188 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 3 



the measurement of flux density down to less than 1 gauss. The noise 



level is equivalent to 0.2 gauss. 



c. Special Search Coils, coils for use in inhomogeneous fields. 



The determination of the local field strength in inhomogeneous 



fields requires a small search coil 

 so that the field is practically 

 homogeneous over the cross- 

 sectional area of the coil. Such 

 coils are inherently insensitive; 

 the signal level at the output 

 terminals of such coils is in general 

 too small for measurement in mag- 

 netic fields of low field strength. 

 The difficulty can be overcome by 

 using the flux ball described by 

 Brown and Sweer. 1 



The method is based upon the 

 fact that "the value of the mag- 

 netic field strength at a point is 

 equal to the volume average of 

 the field over the interior of any 

 sphere centered at that point and 

 not containing any source of the 

 field" (Brown and Sweer). A 

 sphere of this type surrounding 



the point P is shown in Fig. (3-1)9. The net flux linked with the coil 



Fig. (3-1)9. Flux ball. 



O = %-nr z nB z center 



is where r is the radius of the sphere, n the number of turns per unit 

 of length measured along the axis, and -S ZiCen ter i s the z component 

 of the flux density at the center of the sphere if the axis is in the 

 z direction. 



For increased sensitivity, the entire sphere is filled with windings 

 (solid flux ball) made up of cylindrical coils of different lengths and 

 diameters as shown in Fig. (3-1)10. Such a coil may be thought of as 

 consisting of concentric spheres; the net flux linked with the solid 

 flux ball is the sum of those linked with each sphere. The entire flux 

 linked with the solid flux ball 



O' 



4 1 



15 "" WD, T ^ center 



P z 



1 F. W. Brown, Jr., and J. H. Sweer, Rev. Sci. Instr., 16, 276 (1945). 



