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The Absorption of Electromagnetic and Ultrasonic Energy /I I : 3 



from 0.1 to 3 /ufd/cm 2 . Few values lie outside of the range 0.8 to 1.1. 

 One low measurement of 0.01 /xfd/cm 2 has been obtained for a frog 

 nerve, but other nerve measurements are in the 0.6 to 1.2 /xfd/cm 2 range. 

 Values for the leakage areal resistance of the cell membrane vary from 

 25 to 10,000 ohm -cm 2 or higher. Nerve and muscle measurements 

 have yielded both extremes. 



Similar considerations apply to the electrical characteristics of whole 

 tissues. Their impedance is hard to separate in terms of cellular 



Figure 2. The frequency dependence of the dielectric constant 

 of muscle. Note the three regions labeled with Greek letters 

 indicating three types of relaxation. After H. P. Schwan and 

 G. F. Kay, "Conductivity of Living Tissues," Annals of the New 

 York Academy of Sciences 65: 1007 (1957). 



parameters but may be represented as a lumped resistivity and capacity. 

 The ratio of the capacity to that of a vacuum is the dielectric constant. 

 A plot of effective dielectric constants against frequency has the shape 

 shown in Figure 2. It should be borne in mind that a variety of effects 

 contribute to this general shape. 



The region labeled f3 is the one related to the change from conductance 

 around the cells to conductance through the cells. The complex shape 

 of the curve indicates a variety of cell sizes and shapes. The region 

 labeled y is due to molecular relaxations discussed below. The low 

 frequency changes labeled a indicate some other type of phenomena 

 which is not clearly understood. Similar low frequency changes in 

 resistivity can be seen in Figure 3. Although neither can be explained 

 clearly, it appears likely that both are due to some common mechanism. 

 Moreover, the low frequencies at which these occur indicate that 

 comparatively large pieces of material are involved. 



All molecules tend to become polarized in an electric field. In an 



