VIII. BIOELECTRIC MEASUREMENTS 247 



from one electrode to the other, and assume that any potential meas- 

 ured then is due to electrode differences or to spurious potentials, 

 and then to use this value as a zero point for the actual measurement. 

 The fallacy of this procedure can be seen by glancing at equation 

 (3). The salt bridge merely changes the value of Ri and eliminates 

 part of El, whereas the important consideration is the relation be- 

 tween the actual value of Ri and R2 of which this short-circuiting 

 procedure may give no indication. 



It is impossible to outline any procedure that can be used to in- 

 sure elimination of these errors in all cases. The investigator must 

 be fully aware of the possible sources of error, draw an equivalent 

 circuit for the particular set-up being used, and show from this that 

 all sources of error have been removed. Until this is done, it is im- 

 possible to have any faith whatever in the measurements. 



5. Insulating Materials 



When using insulating materials for biological work it is usually 

 necessary to develop criteria of excellence different from those used 

 in the physical sciences. In the latter, volume resistivity is usually 

 rather important, since surface conductance can be kept small by 

 keeping the insulator clean and dry. However, this is very difficult 

 if not impossible in biological work, so volume resistivity is of very 

 little importance. An}^ insulator impervious to water will in general 

 have a high enough volume resistivity to be acceptable for biological 

 work. Glass and quartz, for example, have a high volume resistivity 

 but are readily wet by water and so are bad from the biological point 

 of view, especially if they are clean. Perhaps the best substance 

 knowTi is paraffin, which has both a very high surface and volume 

 resistivity and is not wet by water. However, it is rather inconven- 

 ient to use in many cases. Paraffin-coated glass has often been used, 

 but extreme care must be used, since in the course of time a film of 

 water will work between the glass and the paraffin and cause trouble 

 while the coating appears to be in good condition. 



Glass coated with petroleum jelly or other heavy mineral oil is 

 very good, although the coating must be renewed from time to time. 

 This can be done merely by wiping the glass with a cloth saturated 

 with petroleum jelly. Some of the plastics such as Incite or poly- 

 styrene are quite good by themselves, and very good when coated 

 with petroleum jelly. 



