no 



EXPLORATION GEOPHYSICS 



intensity of the earth's magnetic field with sufificient accuracy for certain 

 kinds of geophysical work, though not for close or accurate magnetic inves- 

 tigations. It can be used where magnetic anomalies exceed 250 gammas. 

 This instrument is of aid in tracing some types of dikes, the limits of in- 

 trusive bodies, and the investigation of iron ore deposits. The general 

 precision of the instrument is comparable to the accuracy obtained by the 

 Brunton in traverse survey work. 



The Wilson attachment consists of a Brunton compass holder, and an 

 arm graduated in millimeters. Slideable along the arm is a magnet holder, 



Fig. 36. — Wilson magnetometer attachment, a, Brunton compass; b, graduated arm; c, calibrated 

 magnet; d, vernier scale; e, vernier adjustment knob with rack and pinion. (Courtesy of Wm. 

 Ainsworth and Sons.) 



equipped with a vernier reading device. (Figure 36.) The magnet holder 

 is moved along the arm by a rack and pinion. The magnet arm makes an 

 angle of 120° measured clockwise from the N. end of the compass box. 



The instrument may be used on a flat board or a plane table, or mounted on a 

 special tripod. At a station, the instrument is set up and leveled. The zero of the 

 graduated circle of the compass is set at the index on the N. end of the compass box. 

 The compass is then oriented to read zero. A small auxiliary magnet of known 

 moment is placed in the magnet holder on the arm and brought up with its S. pole 

 toward the compass until the compass needle reads exactly 30°. In this position the 

 needle is perpendicular to the axis of the auxiliary magnet. The distance r on the 

 extension arm is read with a vernier to the nearest tenth of a millimeter. 



In this relationship the inverse cubes of the distances r are proportional to the 

 horizontal intensities at a series of stations. The formula applicable when the instru- 

 ment is so used is 



H = - 



4M 



1 



2r= 



+ 



16r 



l1 



(43) 



Where //= horizontal magnetic intensity, in gammas, 



M =■ the magnetic moment of the auxiliary deflector magnet, 

 rz=: the distance in cm. between the center of the auxiliary magnet and the 



compass needle, and 

 L z= the length of the auxiliary magnet. 



