Chap. 7] 



GRAVITATIONAL METHODS 



133 



that of repeat stations about 0.5 milligal.^^ The drift in one day is 1 to 

 1.5 miUigals. 



The Mott-Smith gravimeter^'' (Fig'. 7-34) is essentially a torsion-wire 

 gravimeter with astatization. The torsion fiber (2) is made of fused 

 quartz and carries a weight arm (1) to which is attached the pointer (3). 

 A negative restormg force is suppUed by a 

 fiber (4) passing through the axis of rota- 

 tion of the weighing arm. Deflections of 

 the system are read by a microscope. 



Gravimeter readings are affected by tem- 

 perature, air pressure, humidity, abrupt 

 changes and slow drift of the base reading. 

 These effects must be corrected for and be 

 determined by experiment; furthermore, the 

 sensitivity (scale value) of the instrument 

 must be known. The effect of temperature 

 is complex, cannot always be calculated, 

 and is generally determined by experiment. 

 In most gravimeters some sort of temper- 

 ature compensation and thermostat protec- 

 tion is provided. If this were not done, the 

 temperature effect would be tremendous. 

 In a spring gravimeter a change of 1°C. 

 would produce an apparent change in grav- 

 ity of 200 to 300 milligals. In a volumetric 

 gravimeter the same temperature change 

 would correspond to an apparent gravity 

 anomaly of 3000 to 4000 milligals. 



Changes in air pressure affect some 

 gravimeters, depending upon construc- 

 tion of the case and the weight of the 

 moving member. The buoyancy in- 

 creases with barometric pressure, and its 

 effect on the reading is determined by 

 experiment. Changes in humidity may 

 produce large effects in gravimeters whose 

 mass is small, since water condensation will 



Fig. 7-33. Thyssen gravimeter 

 (schematic). 



"A. Schleusener, Zeit. Geophys., 10(8), 369-377 (1934); Oel vmd Kohle, 2(7), 

 313-318 (1934). St. v. Thyssen and A. Schleusener, Oel und Kohle, 2(8), 635-650 

 (1935); Beitr. angew. Geophys., 6(1), 1-13 (1936). St. v. Thyssen, Zeit. Geophys., 

 11(3), 131-133 (1935); 11(4/5), 212-220 (1935); Beitr. angew. Geophvs., 5(2), 178-181 

 (1935); 5(3), 303-314 (1935); 7(3), 218-229 (1938). A. Berroth, Oil Weekly, 76(13), 

 33-37 (March 11, 1935). F. Lubiger, Beitr. angew. Geophys., 7(3), 230-244 (1938). 



"Geophysics, 2(1), 21-32 (Jan.. 1937). 



