298 MAGNETIC METHOD [Chap. 8 



(r). The units corresponding to the ohm are the oerstedt^ (gilberts per 

 maxwell) and the rel (ampere turns per maxwell, or 1.257 oerstedts). 

 The reciprocal of resistivity corresponds, in a magnetic circuit, to the 

 reciprocal of reluctivity (reluctance for unit dimension) and is called "per- 

 meability," y. Hence, for a magnetic conductor of length I and area S, 

 the reluctance 



and the flux <^ = — p^ so that 



I = ..f . (8-3) 



where M is magnetomotive force. This equation states that flux density 

 is equal to permeability times magnetic potential gradient. It is analogous 

 to equation (10-5), which indicates that electric current density is 

 equal to conductivity times electrical potential gradient. 



The magnetic potential gradient is the magnetic field intensity H, meas- 

 ured in gilberts per centimeter, or gauss. The flux density $/S is called 

 induction, designated by the symbol B, and measured in maxwells per cm^, 

 or also in gauss. Hence from (8-3) 



B = vH. (8-4) 



The permeability in vacuo is 1. Bodies which increase the number of 

 field lines per unit area have permeabilities greater than 1 and are called 

 paramagnetic, while those of permeabilities less than 1 are diamagnetic. 

 Since a unit pole radiates 47r lines of force, $ = 47rm where m is pole 

 strength. Further, the flux in the magnetized substance may be consid- 

 ered to be the sum of the flux due to the substance alone and the flux in 

 air. Recalling that B = yH and that for air y = 1, we have for the flux 

 per unit area 



B = 1^ + H. (8-5) 



The ratio m/S is the surface density of magnetization or intensity of mag- 

 netization and designated by the script letter ^. It is also the magnetic 



' As far as the geophysicist is concerned, the ruling of the International Electro- 

 technical Commission at Oslo in 1930 created no little confusion by adopting the 

 name oerstedt for gauss. Since the gauss unit for field intensity is so firmly en- 

 trenched in the geophysical literature, very few geophysical authors since 1930 

 have adopted the new unit. 



* See footnote, p. 295. 



