298 EXPLORATION GEOPHYSICS 



equipotential surface is inversely proportional to the distance separating adjacent equi- 

 potential surfaces. In other words, the greater the gravity force, the closer togetlier 

 are the equipotential surfaces. 



Since it has been illustrated previously that the force of gravity is greatest over 

 the crest of an anticline, it follows that equipotential surfaces are closer together over 

 the crest of an anticline, than over the flanks and the adjacent synclines. It also follows 

 from the above that it takes two equipotential surfaces to define the gravity force 

 at a point. 



Referring to Figure 159, gi = — — or gt = — — 



hi ht 



where only the magnitude of the gravity force is considered. 



The gravity force g could, of course, be determined without two such surfaces if 

 the potential were a simple enough function to be expressed mathematically. It would 

 then only be necessary to differentiate U with respect to the direction h. 



For + h measured upward g =■ — rrr 



o h ' 



"rS 7 J 



For + h measured downward £r = + .;— - 



oh 



In both cases the direction of g is indicated in the sign (=t=/t direction respectively). 



Field Instruments. — In the United States use has been made 

 chiefly of three types of Askania torsion balance and two types of Siiss- 

 Rybar torsion balance. The instruments are usually designed either for 

 visual observation or photographic recording. The accuracy of the instru- 

 ments is of the order of 1 to 3 Eotvos units. (1 Eotvos unit = 10"'' 

 cm. /sec-.) 



To conserve space, the discussion of field instruments given here will 

 be limited to a description of the Eotvos-Askania torsion balances.* 



Large Torsion Balance 



A vertical cross section of the large type instrument is shown in Figure 

 160. The torsion balance proper, with its two suspension systems, is 

 mounted in the upper part of the instrument. The torsion wires are made 

 of specially treated platinum-iridium and have a diameter of 0.04 mm. and 

 an approximate length of 56 cm. Each torsion wire is held tightly liy 

 clamps. The upper end is fastened to a torsion head, and the lower end to 

 a small vertical rod attached to a balance beam and carrying a small mirror 

 for photographic recording of the position of rest or equilibrium of the 

 beam. One end of each beam carries a gold weight of about 42 g. ; the 

 other end has a small hook from which a lead weight of the same mass as 

 the gold weight is suspended by a thin brass wire some 60 cm. or so in 

 length. To minimize temperature fluctuations, the suspension systems are 

 enclosed in three casings which are insulated from one another. The pro- 



* For a description of the Rybar instrument the reader may refer to the catalogue 

 of the Siiss-Rybar manufacturing concern. A description of a gradiometer and the 

 operating technique to be used with it may be obtained from L. Oertling, Ltd., London. 



