Chap. 7] 



GRAVITATIONAL METHODS 



129 



5. Horizontal seismo-gravimeters resemble in principle the well-known 

 Wiechert astatic seismograph. In the Ising gravimeter a quartz fiber is 

 stretched between the prongs of a fork-shaped support and forms the 

 horizontal axis of rotation of an inverted quartz rod fused rigidly to the 

 fiber (Fig. 7-29). The pendulum 

 assembly is mounted on a heavy 

 metal block hung from two leaf 

 springs in such a manner that it 

 can be turned slightly about an 

 axis parallel to the fiber by tight- 

 ening a spring attached to one 

 side of the block. When the block 

 is tilted at an angle (p, the pen- 

 dulum is deflected from its verti- 

 cal position by the angle 6. In 

 the position of equilibrium, 



7]6 = Kid + ip) 



or 



> (7-3 la) 



= A, 



Assuming that the gravity at 

 a (base) station is g^ and that a 

 tilt, (p, has produced the deflec- 

 tion, ^0, the labilizing force at 

 that station ko = v^o/i'P + ^o). 

 If, at another station with the 

 gravity gi, the same tilt angle is 

 used, the labilizing force is ki = 

 ■ndi/{<p -{■ 6i). Then the difference 

 in gravity is A^ = g{Ki - kq)/kq 

 or, in terms of (small) deflection 

 angles for constant tilts. 



Afif = <7 



- 1 



(7-316) 



cm 



Fig. 7-29. Ising astatic gravimeter 

 (section). 



The deflection of the inverted 

 pendulum is read with a micro- 

 scope, and the tilt of the block is measured with the micrometer screw 

 that controls the tension of the tilt spring. In the instruments described 

 in the published reports the mean error was 0.5 to 0.6 milligal. 



The Lejay-Holweck pendulum with some modifications also could be 

 used as a static gravimeter to operate like the Ising instrument. 



