GLASS MICROCAPILLARY ELECTRODES 



Again a bank of resistances can be used to find that resistance which reduces 

 a deflection to half its initial value. 



A method used by Frank and Fuortes^^, illustrated in Figure 35.6c, does 

 not require an external source of voltage. The voltage used is that already 

 present in the circuit, which may be due to such factors as the potential of 



(a) 



(b) 



Figure 35.6 Arrangements used for measuring 

 microelectrode resistance 



(c) 



the membrane electrode and junction potentials. A three-position switch 

 is connected between the input and earth, in which the first position is 

 open, the second connected through a known resistance i?(5-10 MO) to 

 earth, and the third linked directly to earth. The input is connected to each 

 position in turn and the potential differences noted. The electrode resistance 

 is obtained either from the relationship 



or 



R. = R 



K = R 



(^1-3 - ^2-3) 

 ^2-3 



(1^^1-2) 



iV^- 



V,-o) 



Connection between electrode and amplifier 



The simplest method of connecting the electrolyte within an electrode 

 to the input of an amplifier is to insert a silver wire previously coated 

 electrolytically with silver chloride. However, a preferable method is to 

 connect the electrode to the metal junction through a Ringer-agar bridge: 

 this will ensure a more stable electrode potential for the Ag-AgCl-electrolyte 

 junction, as the electrode surface can be formed and left in place. The 



544 



