VIII. BIOELECTRIC MEASUREMENTS 



245 



solution of potassium chloride isotonic with the sohitions to be meas- 

 ured. The tube is sealed with a rubber stopper and contact is made 

 through a camel's hair brush or cotton wick. Chloride is plated on 

 after the electrode is finished. These should always be made and 

 used in pairs. They are very convenient to use and may be stored 

 for months or even years and used on a moment's notice. 



Silver wire 



Rubber 

 ^ stopper 



Glass 

 tube 



Camel's hair 

 brush 



Fig. 4. Convenient work- 

 ing electrode for biological 

 investigations. 



/?, £-, 



r-^VAA/VM|- 



L^WW^— I 



-O £■ O- 



Fig. 5. Equivalent circuit for po- 

 tential measurements, showing magni- 

 tude of error when leakage is present. 



3. Salt Bridges 



It is usually desirable to isolate the electrode itself from the solu- 

 tion being measured, and this is done by means of a salt bridge. It is 

 merely a means of making electrical contact between two solutions 

 without introducing electrodes and their accompanying complica- 

 tions. It usually consists merelj^ of an inverted U tube filled with 

 molar potassium chloride solution, with one end dipping in each of 

 the solutions. It will be seen from equation (1) that under these 

 circumstances there will be a licjuid junction potential between the 

 solution of the salt bridge and the other solutions. However, it will 

 be seen that the potential is proportiijnal to the difference in mobility 

 between anion and cation. The mobility of K+ and ('1~ are almost 

 identical, so a jjotassium chloride solution can add nothing to a li(iuid 

 JLUiction potential. Thus if two solutions are joined electricall}' by a 

 potassium chloride salt bridge, they will have no more of a potential 

 between them than they would have if it were possible to place the 

 two solutions in direct contact. 



