Chap. 10] 



ELECTRICAL METHODS 



641 



In terms of the displacement current and by substituting dE/ dt = 



jcoE, orE = — — -, eq. (lO-lOo) may be written: 

 (t) dt 



_ dE ATff dE 



at CO at 



dE/ . 4x 



4x(t\ 



so that 



/r = K 



,dE 

 dt' 



(10-lOd) 



where k' = k — j4ir<r/« is the apparent dielectric constant (see Fig. 10-56). 



Diehctrk ^q 

 CinsM 



8 12 16 29 



Moisture Canttnt ■ % 



24 



a 



Fig. 10-6. Apparent dielectric constant as a function of moisture content (after 



Smith-Rose). 



Like the conductivity, the dielectric constant of an aggregate increases 

 with water content, owing to the large dielectric constant of water. Smith- 

 Rose gives as dielectric constants for dry soils, 2.6 to 2.8; for soils with 

 3.6 per cent moisture, 2.3 to 5.6; and for soils with 16 to 30 per cent water, 

 18 to 30. Fig. 10-6 shows the change of dielectric constant with moisture 

 for a frequency of 1200 kc. 



