Sensory Faradic Threshold. 75 



determine, according to Martin's formula, the number of /3 units 

 required for stimulation. As the tissue resistance is without doubt 

 almost entirely due to the outside horny layer of skin, it is regarded as 

 outside resistance like the resistance of the secondary coil of the induc- 

 torium, or the resistance of any unit put in the secondary circuit. The 

 measurement of the tissue resistance is accomplished in practice by 

 balancing on a Wheatstone bridge. In the very simple form used, the 

 secondary of a Porter inductorium was employed as a source across a 

 slide wire. A telephone was connected to the slide and also at a point 

 between the known arm of the bridge, 20,000 ohms, and the arm which 

 contained the tissue to be measured. The balance point was extremely 

 indefinite. Practically it was necessary to endeavor to match inten- 

 sities of tone on both sides of what seemed to be the indicated balance 

 position for the slide. This method must be called in question, since 

 the tone on one side of the balance position of the bridge has quite a 

 different quality from that found at the other end. These quality 

 differences play a role from the standpoint of intensity when the tele- 

 phone is used as an indicator in the bridge, because of the characteris- 

 tics of the telephone membrane. It was of course recognized that the 

 tissue has not only resistance but capacity and that there was present a 

 fluctuating body current. At the time of experimentation we were 

 under the misapprehension that it was not practical to balance for 

 capacity along with the resistance. 



The tissue-resistance measure was usually about 5,000 ohms. The 

 fingers were immersed to a depth of 2 cm. and this condition was kept 

 quite constant, as it was recognized that theoretically the amount of 

 tissue immersed would change the resistance value. To balance about 

 5,000 ohms tissue resistance against a known resistance of 20,000 ohms, 

 as was done in this case and also in the measurements of the previous 

 series, 1 is theoretically not good practice. The balance position on the 

 slide wire under these conditions is close to one end of the bridge. The 

 observational error at the end of the slide wire of the bridge is propor- 

 tionately quite large, as a change of 2 cm. in the determined balance 

 point would here be of much more importance on the resistance figure. 

 The employment of a known resistance of 20,000 ohms was an expedi- 

 ent, as in practice the balance point was much sharper than when using 

 a known resistance of 5,000 ohms in its place. Probably this condi- 

 tion was due to a larger proportional effect of the finger capacity with 

 a lower resistance. 



Another point to be criticized in the technique is the order in which 

 the measurements were made. As the tissue resistance is due to the 

 outside horny layer of skin, evidently there will be a decrease in resist- 

 ance when the finger tips become somewhat soaked with salt solution. 

 It is regrettable on this account that the order of our procedure in refer- 



1 See Dodge and Benedict's report, p. 137. 



