206 The Absorption of Electromagnetic and Ultrasonic Energy /I I : 2 



Supplement to TABLE I, Appendix C 

 Quantity Symbol Defining Equation Units 



In a vacuum, B is the same as H, and E is the same as D. The 

 quantities H and D are generated by currents and fixed charges, respect- 

 ively; accordingly, they do not include the specific medium such as 

 tissue or cell. On the other hand, B and E represent the force on a unit 

 current or charge, respectively; they do include the effects of the 

 medium. 



The properties of the medium itself, then, are included in the pro- 

 portionality factors /x and e. Likewise, the conductivity a represents an 

 additional proportionality factor depending only on the medium, not 



on the size of E, H, and^so on. 1 Thus, one way of characterizing the 

 electromagnetic behavior of a medium is to give values for /x, e, and a. 

 For all biological cells and solutions of most compounds of biological 

 interest, [i is less than 0.01 per cent different from /x , the permeability 

 of free space. Studies utilizing these small differences are described in 

 Chapter 28. For the material in the current chapter the approximation 



is well within experimental limits of error for all biological cells. 



The basic equations describing electricity and magnetism are known 

 as Maxwell's equations. These allow anyone adept at mathematics to 

 predict the existence of propagated electromagnetic waves with wave 

 velocity in free space 



1 



Co = 



V'n, 



€ 



and velocity in a medium 



VV 



Because e for all cells, suspensions of cells, and tissues is much larger than 

 e , c is smaller than c . 



The conductivity cr for the axon and muscle-fiber membranes varies dramati- 

 cally with the potential difference across the membrane. (See Chapter 4.) 



