in the supply magazine. 



As an alternative design, the recording camera can be provided with 

 a drive in which the paper is fed through rubberized rollers which rotate 

 at a constant speed. Also, instead of the paper cutter mounted on the 

 magazine, an externally-operated device is sometimes substituted which 

 will either perforate or cut off the record, as desired by the observer. 



The multiple galvanometer is carefully mounted so as to be practically 

 shock-proof and free from disturbances by truck or other vibrations 

 transmitted through the oscillograph case. The control switches usually 

 are mounted on the lower edge of the front panel or on a separate panel 

 external to the oscillograph. Because of the high recording speeds neces- 

 sary for seismic work and to avoid the use of paper of too high a sensitivity, 

 the galvanometer lamp is operated at a brightness well above its rated 

 value. However, the lamps are so wired that their brightness is increased 

 to the required intensity only when the drive motor is operated, which 

 greatly prolongs their life. 



Recording Papers and Photographic Developing. — The recording 

 medium used in the photographic methods consists of a sensitized material 

 deposited on a paper or film support. The paper should be of a good 



t C. D. McClure, "Timing Device for Recorders," U. S. Patent 2,424,622, July 29, 1947. 

 E. W. Kammer, "Optical Time-Base Generator," U. S. Patent 2,428,369, Oct. 7, 1947. 

 G. W. Rusler, "Timing Fork Construction," U. S. Patent 3,433,160, Dec. 23, 1947. 



