VIII. BIOELECTRIC MEASUREMENTS 263 



tions. AVhen the measurement of the resistance of a biological ma- 

 terial was made, however, many difficulties were encountered that 

 had not been met in physical systems. The resistance of a tissue 

 was found to depend upon the magnitude of the current, the time it 

 had been flowing, and even upon the direction of flow. Further, if an 

 electromotive force were applied to a tissue and then removed, a po- 

 tential difference would persist for a long time thereafter. The situa- 

 tion was further complicated by the fact that all these phenomena de- 

 pended upon the composition and previous history of the electrodes 

 used. All this was completely contrary to Ohm's law and to the be- 

 havior of electricity in physical systems, so it was felt for a time that 

 a completely different set of laws would have to be used for biological 

 systems. 



These difficulties did not prevent a number of investigators from 

 making measurements in an effort to obtain some empirical correla- 

 tion between resistance and structure or function. The effects of 

 such things as salts, injury, narcotics, physiological activity, etc. 

 were tried. In general it was found that the resistance of highly or- 

 ganized and oriented tissues such as skin or muscle is different in dif- 

 ferent directions, but that the resistance is decreased and is the same 

 in all directions when the tissue is dead. 



In the latter part of the nineteenth century Kohlrausch intro- 

 duced the practice of using alternating currents for measuring the re- 

 sistance of electrolytic solutions in order to eliminate the electrode 

 difficulties. When this idea was tried on biological materials it was 

 found that the resistance was independent of the type of electrode 

 used and, for small currents, independent of the strength of the cur- 

 rent. Under these conditions. Ohm's law could be considered valid 

 and a logical interpretation could be attempted. 



These measurements Avere made by means of a Wheatstone bridge, 

 and it was found that a true balance of the bridge could not be ob- 

 tained unless a capacitor was placed in an arm of the bridge adjacent 

 to the tissue. This meant that there was a capacitance associated 

 with the tissue, which disappeared with the death of the cells. 



It was next found that the resistance of biological material, unlike 

 physical substances or electrolytic solutions, depends upon the fre- 

 quency of the alternating current which is used to measure it. Hober, 

 in 1912, showed that the resistance of blood is very much lower at 

 very high frequencies {ca. 10,000,000 cycles per second) than at low 

 frequencies. Further, he showed that, when the cells are killed, the 



