Chap. 8] 



MAGNETIC METHOD 



319 



Magnetic instruments may be divided into (1) prospecting magnetome- 

 ters, (2) instruments for regional magnetic surveys, and (3) observatory 

 instruments. 



A second classification is based on construction principles and includes 

 the following groups: (1) Instruments for finding the direction of the field 

 b}^ determining (a) the rest position of a magnet capable of rotation about 

 a vertical axis (declinator, compass), a horizontal axis (inclinator, dip 

 circle, dipping needle), or both horizontal and vertical axes (Swedish mining 

 compass) ; (&) the zero induction position of a rotating coil (the earth induc- 



FiG. 8-17. Action of three fixed-force components on moving magnetic system. 



tor and earth inductor compass) ; or (c) intensity ratios as in iron-induction 

 inclinometers. (2) In the second group are instruments for the determina- 

 tion of the total intensity or its components (a) by measuring the period of 

 oscillation of a magnet in a horizontal or vertical plane; (6) by measuring 

 the current induced in a rotating coil; (c) b}^ measuring the induction in 

 iron bars; or ((/) b}'^ using a comparison force of some kind. These may be 

 produced by (Z) magnets or coils, (m) elastic suspensions, (n) gravitj^, and 

 (o) moving electrons. 

 A general theory of magnetic instruments may be derived from the action 



