11:2/ The Absorption of Electromagnetic and Ultrasonic Energy 207 



When an electromagnetic wave falls upon a cell or group of cells (or 

 any conductor, for that matter) a current will flow and energy will be 

 dissipated. It is usually not possible to restrict the effects of a wave to 

 an area smaller than a wavelength in diameter. Even with electro- 

 magnetic wavelengths of 5 cm in air the wavelength in the tissues will 

 be about 1 cm. Thus, it is hard to confine electromagnetic waves to a 

 small volume within tissues or cells. 



The names given to the various parts of the electromagnetic spectrum 

 are listed in a table in Chapter 21 on Spectrophotometry. That chapter 

 also discusses atomic and molecular absorption of electromagnetic waves 

 which are more important for higher frequencies. In the present 

 chapter, radiofrequency and microwave absorptions and dielectric 

 constants will be emphasized. These show neither sharp spectral bands 

 nor rapid variation with wavelength. The frequencies included in this 

 chapter range from 10 cps to 10 10 cps. , 



In these spectral regions the tissues and cells may be characterized by 

 an impedance (or an impedance of a unit crosssection) . In Appendix C, 

 it was noted that the impedance Z was defined by 



The impedance per unit length of a unit crosssection is 



It has the units of ohm -cm. In Appendix C, it was also noted that 

 the definition for Z was meaningful only if one used sinusoidal 

 currents or if one used the Fourier transform of the current. Thus it is 

 convenient to separate Z into two parts, R and X. In the complex 

 notation, 



Z - R + jX 



The resistance R gives the ratio of the part of the potential in phase with 

 the current to the current. The reactance X gives the ratio of the 

 potential 90° out of phase to the current. The real part of Z' is the 

 resistivity p. For d-c, 



1 

 P = a 



The resistance R and reactance X can be used instead of the con- 

 ductivity a and dielectric constant e to characterize the cells and tissues. 

 Either pair will completely describe the electromagnetic properties of 

 the biological system (with the unwritten addition that 



