302 



EXPLORATION GEOPHYSICS 



attached to a bimetallic strip for recording the temperature. (A one-half 

 mm. displacement of the spot of light represents a temperature variation 

 of about 1°C.) The other mirror is fixed, and the spots of light reflected 

 by it are used as fiducial marks in measuring the displacements of the spots 

 of light reflected from the two balance mirrors. A copy of a photographic 

 record obtained with this balance is shown in Figure 161. The start of the 

 plate is at the bottom of the figure. The instrument is turned until the first 

 stop is reached. Then it is manually turned and oriented, using the compass 

 until the north direction coincides with this stop and the regular runs com- 

 prising three dots are recorded. 



The time required for the beam to come to rest in any azimuth, i.e., the 

 observation period, is approximately 60 minutes. Thus the time required 

 to obtain data for one station, i.e., readings in 3 azimuths, is 3 hours. 

 (Actually, it is common practice to have a repeated observation at the first 

 azimuth to obtain better accuracy so that the total time per station is 4 

 hours.) 



The overall height of the large torsion balance is 183 cm. (72 in.), and 

 its weight when set up for measurement is about 130 pounds. An exterior 

 view of the balance is shown in Figure 162. 



Intercepting mirror 

 Scale 

 Lamp 



Fixed mirror 

 Mirror on support , 



Compass . 

 Balance beam 



Contact clockwork 



Lower wei| 



Dark slide support 



Torsion head 



Platinum thread 



Upper weight 



Level 



Balance beam mirror 



Stop 



Driving gear 

 Toothed crown 



Protective tuht 



lill 



ITTM I I ^ I I ' ' iiiiniii 

 I 

 Fig. 163. — Z-beam torsion balance (in section). (Courtesy of American Askania Corporation.) 



