MAGNETIC METHODS 



87 



Under assumptions as made previously for Case I, except that a is 

 measured from the horizontal (or parallel to S' N'), the intensity of the 

 magnetic field at the needle due to the deflector magnet is : 



and the force at the needle 



F' = mH' - mU'/d^ 

 The couple acting on the needle is 



2ml M' sin a MM' sin a 



(23) 



d^ 



d^ 



(36) 



If a = 0°, sin a = and the couple = 0. 



If a = 90°, sin a = 1, and the couple is a maximum; then 



couple = 



MM' 

 d^ 



(37) 



Fig. 21.- — Action between two 

 small magnets: Position II of 

 Gauss used in calibrating a hori- 

 zontal magnetometer. 



Case HI — 3rd Position of Gauss. — The axis of rotation of the needle 

 passes through the midpoint of the deflector, and is normal to it. (Figure 

 22.) This position is seldom used in calibration and therefore the develop- 



