BALANCE 



GRAVITATIONAL METHODS 325 



The constants 5" and T are close to 1, and are so used, where the wires in the two 

 balances have torsional coefficients of nearly the same value. The constants for Askania 

 instrument No. 529, in 1930, were : 



O = 2.61 ; P = 4.52 ; Q = -5.07 ; R = -8.78 ; S = 1.0206 ; and T = 1.0073. 



This will serve to show the magnitude of these constants expressed in Eotvos units 

 or 1 X 10~'* c.g.s. At the date on which the above constants were figured, the values of 

 a and b and the torsional coefficient r of the two balances were : 



Balance I : a' = 0.0573 ; b' = 0.1098 ; r = 0.597 

 Balance II : a" = 0.0561 ; b" = 0.1090; r = 0.600 



When a torsion wire is broken and has to be replaced, or if the physical dimen- 

 sions of the instrument are altered, these constants have to be recalculated. 



Reading Torsion Balance Plates. — Figure 183 represents a photographic 

 plate obtained with a standard double beam torsion balance with readings 

 taken in 3 azimuths. The plates are usually GYz X 9 cm. and are sensi- 

 tized with a high speed emulsion. Where 

 the daily temperatures are high, a special 

 tropical emulsion may be used which will 

 not be adversely affected by warm devel- 

 oping solutions. 



There are four sets of dots on such 

 a plate. Those on the left (Figure 183) 

 form a straight line. They are laid down 

 on the plate by the light ray from the 

 fixed mirror to form a reference line. 



From this line of dots, scale readings Fig. 183.— Torsion balance photographic 



, ',. , ° plate. 



ni, n^, and ns, corresponding to the posi- 

 tion of rest of the beam, are obtained for its orientation in azimuth or 

 positions 1, 2, and 3. The values are read by placing the special glass 

 scaHng plate, graduated in 3^ mm. divisions, over the photographic plate, 

 and reading the scale divisions to tenths. The zero line of the scaling plate 

 is carefully set on the dots of the reference line and the distance is scaled 

 to the dot for a given rest position of the balance in each orientation. 



In Figure 183, the second line of dots shows the rest positions for 

 balance II in its orientation positions 1, 2, and 3, with check readings, or 

 dots, for the first two of these positions. The third line of dots gives 

 the rest positions of balance I. The fourth line shows temperature change 

 by means of a metal spring thermometer which operates a mirror in the 

 optical system of balance I. 



Where the temperature dots shift to the right an increase of temper- 

 ature is indicated. If the shift is erratic, the station should be repeated. 

 In the sample calculation given later, no temperature correction has been 

 used, although some organizations make temperature corrections in torsion 

 balance surveys. Temperature change affects the torsional coefficient of the 

 torsion wire and also causes a shift in the rest position of the beam. Torsion 

 wires are carefully selected and heat-treated before they are used, to 



