150 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



The general behavior of complex dielectric materials as a function 

 of the applied frequency is shown in Fig. (1-7)15. The dielectric con- 

 stant decreases with increased frequency, while the loss (conductive 

 component) increases. 1 The location of the point of inflection de- 

 pends upon the sample material. Increase of the temperature of the 

 sample has the effect of shifting the point of inflection toward higher 

 frequencies. The inflection can take place in a narrow frequency 

 region, but in most cases it extends over a frequency range of several 



10' 



"4= c 



'10' 



go 



-K) 2 





£ § 



JD O 



.5? ■~~ 



Q 



00 

 80 

 60 

 40 



20 







10 



10" 



10 6 

 Frequency 



10 



12 



Fig. (1-7)15. Variations of conductivity and dielectric 

 constant of dielectrics (generalized curve) and of water. 



decades. The characteristic curves for water are shown in the same 

 diagram. It can be seen that under certain circumstances measure- 

 ments at two different frequencies can lead to a better definition in 

 moisture determination. 



b. Nuclear Magnetic Resonance Method. If hydrogen nuclei are 

 brought into a magnetic field, they will be oriented in two different 

 directions, depending upon their angular momentum. The two orien- 

 tations correspond to two slightly different energy levels; therefore, 

 a transition from the lower to the higher energy level will require the 

 absorption of energy. The energy can be supplied from an electro- 

 magnetic field. The frequency at which energy is absorbed by proton 

 nuclei is/ = 4,258 x B, where B is the flux density of the magnetic 

 field. For practical magnetic fields of the order of several thousand 

 gauss, the frequency is in the megacycle range. The energy absorp- 

 tion is measured by an electrical system (3-4); the absorption\curve 

 has the form of a narrow resonance curve ("absorption line"). 



1 P. O. Schupp, Wiss. Veroffentl. Siemens -Werke, 17, 1 (1938). 



