GRAVITATIONAL METHODS 



377 



— an arrangement which gives an optical 

 magnification of 60. The meter has very 

 Httle damping. The spring material and 

 construction, and the material and length 

 of the tube surrounding the spring, are so 

 chosen that the temperature effect is very 

 small. In normal operation the entire 

 meter is enclosed in a thermally insulated 

 box, but usually w^ithout thermostat con- 

 trol. The instrument is very sensitive to 

 level ; consequently, in practice, two nearly 

 identical units are employed with opposite 

 orientation, i.e., with masses mounted at 

 approximately the same level but on oppo- 

 site sides of co-axial knife-edges. This 

 expedient is employed in several other 

 meters which are sensitive to level. 



"^Zero Length^' Spring Gravimeter. 



— Another gravimeter utilizes a "zero 



length" spring.f Figure 223 shows a dia- 

 gram of the essential part of the meter. The 



spring 6" is so wound that its elongation 



is equal to the distance between the points 



where it is attached ; that is, if one defines 



the initial length as the actual physical 



length minus the elongation, this type of spring has zero initial length. 

 The weight W is fixed to the arm DC which is pivoted at D. The 



spring 5 is attached to the arm at the 

 point C and to the frame at the point F 

 vertically above the pivot D. The dis- 

 tances a and b are about 45 cm. and 

 the weight IV about 2.5 kg. 



The period of the spring can be 

 given almost any desired value by 

 choosing a spring material which has 

 an appropriate constant. For periods 

 of about one minute the system tends 

 to become unstable and hence very 

 sensitive to variations in the torque 

 exerted by the weight. 



The meter is damped and employs 

 an additional spring in order to return 

 the mass to its standard position mak- 



t L. J. B. La Coste, Jr., "A New Type of Long Period Vertical Seismograph." Physics, Vol. 

 5, 1934, p. 178. 



Fig. 222.- — Diagrammatic sketch of 

 Thyssen gravimeter. (Zeitschrift fiir 

 Geophysik.) 



Fig. 223. — Diagrammatic sketch of spring 

 gravimeter. (After La Coste, Physics.) 



