378 



EXPLORATION GEOPHYSICS 



ing the instrument a null reading type. The instrument is compensated 

 for small changes in temperature. 



Wright Gravimeter.X — The Wright meter (Figure 224) comprises 

 primarily two helical elastic springs which taper from opposite ends 

 toward a common center portion to which is attached a relatively short 

 aluminum rod. The rod carries a weight at its outer end and a mirror at 

 or adjacent its inner end on the axis of the two helical springs. The helical 

 springs are attached at their outer ends to a frame 12 which is rotatably 

 mounted. The frame, together with the springs, may be rotated by means 

 of a shaft extension terminating in a knurled head 14. Clamps are pro- 

 vided for preventing serious vibrations of the springs during transporta- 

 tion. Also, to minimize temperature variations within the case, and hence 

 changes in the elastic constants of the spring, the case is thermally in- 

 sulated by use of several outer casings. 



The operating procedure may be summarized as follows. At a given 

 location (base station) the frame and springs are rotated by means of the 



knurled head 14 until the alu- 

 minum rod and weight W are 

 in an approximately horizontal 

 position (standard reference 

 position). The standard po- 

 sition of rest is determined by 

 means of an autocollimating 

 telescope sighted on the mirror 

 M. The reference angle is read 

 off the graduated circle 15 at- 

 tached to the axis of rotation 

 of the frame 12. The difference 

 in the readings of the gradu- 

 ated circle for the two horizon- 

 tal positions of the aluminum 

 arm is a measure of the elastic 

 deformation set up in the 

 springs by the mechanical 

 torque exerted by this arm in 

 response to the gravity pull on weight W. The instrument is then moved 

 to another location (field station) and the procedure repeated. The rela- 

 tive difference in the readings of the horizontal positions at the field 

 station as referred to that at the base station is a measure of the relative 

 gravity at the field station. 



Boliden Gravimeter. t — ^This instrument is a static type wherein the 



Fig. 224. — View of the Wright gravimeter. (U. S. 

 Patent 1,579,273.) 10, helical elastic springs; IV, mass; 

 M, mirror; 13, supporting frame of helical springs; 14. 

 adjustment knob; 15, graduated circle; 16 and 17, ver- 

 nier scales; 18, autocollimating telescope; 23, light; 

 27, totally reflecting prism. 



t F. E. Wright, U. S. Patent 1,579,273. 



t H. Hedstrom, "A New Gravimeter for Ore Prospecting," A.I.M.E. Geophysical Prospecting, 

 Tech. Pub. 953, pp. 12-15 (Feb., 1938). 



