7(i EXPLORATION GEOPHYSICS 



It is pole strength per unit cross section of the magnet, taken at right angles 

 to the direction of magnetization. In equation form : 



/ = f (8) 



where m = magnetic poles and A is end area. 



The quantity / also equals magnetic moment per unit volume. This 

 relation can be derived from Equation 8, by multiplying the numerator 

 and the denominator of the second term by L', the distance between the 

 poles of the magnet. mU = magnetic moment M. AL' = volume of the 

 magnet or V. Therefore 



_ mU _ M 

 AU" V 

 or 



The strength of a magnet is conveniently represented by its magnetic 

 moment per unit volume. Example : a magnet with a cross-section of 5 

 sq. cm. and a pole strength of 15 will have an intensity of magnetization 

 of 3. Intensity of magnetization is also called polarization. 



An iron bar or a bar of magnetic material, when placed in a magnetic 

 field, becomes magnetized, a property which is known as magnetic suscep- 

 tibility. Susceptibility, defined quantitatively, expresses the ratio of the 

 intensity of magnetization acquired by a substance to the strength of the 

 magnetizing field acting on the body. The letter k (or the Greek letter 

 kappa) is used to denote susceptibility. 



k=jj (10) 



Equation 10 yields the following : 



I = kH 



and 



-=i 



Theoretically any material that has a magnetic susceptibility greater 

 than zero will become a magnet when placed in a magnetic field in air. 



A ferromagnetic or a paramagnetic substance in a magnetic field shows 

 a susceptibility greater than zero. A cubic centimeter of such a substance 

 has an appreciable magnetic moment. For a diamagnetic material the sus- 

 ceptibility is less than zero, or negative. In a magnetic field, a cubic centi- 

 meter of it will be magnetically weaker than its surroundings. 



Volume and mass susceptibility. — The magnetic susceptibility of 

 substances can be referred to either unit volume or unit mass. As defined. 



