370 EXPLORATION GEOPHYSICS 



ference of 1 part in ten million would represent a movement of the mass 

 (or an increase in the length of the spring) by only 30 • 10""^ cm. 



Such a small movement is not easy to detect, and is extremely difficult 

 to incorporate into a repetitive, reproducible measurement. By various 

 arrangements to magnify extremely small motions, as is brought out in the 

 descriptions of gravimeters, the problem is solved for practical purposes. 



Instrumental Problems in Gravimeter Construction 



Many instrumental difficulties v^ere encountered in the development 

 of the gravimeter. The chief problem was measurement of the minute 

 displacement of the mass produced by a change in the gravitational force 

 of attraction. The measurement must be made with an accuracy of about 

 one ten-millionth of an inch. Several systems have been developed which 

 possess the required sensitivity. The most satisfactory method employs a 

 compound spring system which is mechanically equivalent to a simple 

 spring of very large extension. Compound spring systems increase the 

 displacement some ten to one hundred times that of a practicable simple 

 spring. A sensitive optical system, working in conjunction with the com- 

 pound spring, constitutes the essential means for measuring the displace- 

 ment to the required accuracy. 



Another difficulty encountered in the gravimeter development was the 

 variation in the displacement of the spring system due to changes in tem- 

 perature. This variation is caused primarily by: (a) thermal expansion 

 and (b) changes in the elasticity of the spring material. The first efifect 

 is large due to the high coefficients of expansion of the materials most 

 commonly used for springs. For example, a steel spring may increase in 

 length from one-half to two parts in ten thousand per degree centigrade 

 rise in temperature. Thus exposure of a gravimeter having a steel spring 

 to ordinary temperature variations would result in displacements due to 

 temperature variations which would be very large in comparison to those 

 due to gravity. Temperature efifects have been minimized by a number 

 of methods, chief of which may be mentioned: (1) compensation by use 

 of bimetallic materials similar to those employed in the new temperature- 

 compensated magnetic systems, (2) maintenance of constant temperature 

 by use of electric thermostats, (3) utilization of spring materials having 

 a very low temperature coefficient. Usually, all three of these methods 

 are emj^loyed. The most effective control, however, is the use of a well- 

 insulated cabinet provided with accurate thermostatic control. Such 

 equipment can maintain the temperature constant within one-hundredth 

 to one five-hundredth centigrade degree. 



The most troublesome difficulty to be overcome was the imperfect elas- 

 ticity exhibited by all elastic materials. This imperfection appears in two 

 distinct manners termed creep and elastic-after-effect or elastic lag. Creep 

 is simply a gradual yielding of any "solid" material when under load. This 

 effect is usually negligible in ordinary engineering applications but must be 



