266 HOWARD J. CURTIS 



may be quite useful for some limited studies. However, if at all 

 possible the frequency should be variable over at least a limited range. 

 Electrode troubles are best diagnosed by varying the frequency. 



In order to obtain a complete and accurate measurement of the 

 impedance of biological systems it is necessary to have a bridge with 

 auxiliary oscillators, amphfiers, etc. capable of making accurate im- 

 pedance measurements over a frequency range from a few cycles per 

 second to several million cycles per second. Further, it must be 

 capable of covering an impedance range from a few ohms to several 

 hundred thousand ohms. No commercial equipment is available 

 that will begin to do this, and the investigator must build his own. 

 A complete description of a satisfactory bridge, oscillator, and ampli- 

 fier has been published by Cole and Curtis (6). The impedance sen- 

 sitivity of this bridge can be made 0.001% with an absolute accuracy 

 over the entire frequency range of 0.1%. Such measurements 

 should not be attempted with a bridge inferior to this, since often 

 important quantities occur that are measured as small differences 

 between much larger quantities. 



In addition to the bridge it is necessary to have carefully designed 

 conductivity cells for the particular application in question. At 

 least one conductivity cell must usually be constructed for each 

 problem undertaken. In general there are two problems involved 

 in the design of a conductivity cell. First the conductivity cell must 

 be of the proper size and shape so that current will flow through the 

 tissue in a way such that the data obtained will be susceptible to anal- 

 ysis. Second, it must be designed in such a way that electrode ef- 

 fects are kept to a minimum. 



Electrode effects are probably the most troublesome feature of 

 these measurements and certainly at low frequencies place a definite 

 limit on the accuracy attainable. There is always a certain resist- 

 ance encountered in going from the electrode into an electrolytic 

 solution and this resistance has a capacitance associated with it. 

 These are known as electrode resistance and capacitance, respec- 

 tively, and both are inversely proportional to the frequency. For this 

 reason they are known also as polarization resistance and capacitance. 

 At low frequencies it is easily possible to have the polarization capaci- 

 tance far larger than the capacitance of the biological system. At 

 higher frequencies the electrode effects become negligible. Polariza- 

 tion effects will be smaller the larger the resistance between the elec- 

 trodes and the greater the area of the electrodes. 



