VIII. J3IOELECTRIC MEASUREMENTS 267 



The type of electrode is enormously important here, and one 

 electrode may exhibit polarization effects a thousand times greater 

 than another. The best electrodes are made of platinum and cov- 

 ered with a dense layer of platinum black. This will minimize elec- 

 trode effects but by no means eliminate them. These electrodes are 

 not reversible except under special conditions and therefore they may 

 have a d.c. potential difference between them. This causes no trouble 

 for impedance measurements but means that in general it is not pos- 

 sible to use the same electrodes for potential and for impedance. If it 

 is necessary to use the same electrodes for both, silver-silver chloride 

 eiecfcrodes are about as good a compromise as can be found. 



4. Cell Constants as Measured by Impedance 



The impedance of cells or tissues depends upon a good many dif- 

 ferent characteristics of the cells or tissue, and therefore we may use 

 impedance measurements to determine some of these characteristics. 



Volume Concentration of Cells. This can be measured quite 

 accurately by the impedance method provided the cells are of a 

 known geometrical shape and are reasonably uniform. It is ob- 

 tained from the extrapolated zero frequency resistance, and the 

 necessary equations have been worked out for the case of suspen- 

 sions of spherical or spheroidal cells such as blood by Fricke (14), 

 and for suspensions of cylindrical cells such as nerve and muscle by 

 Cole and Curtis (11). For these special cases, the accuracy for de- 

 termination of volume concentration would probably be better than 

 any other known method, up to volume concentrations so high as to 

 cause deformity of the cells. However, there are some inherent er- 

 rors such as effects of stirring, surface conductance, etc. that limit the 

 accuracy at the present time. For tissues in general it can be said 

 that the zero frequency resistance would be proportional to the vol- 

 ume concentration of cells but no quantitative relations have been 

 worked out and indeed it is not to be expected that any great accu- 

 racy could be obtained by this method. 



Thus by using a single low frequency, the impedance method 

 can be used for the routine determination of volume concentration 

 of blood or other cell suspensions, the fat content of milk, etc. It has 

 also been used for measuring the moisture content of soils, wood, 

 etc. A frequency of about 1000 cycles is high enough to minimize 

 electrode errors in most cases and low enough to be practically equal 

 to the zero frequency resistance of most cell suspensions or tissues. 



