GRAVITATIONAL METHODS 



277 



Apparatus 



Relative gravity measurements are usually effected with the aid of a 

 so-called invariable type compound pendulum. The pendulum as used in 

 geophysical applications consists of two or more bobs or masses, sup- 

 ported by knife-edges and having lengths such that they swing in syn- 

 chronism.* The bobs are released simultaneously so that their horizontal 

 components of reaction on the support are of opposite phase and cancel 

 each other. This minimizes the tendency of the system to sway upon its 

 support. 



Early Pendulum Prospecting Equipment. — Figure 135 shows pendulum pros- 

 pecting equipment which was typical in the late 1920's. The two pendulums shown at 

 the left of the figure are made of invar. 



Von Sterneck-Askania 4-Pendiilum Apparatus. — The Von Sterneck 

 type of 4-pendulum apparatus is illustrated in Figures 136, 137, and 138. 

 The apparatus consists of a pendulum base frame ; tripod ; 4 pendulums ; 

 and pendulum hood. The base frame 

 fits into the tripod which also accom- 

 modates the pendulum and the hood. 



The tripod contains all the equip- 

 ment necessary for clamping; setting 

 the pendulum into oscillation ; adjusting 

 the amplitude, etc. On the tripod is 

 mounted an optical bridge which reflects 

 the light coming from the point of 

 observation via the four pendulum 

 mirrors. 



The pendulum tripod is supported 

 by 3 level screws which rest on the base 

 frame and are used for leveling and 

 locking the apparatus. A disk is pro- 

 vided for starting the oscillation of one, 

 two, or all four of the pendulums. 

 A crank facilitates the releasing and 

 clamping of the pendulums. The pen- 

 dulums are supported by agate bearings 

 and each pendulum swings in a separate 

 chamber. (Figure 136.) The pendulum 

 bridge consists of 6 prisms, each of 

 which can be turned and tilted for 

 adjusting purposes. 



The pendulums are made of invar 

 material and are gold-plated for pro- 

 tection against corrosion. (Figure 137.) 



*A pendulum comprising two bobs is also described as a 2-pendulum apparatus; 

 a pendulum comprising three bobs as a 3-pendulum apparatus, etc. 



Fig. 137. — Von Sterneck type invari- 

 able pendulum. (Courtesy of the Askania 



Corporation.) 



