160 



EXPLORATION GEOPHYSICS 



in Figure 68, is a miniature magnetometer developed for measuring flux 

 density and determining direction of flux in permanent magnets, electro- 

 magnetic coils, etc. It may be used for 

 checking flux density and flux gradients in 

 air gaps, and for measuring flux density in 

 iron structures. The instrument comprises a 

 very small magnet fastened to a delicately 

 pointed shaft to v^hich a pointer is attached. 

 The deflecting torque of the pointer results 

 from direct interaction betw^een the flux 

 under investigation and the small magnet. 



Flux density is measured by rotating the instru- 

 ment case until maximum reading is obtained on 

 the calibrated scale. This point is reached when the 

 probe magnet flux is at a right angle to the flux 

 being investigated, and the scale can be read directly 

 in gauss. The probe diameter varies from .090 of 

 an inch to as little as .052 of an inch. 

 To measure the flux density in the iron position of magnetic circuits, a small hole 

 (0.09 inch in diameter) is drilled in the iron structure. If this hole is small in propor- 

 tion to the iron, then 



Fig. 68. — Sectional view of 

 gauss meter. (Courtesy of Gen- 

 eral Electric Company.) 



Flux density = // = 



1 



[-i] 



H, 



(64) 



From known data on the BH curve of iron (Figure 72i), flux density of the structure 

 is easily computed. Thus, detailed information can be obtained about flux distribution 

 in large and often irregularly-shaped iron structures used in generators and other large 

 electric apparatus.f 



Magnetic Properties of Rocks 



The magnetic character of minerals and rocks and their geologic 

 history govern the functioning of the magnetic methods. Magnetic sus- 

 ceptibility bears the same relation to magnetic field studies as does density 

 to gravity studies. Susceptibility, hovi^ever, is not as simple or as easily 

 measured a physical property as density. 



The magnetic susceptibility of a given rock or formation depends on 

 a number of factors, some of which are not too well understood. These 

 factors may be classed broadly as mineralogical and geological. 



Mineralogical Factors. — Most substances react in a definite manner, 

 some rather mildly, to a magnetic field. A few minerals, because of their 

 marked magnetic properties, are set apart and called ferromagnetic. We 

 will be largely concerned with these latter in discussing rock susceptibility. 



In interpreting magnetic data it is assumed that the minerals, rocks 

 and formations owe their magnetization to induction in the earth's magnetic 

 field. In the north temperate zone this is a weak field of about 0.6 gauss 



t General Electric Co., publication G.E.C.-238, Mar., 1938. 



