VIII. n 1 O E L E C T R I C M 10 A S U II 10 M E N T S 265 



through the intercellular fluid. (2) It can flow through the intercel- 

 lular fluid to the cell, through the cell membrane by ionic conduction, 

 through the intracellular fluid, through the membrane on the other 

 side, and on to the other electrode. (3) It can flow through the inter- 

 cellular fluid to the cell, across the cell membrane by induction by 

 virtue of the membrane capacitance, through the intracellular fluid, 

 out the other side by induction, and on to the other electrode. On 

 this basis it is possible to, draw the equivalent circuit, and this has 

 been done in Figure 1 la. It will be seen that the circuit of Figure 1 la 

 can be reduced to that of Figure 116 by simple combination of series 

 and parallel elements. This latter circuit is a i-easonably close ap- 

 proximation to the impedance of all cells and tissues at all frecjuencies. 



_J 



^o 



f^i 



^n,^ 



(a) (Z>) 



Fig. 11. Equivalent circuit of a suspension of cells: 

 (a) complete circuit; (6) reduced circuit. 



The object of all impedance measurements then is first to assign exact 

 values to the three elements of Figure 116, and then to interpret these 

 values in terms of the structure of the cell. Neither objective is by 

 any means simple, and each will be considered separately. 



3. Methods of Measurement 



Complete discussion of the apparatus used for making impedance 

 measurements is beyond the scope of this book, and it will be possible 

 here only to indicate some of the major requirements that must be 

 met before impedance measurements are attempted. 



First it is necessary to have a very accurate alternating current 

 Wheatstone bridge. If only low frequencies of about a thousand cy- 

 cles are needed, it is possible to purchase a conductivity bridge, since 

 several good ones are available commei'cially. Such measurements 



