132 



GRAVITATIONAL METHODS 



[Chap. 7 



Fig. 7-31. Action of trifilar 

 gravimeter. 



Objective , 

 Lens I 

 MttPUh 



6round 6k.t-^ 





 Light 



Fig. 7-32. Truman gravimeter. 



changes in gravity brought about by 

 the changes in attraction of the moon 

 with an accuracy of about 0.001 milh- 

 gal. A portable instrument for gravity 

 exploration has not yet been developed. 



The Truman gravimeter (see Fig. 7- 

 32), used chiefly by the Humble Oil 

 Company and associated companies, 

 is similar in construction to a Ewing 

 astatic vertical seismograph. The beam 

 consists of a right triangle with the 

 right angle at the axis of rotation. 

 The mass is attached to the end of this 

 triangle, and near it a vertical coil 

 spring supports most of the mass and 

 the beam. An astatizing spring is at- 

 tached to the other corner of the tri- 

 angle, approximately below the point 

 of suspension. The accuracy is of the 

 order of 0.5 milligal. 



The Thyssen gravimeter (Fig. 7-33) 

 is likewise a beam-type vertical seis- 

 mometer. The beam is horizontal and 

 suspended in the center on a knife 

 edge, the balancing spring being housed 

 in a tube surrounded by a water jacket 

 expanding downward with an increase 

 in temperature to offset the increase 

 in spring length. An astatizing arm 

 is fastened to the center of the beam 

 which is made of fused quartz and has 

 a length of 15 to 20 cm. Two beams 

 are arranged side by each in antiparal- 

 lel arrangement as in a torsion balance. 

 They are clamped by a mechanism 

 operated from the top of the instru- 

 ment and are read separately by the 

 optical arrangement shown in the 

 figure. The zero position is calculated 

 from reversal readings. The mean 

 error of one station is 0.25 milligal, 



" A. B Bryan, Geophysics, 2(4), 301-308 (Oct., 1937). 



