556 Subsurface Geologic Methods 



Table 28 shows data on the type "W" instrument and barrels. 



* Barrels are interchangeable with either wire line, sand line, or "go devil" con- 

 nections. Sinker bars are also available. 



A few special to 15- and to 30-degree-range instruments are 

 available for use in high- drift-angle wells. 



Figure 278. 

 Type "M" 

 drift-indi- 

 cator disc. 



The type "M" drift indicator (fig. 277) is photographic and makes a 

 permanent record on a sensitized disc (fig. 278). A contact-timing watch 

 similar to that used in the "W" instrument is set and wound in one opera- 

 tion by turning the stem of the watch. The time setting and functioning 

 of the watch are observed through windows in the case. When the time 

 set has elapsed, an electrical contact illuminates the bulb in the instrument 

 with current from small flashlight batteries. This light is concentrated 

 into a beam by an optical system of two lenses mounted within the 

 plumb-bob tube. Since the tubular plumb bob is suspended in a universal- 

 type mounting, it points downward whenever it is motionless. This spot 

 of concentrated light is directed upon the record disc, which has been 

 centered in the disc cup. Concentric rings representing each degree of 

 drift are printed upon the face of the disc. Sixty to ninety seconds of 

 exposure is sufficient to print a circle with a small dot at its center upon 

 the disc where the concentrated light has struck. The amount of drift is 

 read by counting the number of rings from the center and interpolating 

 between rings to the dot made by the spot of light. The acceptance of an 

 incorrect reading is prevented by the slow exposure qualities of the disc. 

 If the plumb bob is in motion, no mark is made upon the disc. Discs 

 are not developed or fixed, but are paper- or foil-wrapped and kept in 



