Chap. 7] GRAVITATIONAL METHODS 87 



It has been proved by a number of experiments that the gravitational 

 constant does not change with the chemical or physical nature of the 

 masses used. By the measurement of the gravitational constant, not 

 only is the proportionality factor in Newton's law determined, but an 

 experiment of greater physical significance is made. Since gravity is the 

 earth's attraction upon a mass of 1 g, and since, from Newton's law. 

 Me = igRm)/k, (g is gravity. Me the mass of the earth, and Rm its mean 

 radius) it is seen that determination of the gravitational constant is equiva- 

 lent to weighing the earth. As the earth's volume can be calculated, its 

 mean density, Sm, may be obtained from the gravitational constant: 



3 g,, 1.0014 . . 



*'" - 4^ • T • ~R;r ' ^^'^^ 



where g^ = 980.616 cmsec"^, and Rm = 6.371 10* cm. This relation 

 yields 5.53 for the mean density of the earth. 



%<';i>\'r. 



w-' 





Fig. 7-4. Eotvos gravity compensator (adapted from Jung). 



To increase the effect of gravitating masses upon the torsion balance, 

 Eotvos^^ designed the gravity compensator and the gravity multiplicator. 

 The instruments incorporate a regular torsion balance of the first type 

 (curvature variometer), provided with four sector-shaped deflectors whose 

 position may be changed by rotation about a horizontal axis (see Fig. 7-4). 

 In vertical position the attraction of the deflectors is a minimum; when 

 arranged in horizontal direction, it is a maximum. If the balance beam 

 is in the center of the case, the attraction of the deflectors is zero because 

 of their symmetrical disposition; however, if a small deflection, <p, is pro- 

 duced by an outside mass whose attraction is to be measured, the de- 

 flectors become effective since they are now unsymmetrically disposed 



" R. V. Eotvos, "Untersuchungen ueber Gravitation und Erdmagnetismus,' 

 Ann. d. Phys. und Chem., 69, 392 (1896). 



