GLASS MICROCAPILLARY ELECTRODES 



coats of 'Formvar' dissolved in chloroform can be used. A suitable form 

 of electrode holder is shown in Figure 35.7b, which while supporting the 

 microelectrode also connects the surface conducting layer to the cathode. 



Polythene 

 or PTFE 



Silver wire 



Rubber tube 



Sliding 

 shield 



Spring 



Coated electrode 

 held in a groove 



(b) 



Figure 35.7 Screened microelectrode holders 



Discussion on the process of drawing a capillary electrode 



The process of drawing a capillary from a wider tube resembles that of 

 the extension of a rubber tube; in both, as the length increases the diameter 

 decreases. The elastic properties of glass, however, change greatly during 

 a draw, as it passes from the fluid to the solid state. At high temperatures 

 the elasticity, as measured by Young's modulus, is very low, and mounts 

 rapidly on decreasing the temperature and approaching the solid state. 

 The electrode is formed at a time when most of the physical properties of 

 glass including viscosity, surface tension tensile strength and heat capacity 

 are changing rapidly. 



The 'shoulder' of an electrode {Figure 35.4b), is the point of most rapid 

 taper, and the glass, because of the mass at this point, probably remains at the 

 highest temperature throughout the process of pulling. The thin capillary 

 drawn from the shoulder quickly loses heat, and in the short time of the 

 pull it solidifies, as there is no evidence of sagging in most usable electrodes. 

 The shank of the electrode is formed as the elasticity and viscosity of the 

 glass is rising, so that the cooler the glass the more rapid will be the degree 

 of taper obtained for the same rate of extension. Thus the cooler the glass, 

 the shorter the length of the electrodes. 



The ultimate tip of the electrode is probably formed from glass at the 

 lowest temperature, near the transition point. The final tip will be formed 

 when the force of extension exceeds the tensile strength of the terminal 



546 



