MAGNETIC METHODS 



115 



resultant field. That is, the magnet is acted on by two opposing couples 

 and comes to rest in an azimuth where the couples are equal in magnitude. 

 The magnetic couple (equal to the product of the magnetic moment M' 

 of the needle times the sine of the angle /? between M' and T) tends to 

 rotate the needle in the direction of the earth's field, while the gravitational 

 couple (equal to the product of the weight of the counterweight times d, its 

 perpendicular distance from the center of gravity of the needle) opposes 

 this rotation. 



The equilibrium condition for a dip needle is given by the equation: 



or 



where 



Wd = TM' sin p 



TM' _ d 

 W ~ sin y8 



Wd = couple due to the counterweight. 



T — total earth's field. 

 M' — magnetic moment of needle. 



y8 = angle between T and M' . 



(45) 



Fig. 41. — Setting of Hotchkiss Superdip. (After Stearn, A.I.M.E. 

 Geophysical Prospecting, 1932.) 



The characteristic feature of the Hotchkiss Superdip is that d and 

 sin 13 are made to vary proportionately in such manner that d is a max- 

 imum when sin ^S is 1. This could be achieved in an ordinary needle only 

 for the unusual case of inclination equal to 90°. It is accomplished in the 

 Hotchkiss Superdip by an ingenious arrangement employing a duplex 

 needle assembly which consists of an adjustable counterarm (provided 



