Sec. 1-7] 



MECHANICAL INPUT TRANSDUCERS 



151 



The intensity of the absorption line, or the height and slope of the 

 resonance curve, depends upon the number of nuclei present in a 

 sample. This effect forms the basis of a quantitative determination 

 of water, i.e., proton nuclei in organic substances. 1 The sample is 

 placed in a container inside a coil which is supplied by rf current. 

 Coil and sample are in highly homogeneous magnetic field, Fig. 

 (1-7)16. The variation of the rf energy absorption, in response to a 



Fig. (1-7)16. Coil 

 and sample of 

 an arrangement 

 for the measure- 

 ment of humid- 

 ity from nuclear 

 magnetic reson- 

 ance [fro?nJ. M. 

 Shaw and R. H. 

 Elsken, J . Appl. 

 Physics, 26, 313 

 (1955); by per- 

 mission]. 





\ 



O 



E 



^ 1 



4 6 8 10 '12 14 16 

 Water content, 

 g of water/100 g dry solid 



Fig. (1-7)17. Typical moisture- 

 calibration curve for dehydrated 

 potatoes [from J . M. Shaw and R. 

 H. Elsken, J . Appl. Physics, 26, 

 313 (1955); by permission]. 



variation of the frequency of the rf source or variation of the mag- 

 netic flux density, is measured by an rf bridge system. 



The method has been used for organic matter (dehydrated vege- 

 table tissue) in a humidity range (mass of water to mass of dry tissue) 

 varying from 1 to 5, for wood having between zero and 1 gram of 

 water per gram of dry material, for dehydrated potato powder, 

 wheat kernels, and starch. A typical calibration curve is shown in 

 Fig. (1-7)17. 



As in other applications of the nuclear-resonance method, a high 

 degree of magnetic-field homogeneity is required. Inhomogeneity 

 can cause considerable error, more for granular substances than for 

 homogeneous liquid or gaseous substances, because different grains 



1 T. M. Shaw and R. H. Elsken, J. Chem. Phys., 18, 1113 (1950). 



