402 



MAGNETIC METHO© 



[Chap. 8 



the transverse (9t) induced components are considered, 

 AH = 2 (Bt log. ^^ - BcA 



-K 



At = 2(^/^ + ^tlog< 



)■ 



(8-636) 



By addition of the corresponding components of the remanent mag- 

 netization {^i and dt), these formulas change to 



AH 



loge ^ (kZo + B'l) 



n 



r 



At = 2 j <p{kZq ± 





1 



(8-63c) 



where, for the transverse magnetization also, the demagnetizing effect 

 (N = demagnetizing factor) has been included. Hp is the horizontal 

 component perpendicular to the strike. N and Bi are usually important 

 for fairly high susceptibility only. In approximation, formula (8-63c) 

 may also be used for dipping ore bodies provided, of course, that the mag- 

 netizations B'l and d[ , as indicated, are reckoned in the longitudinal and 

 transverse direction of the dike or vein. 



F, Model Experiments in Magnetic Interpretation 



The value of model experiments in magnetic interpretation lies in the 

 fact that the magnetic fields of geologic bodies can be ascertained with 

 greater accuracy by experiment than they can be calculated. This ad- 

 vantage is enhanced by greater speed and the possibility of simulating 

 irregular distributions of magnetic materials. The earliest application of 

 model experiments was probably made in Sweden, where the use of the 

 magnetic method in the location and depth determination of magnetic 

 ore bodies necessitated accurate interpretation methods. Lundberg and 

 Sundberg state that such experiments have been carried out continuously 

 since 1875 by students of the Royal Institute of Technology in Stockholm. 



Probably the first description of model experiments was given by 

 Thal^n^" who used a magnetic bar in a vertical position. Above the bar 

 was a board which he rotated about a vertical axis coincident with the 

 axis of the bar. At intervals of 20°, declination and horizontal intensitj^ 

 were determined with a Thal^n magnetometer. Similar model experi- 

 ments were carried out by Uhlich. 



'^ In JernkontoreVs Annaler (1879), translated by B. Turley {Untersuchung von 

 Eisenerzfeldern durch magnetische Messungen [Leipzig, 1879]) p. 56-64. 



'* Aufsuchung magnetischer Erzlagerstaetten, pamphlet issued by Jahrbuch fuer 

 das Berg- und Huettenwesen in Sachsen (Freiberg, 1902). 



