236 H o ^^' A R D j. c u r t i s 



the conventional elements of resistance, capacitance, inductance, 

 and electi'omoti\e force, which from all points of view is identical 

 with the circuit under investigation. If one has drawn a correct 

 eciuivalent circuit, it should be possible to construct the circuit from 

 conventional components and demonstrate that electrically it is 

 identical to the unknown. Indeed, if the experimental circuit and 

 its equivalent circuit were placed in identical sealed boxes with only 

 the two lead wires protruding from each, it would be impossible by 

 any electrical measurements to distinguish between them. This 

 does not implj^ that there is only one possible equivalent circuit. 

 There may be a number of different equivalent circuits for any par- 

 ticular experimental circuit that will fulfill the above criteria, and 

 the one that is chosen is decided by its usefulness. 



An equivalent circuit may be either drawn from the known be- 

 havior of the components of the experimental circuit, or constructed 

 from impedance and potential measurements. In the first case, the 

 circuit is obtained by the laws of combination of series and parallel 

 circuits as demonstrated in elementary physics texts. The latter 

 case is the subject of this chapter. 



In general it is important to draw the equivalent circuit for one of 

 two reasons. The first is to prevent errors of measurement and to 

 make sure that the measurements actually represent what they are 

 supposed to. The second is to aid in the interpretation of the meas- 

 urements. 



While it is usually sufficient to consider only the experimental 

 circuit when drawing an equivalent circuit, it should never be for- 

 gotten when making measurements that the measuring equipment is 

 an integral part of the entire circuit and when necessary its equivalent 

 circuit should be included in the entire equivalent circuit. An ex- 

 ample will serve to illustrate the importance of the concept. Many 

 investigators have tried to produce cellular changes by subjecting 

 cells to an electric field. A cell suspension is usually placed in a test 

 tube or other suitable glass container, which is then placed between 

 tiie poles of a high d.c. potential source. Since we are dealing with 

 direct currents we can ignore the membrane capacitance of the cells 

 and represent the cell suspension by two resistances in parallel . The 

 glass walls of the test tube will act as a dielectric so there will be a 

 capacitance between the polepiece and the suspension on each side. 

 Thus the equivalent circuit can be represented by the circuit of Fig- 

 ure 1. From this it will be seen that there can be no flow of direct 



