GRAVITY PENDULUMS 



IN THE 19th CENTURY 



The history of gravity pendulums dates back to the 

 time of Galileo. After the discovery of the variation of 

 the force of gravity over the surface of the earth, gravity 

 measurement became a major concern of physics and 

 geodesy. This article traces the history of the develop- 

 ment of instruments for this purpose. 



the authors: Victor F. Lenten is Professor of 

 Physics, Emeritus, at the University of California at Berke- 

 ley and Robert P. Mjilthauf is Chairman of the Depart- 

 ment of Science and Technology in the Smithsonian 

 Institution s Museum of History and Technology. 



Tin intensity of gravity, or the acceleration of a 

 (reel) falling body, is an important physical quan- 

 tn\ for the several physical sciences. The intensity of 

 gravity determines the weight of a standard pound 

 or kilogram as a standard or unit of force. In physical 

 experiments, the force on a body may be measured 

 b) determining the weight of a known mass which 

 serves to establish equilibrium against it. Thus, in 

 the absolute determination of the ampere with a cur- 

 rent balance, the force between two coils carrying 

 current is balanced by the earth's gravitational force 

 upon a body of determinable mass. The intensity of 

 gravity enters into determinations of the size of the 

 earth from the angular velocity of the moon, its 

 distance from the earth, and Newton's inverse square 



law of gravitation and the laws of motion. Predic- 

 tion of the motion of an artificial satellite requires an 

 accurate knowledge of gravity for this astronomical 

 problem. 



The gravity held of the earth also provides data for 

 a determination of the figure of the earth, or geoid, 

 but for this problem of geodesy relative values of 

 gravity are sufficient. \i g is the intensity of gravity 

 at some reference station, and A,? is the difference 

 between intensities at two stations, the values of 



gravity in geodetic calculations enter as ratios 



g 

 over the surface of the earth. Gravimetric investiga- 

 tions in conjunction with other forms of geophysical 

 investigation, such as seismology, furnish data to test 



PAPER 44: DEVELOPMENT OF GRAVITY PENDULUMS IN THE 19TH CENTURY 



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