194 



GRAVITATIONAL METHODS 



[Chap. 7 



Fixed point mirror 

 Mirror 



Lower weighi- 



-PhotagraphK attachment 

 -Torsior) head 



-Platinum wire 



'Baiancetvam mirror 



Arreiting meinanism 



'^ Upper meight 



Unn/ng dock Mork 



Gears 



-Tube 



American Askania Corp. 



Fig. 7-66. Large Askania photographic 

 recording torsion balance. 



quartz balance with m = 0.25, 

 h = 20, 1 = 10, K = 6Q,d = 5m, 

 and L = 20. It was soon found 

 that a geologically useful torsion 

 balance cannot be decreased in 

 dimensions beyond certain limits. 

 For instance, the Eotvos-Suess 

 small visual balance (Fig. 7-65) 

 has the dimensions m = 8,h - 30, 

 Z = 10, d = 20m, L = 40 cm. 

 Very effective in the reduction of 

 overall dimensions was Schwey- 

 dar's invention of the Z beam 

 (Fig. 7-67). This balance has 

 the following dimensions : m = 22, 

 h = ^5, I = 20, d = 26-36m, 

 L = 28 cm, X = 19,500, 

 T = 0.23-0.47. Other small bal- 

 ances in which a further reduc- 

 tion in dimensions has been at- 



tempted (Gepege, Haff, Hecker, Rybar) have not 

 been very successful. 



The Z beam has recently been superseded by the 

 tilt beam (Fig. 7-68),** with the following con- 

 stants: m = 40g, /i = 30cm,Z= 10cm,E: = 9150, 



d = 33-43m, f dmhl = 13,800. Torsion balances 



completely deviating in dimensions from those 

 mentioned are : the Tangl balance (curvature vari- 

 ometer floated in water), Nikiforov's short-wire 

 balance (t = 16, L = 2 cm), and the gravity 

 gradiometer previously discussed. 



2. Constants. The instrument constants m, h, 

 I, f , L are readily determined before assembly in 

 the factory by measurement of lengths and 

 weights; they are not subject to change, are 

 usually given in the calibration certificate, and 

 require no recalibration. More involved are the 

 determinations of K and t. In an assembled instrument, K/t may be 

 obtained from oscillations and t from deflections. More convenient is 

 the independent determination of these constants before assembly. 



American Aakania Corp. 



Fig. 7-67. Z-beam 

 Askania balance. 



86 H. Imhof and A. Graf, Beitr. angew. Geophys., 4(4), 426-436 (1934); Rev. 

 Sci. Instr., 6(10), 356-358 (1934). 



