28 : 4/ Magnetic Measurements 



531 



is then determined. The entire apparatus is very small. The magnet 

 is about 2 cm long and each half cylinder holds about 0.5 ml. Instead 

 of allowing the magnet to move, a servo system is employed to hold the 



Mirror 



Quartz Fiber 



Half- eel I 1 



Quartz Fibers 



Brass Counterweight 

 Magnet 



Half -cell 2, 



Moved Down as 

 Indicated to Show 

 Magnet 



Figure 2. The principle of the Rankine magnetic susceptibility 

 balance. After A. S. Brill, H. den Hartog, and V. Legallais, 

 "Fast and Sensitive Magnetic Susceptometer for the Study of 

 Rapid Biochemical Reactions," Rev. Sci. Instr. 29: 383 (1958). 



magnet in place; the servo current is recorded. This instrument is not 

 an absolute one ; it must be calibrated using solutions of known suscep- 

 tibility. This problem is discussed in considerable detail in the reference 

 by Brill and co-workers. The use of a flow system and a rapid mixing 

 chamber makes it possible to resolve short times after the start of the 

 reaction, as discussed in Chapter 26. 



4. Resonance Measurements 



Both of the balances described in the previous section measure the 

 static magnetic susceptibility of a sample. This includes the effects of 

 all the paramagnetic and diamagnetic molecules in the sample. There 

 is no simple way of separating the various contributions. Resonance 

 methods, in contrast, allow one to separate the effects of different para- 

 magnetic species. Resonance methods are based on the quantum 

 mechanical effect that magnetic dipoles are restricted so that their 

 projection on the direction of a magnetic field can have only certain 



