GLASS MICROCAPILLARY ELECTRODES 



The voltage recorded between the two input grids will be the sum of all 

 the voltages in the external circuit, and besides the voltage due to the tissues 

 present will also include the P.D. from metallic contacts of dissimilar metals 

 at switches and junctions, electrode potentials of metals in aqueous 

 solutions, and diffusion potentials in the fluids. When measuring the P.D. 

 it may not be sufficient to determine the value of the potential deflection 

 from the level achieved when both inputs are shorted with a wire to earth, 

 unless care is exercised to reduce unwanted additional potentials. The sum 

 of the total P.D. can be reduced both by the choice of materials forming 

 junctions and by arranging that the unwanted potentials are placed symmetri- 

 cally to oppose and cancel each other. Thus when a switch or other circuit 

 element is introduced in one lead similar wires should be used to establish 

 connection to both ends; and when a single junction is made, another 

 identical one should be placed in the other lead. When a short circuit is to 

 be established it is best placed as near to the point of measurement as possible 

 so as to reduce the number of sites where potentials may be introduced. 



Electrodes — An important source of e.m.f. in a circuit may be the electrode 

 potentials at the metal fluid junctions. To reduce these, two similar junctions 

 are arranged to oppose, with precaution to keep them stable. The junctions 



Amplifier 

 input 



Ringer -agar 



Ringer's fluid 

 Tissue 



Figure 35.8 Arrangement for measuring with microelectrodes 



commonly used for physiological experiments are of Ag-AgCl-Ringer. The 

 Ringer can be made up with 2-3 per cent agar and pairs of electrodes kept 

 together. One electrode is introduced into the fluid of the tissue bath and 

 the other connected to the microelectrode filled with 3 M potassium chloride 

 or Ringer's fluid. In this way fluid junction and electrode potentials are 

 kept at minimal values {Figure 35.8). 



Silver chloride working electrodes can be prepared in any size or shape 

 required and are made from silver wire or platinum electrolytically coated 

 with silver. Silver halide electrodes have been discussed recently by Janz 

 and Taniguchi^^. The commonest form of Ag-AgCl electrode has a 'plum' 

 or purple coloured surface and is relatively stable if carefully prepared with 

 respect to current density and purity of solutions and metals. Silver is 

 deposited on to platinum in a 1 per cent solution of potassium silver cyanide, 

 and the electrode must be washed at least 12 hours in running water to 



548 



