86 



EXPLORATION GEOPHYSICS 



The force acting on the needle F' = mH' = 2M'm/d'. 

 The couple acting on the needle is : 



2M'm 2 I sin a 2M' M sin a 



d^ 



d^ 



(34) 



If a becomes 0°, or with both magnets in the same line, there is no couple 

 acting since sin a = 0. 



If a becomes 90°, sin a = 1 and the couple is a maximum. In this case. 



couple = 



2M M' 

 d^ 



(35) 



DEFLECTOR 



Fig. 20. — Action between two 

 small magnets: Position I of 

 Gauss, used in calibrating a ver- 

 tical magnetometer. 



Ciise H — -2nd Position of Gauss. — This position is involved in the 

 determination of the scale value of the horizontal intensity magnetometers 

 with auxiliary magnets. (Figure 21.) 



