GRAVITATIONAL METHODS 



379 



determination of the relative gravity is based on a change in capacity pro- 

 duced by a vertical displacement of a spring-supported plate. 



A schematic section of the gravi- 

 meter is shown in Figure 225. The 

 movable body is suspended by the plate 

 springs F which are attached to the 

 cross piece of the support E. The top 

 of the movable member forms one 

 plate of a parallel plate condenser, the 

 other plate of which is fixed to a thick 

 steel disc A by an electric insulator B. 

 The contact C connects the condenser 

 with the oscillatory circuit of an ultra- 

 micrometer coupling which registers 

 minute variations in capacity between 

 the two plates. The function of the lower plates (the returning plates) 

 is to provide a means for returning the movable body to the zero (base 

 station) position. This is accomplished by applying a potential to one 

 of the plates, thus causing the movable plate to be displaced due to the 

 electrostatic forces produced by the potential difference between the 



Fig. 225. — Schematic drawing of Boliden 

 gravimeter. (After Hedstrom, A.I.M.E. Geo- 

 physical Prospecting, Tech. Pub. 953, p. 13.) 



plates. 



-Diagrammatic sketch of Haalck 

 gravimeter. (Zeitschrift filr Geophysik.) 



The ultramicrometer coupling is de- 

 signed so-that variations in the anode cur- 

 rent caused by changes in the capacity 

 between the two plates are registered on a 

 milliammeter. To accomplish this, the plates 

 are connected to the grid circuit of an 

 oscillating tube. By choosing the instru- 

 ment constants such that the plate separa- 

 tion of the condenser is 0.002 cm. and the 

 coupling sensitivity 10 ma. per cm., a 

 gravimeter sensitivity of 0.01 milHgal may 

 be obtained. An instrumental sensitivity 

 between 0.01 and 0.05 milligal is claimed. 



Haalck Gravimeter. t — The Haalck meter 

 is a static meter using a sealed volume of gas 

 as the elastic medium. A schematic representa- 

 tion of the meter is shown in Figure 226. The 

 long vertical tube and the lower portions of the 

 two containers Z and Z' are filled with mercury. 

 The upper portion of each of the containers Z 

 and Z' and a portion of the tubes above them are 

 filled with a light fluid, such as toluol. When 

 the value of gravity varies, the density of the 

 mercury column varies correspondingly. Also, 

 the volumes of gas in the two chambers V and 



t H. Haalck, "Ein Statischer Schwerkraftsmesser," Zeitschrift fUr Geophysik, Vol. 7, 1931, p. 

 95; Vol. 8, 1932, pp. 17 and 197; Vol. 9, 1933, pp. 81 and 285. 



