134 



GRAVITATIONAL METHODS 



[Chap. 7 



produce large changes in equivalent mass. In some instruments, calcium 

 chloride in solid form or as supersaturated solution is used to keep the air 

 dry. 



Although the sensitivity of a gravimeter is proportional to the square 

 of the period, determinations of scale values by period observations are 

 not in use (except for qualitative observation while adjusting an instru- 

 ment), since there are numerous other methods with which scale values 



can be determined more easily and accu- 

 rately. Some gravimeters may be calibrated 

 by tilting. For an inclination, v?, the effec- 

 tive gravity changes from ^ to g^ cos ip\ there- 

 fore, the apparent difference in gravity is 



2 



A^ = giX - cos^) = 2g sin^ \'^ ^\' (7-34a) 



Many gravimeters lend themselves readily 

 to scale value determinations by addition of 

 weights (see Fig. 7-35). If the total gravi- 

 meter mass is M, the apparent 

 change in gravity produced by 

 an addition of mass m is 



Mass-Arm 



Fig. 7-34. Mott-Smith gravi- 

 meter (schematic). 



62 



58 



.«54 



c 

 o 



I 50 



c 



(J 



^46 



42 



38 



dm 



(7-346) 



Gravimeters with condenser 

 plates may readily be calibrated 

 electrostatically. If the two con- 

 denser plates have equal surfaces 

 of S square centimeters at a dis- 

 tance of d centimeters, the ap- 

 parent change in gravity brought 

 about by a voltage difference, E, is 



12 3 4 5 



Load, mgrams 



Fig. 7-35. Gravimeter calibration by addi- 

 tion of mass (after Sohleusener). 



Afif = 



sr 



8t d^M 



(7-34c) 



In terms of capacity C = S/47rrf, 

 which is more readily deter- 

 mined,^® the apparent change in gravity is 



A^ = 2ir 



E-C'^ 

 SM 



(7-34d) 



" A. Lindblad and D. Malmquiat, loc. cit. 



