MAGNETIC METHODS 77 



intensity of magnetization of a body is its magnetic moment per cubic 

 centimeter, or / = M/V and volume susceptibility k = I/H, as given. 



To express susceptibility in relation to the mass of a particular mater- 

 ial, another term, specific magnetization Q, is introduced. Specific magne- 

 tization, for Mfhich the symbol X is sometimes used, stands in the same 

 relation to mass susceptibility as does intensity of magnetization to volume 

 susceptibility. Specific magnetization Q is magnetic moment per gram. 

 With M^ = mass, 



(11) 



and mass susceptibility k^ 



(12) 



These are natural and logical units in the c.g.s. system for : volume 

 magnetization / represents 1 cubic centimeter of a material that has 1 c.g.s. 

 moment. Likewise, mass magnetization Q for a substance indicates that 1 

 gram of it has 1 c.g.s. moment. 



Volume and mass magnetization are related in an apparently paradox- 

 ical manner which is shown as follows : 



1 = M/V = iiiL/AL = m/A (13) 



In this equation we have set pole separation as equal to L, the physical 

 length of the magnet. This is true only in a theoretical magnet. Let d = 

 density, and with the other symbols as previously given, we have 



Q = M/W — M/Vd = M/ALd = mL/ALd = m/Ad ( 14) 



In words, intensity of magnetization is pole strength per unit area, and 

 specific magnetization is pole strength per unit area times density. / and Q 

 differ from each other only by the density factor. 



If in the expression Q = m/Ad, both sides of the equation are multi- 

 plied by d, then Qd = m/A. However, as m/A = /, it is apparent that 

 Qd = I. Since / = kH and Q = k'H, volume susceptibility equals mass 

 susceptibility times density, or 



k = k'd (15) 



From similar analysis of the equation Q = I/d, moss susceptibility 

 equals volume susceptibility divided by density, or 



k' = k/d. (15a) 



The usual relations of mass, volume, and density are given in the defi- 

 nition of density. Density equals mass per unit volume; viz., d = M/V. 

 Also, volume x density = mass (in symbols W = V-d) and volume = 



