28 : 3/ Magnetic Measurements 



529 



susceptibility of biologically active molecules. A novel detector with a 

 more rapid response time is a modification of the Rankine balance at 

 the Johnson Research Foundation at the University of Pennsylvania. A 

 third, completely different, method utilizes the phenomenon of electron- 

 spin resonance. It is described in the following section. 



The Gouy balance consists essentially of an ordinary analytical 

 chemistry balance and a strong magnet. It is shown diagrammatically 



Figure I. The principle of the Gouy magnetic susceptibility 

 balance. After P. W. Selwood, Magnetochemistry (New York: 

 Interscience Publishers, Inc., 1956). 



in Figure 1 . The sample extends from the region of maximum magnetic- 

 field strength to a region of minimum magnetic-field strength. Under 

 these conditions, it is shown in texts on electricity and magnetism that 

 there exists a force per unit volume d/Jdv on the sample given by 



df = KH dH 



dv 8x 



where x is the direction in which f is measured. If the sample is im- 

 mersed in a medium of susceptibility k , the foregoing expression becomes 



i - <* - K » )H - X 



(i) 



For a sample extending from maximum field H m to zero field, Equation 1 

 can be integrated to 



{-*(«- *o)H*A 



(2) 



where A is the sample cross section and v is the sample volume. 



By comparing the weight of the sample in the Gouy balance with its 

 normal weight, one finds the excess force designated by^in Equation 2. 

 Then k can be computed. Equation 2 may be rewritten 



f=F(K- K ) 



(3) 



