GLASS MICROCAPILLARY ELECTRODES 



This larger kind of electrode is also easily made from the smaller ones 

 simply by breaking the tip. Controlled damage can be produced by carefully 

 lowering an electrode on to a solid surface with the aid of a micromanipula- 

 tor. The electrode can also be stabbed several times through cotton wool, 

 filter paper or a jelly containing a suspension of fine carborundum powder. 



Another method of breaking the tip is by passing a high-voltage arc 

 discharge between it and a carbon or metal electrode brought close, and this 

 can be done when the microelectrode is filled with conducting fluid. A 

 motor car induction coil may be used. However the method is not a good 

 one as it may cause cracks along the shank. 



A simple and rehable method which has been used to enlarge an electrode 

 tip is to expose it to fumes of hydrofluoric acid in the neck of a bottle contain- 

 ing a 48 per cent solution. A brief exposure of one or two seconds at a time 

 is made, followed by washing in water (Tobias and Bryant^''). Electrode tips 

 can also be ground on a rotating fine abrasive wheel of India or Arkansas 

 stone. The electrode is approached with a micromanipulator at a slight 

 angle, pointing in the direction of rotation. The process can be observed 

 with a dissecting microscope and the point where it makes contact is 

 immediately seen, as the electrode moves laterally. The sound of the abrasive 

 action can also be used as a guide. 



Glass microcapillary electrodes with saline and metallic conductors 



The most common way of reducing the resistance of saline-filled micro- 

 capillary electrodes is to introduce a silver wire as far as possible: if the 

 wire protrudes at the tip both can be cut off. However it is not possible to 

 insert a wire into very fine capillaries, so that electrodes of this type are 

 usually greater than 20 [x diameter. With electrodes too small for the wire to 

 reach the tip, the major portion of the resistance remains. 



Electrodes with metallic conductors as far as the tip, while suitable for 

 recording fluctuating signals, are usually not reliable for measuring steady 

 potential levels as the electrode surface is small and may be subjected to 

 changes in composition of the medium, and to chemical action: this will 

 cause potential drift. Such difficulties were overcome by Hodgkin and Katz^^ 

 by inserting a bright silver wire into the saline-filled electrode from the tip to 

 the fluid in the stem. This wire did not make direct contact with the grid 

 wire. For rapid signals the impedance of the silver wire was less than that of 

 the saline bathing it, which was thus effectively short-circuited. At d.c. it 

 behaved simply as a saline-filled capillary electrode. 



Tomita and Funaishi^^ have described a form of saline-filled capiUary 

 electrode with a silver wire as far as the tip. They have found that the 

 noise of the electrode was greater when it was not filled with saline. It is 

 possible that the saline surrounding the wire near the tip served to increase 

 the effective area of the electrode surface ; alternatively the saline fluid may 

 have increased the value of the parallel wall capacitance and caused a 

 reduction in noise by reducing the high-frequency response. 



Glass microcapillary electrodes with metallic conducting media 



Electrodes of glass can be formed with conductors of various metals. 

 Their value is found in the relatively low noise level and in an improved 



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