MAGNETIC METHODS 



111 



Usually r is large in comparison to L, and the formula may be simplified. 



(44) 



With the auxiliary magnet commonly used and reading r to 0.1 of a mm. it would 

 not be possible to measure H closer than 25 to 50 gammas. Repeated measurements at 

 any one station will usually show a departure of about 50 gammas from the mean 

 value.f 



HORIZONTAL INTENSITY 



MAGNETIC SURVEY 



ACROSS 



THE VALMONT DIKE EXTENSION 



Fig. 37. — Horizontal intensity magnetic survey across the Valmont Dike 

 Extension, Boulder Oaunty, Colorado. Survey made with Wilson attach- 

 ment on a Brunton compass; after J. H. Wilson, 1929. 



Figure 2i7 shows a magnetic traverse across a highly magnetic basalt dike, intruded 

 between shale beds of weak magnetic properties. 



Dip Needle Types of Instrument 



The Dip ISeedle. — A dip needle is another simple magnetic instru- 

 ment. Its center of gravity and center of rotation are at the same point. 

 When in operation, the needle swings only in a vertical plane. Obviously 

 such a needle, when aligned with the earth's magnetic field, measures 

 minimum dip angle and its plane of rotation is in the magnetic meridian. 



The dip instrument is read by suspending it vertically, and orienting 

 it a few degrees to the right or the left of the magnetic meridian. The 

 instrument is then slowly rotated through the magnetic meridian and the 

 minimum angle noted. This is the dip angle. 



t A.A.P.G. Bull.. Vol. 15, No. 11, pp. 1391-1398, Nov., 1931. 



