GLASS MICROCAPILLARY ELECTRODES 



possesses other advantages both chemical and physical. It does not readily 

 dissolve in water or give up its alkali. The volume resistivity is high, given 

 by Morey^ as 3-1 X 10^^ Q. cm, and its surface resistivity, due to adsorbed 

 moisture, is better than many other glasses. Its dielectric strength can be 

 several times that for soda glass, the breakdown voltage being approximately 

 1,000 kV per cm. 



Hand-pulled electrodes — Microelectrodes of wide variety in shape and 

 size can be produced by manual means. 



Microelectrodes are usually made from tubing of 1-2 mm outside 

 diameter, which is prepared from any larger size conveniently available. 

 The lumen of the initial tube should be approximately two-thirds of the 

 total diameter. A Pyrex tube 8-9 mm external diameter and 5-6 mm 

 internal diameter is heated in an oxygen-gas flame and drawn down to 

 about 1-5 mm, in lengths of 1-2 ft. The initial part of the draw is performed 

 relatively quickly until the required diameter is reached, and a steady pull 

 is then maintained to obtain a length of uniform thickness. Two in. lengths 

 of this tubing are prepared. 



For the final stages of drawing a very small oxygen-gas flame is used, 

 which can be obtained from a gas-jet with a bore of approximately 0-3 mm 

 diameter. A short length of tubing is held in both hands and 2 mm in the 

 middle is heated to red heat indicating a temperature at which the glass 

 has softened but not begun to coflapse. The tube is then removed from 

 the flame and rapidly pulled apart, when two electrodes should be formed. 



Whilst drawing, both ends should be kept parallel to the direction of 

 movement, as this will lead to the production of straight electrodes, and 

 will prevent the occurrence of flattening. The length of the electrodes will 

 be determined by the amount of glass heated, the temperature and the 

 speed of the draw. The larger the flame the greater will be the amount of 

 glass heated, thus increasing the length. The temperature of the glass, as 

 indicated by the colour, should determine the time at which the draw is 

 made. If too high, as when approaching white heat, a deliberate pause 

 can be made to allow cooling to occur, and either the glass held stationary 

 or slowly extended a few mm. This can in fact be done routinely as it 

 allows more time for the exact drawing point to be estimated. Overheating 

 however may lead to partial collapse of the tube, resulting in a reduction 

 of the ratio of lumen to wall thickness. A variety of electrodes with varying 

 shank length and diameter can be produced by varying the speed during 

 the draw. Thus a slow pull with soft glass will produce a thick shank, while 

 a rapid movement with cooler glass will yield a more sharply tapered and 

 thinner shank. 



A two-stage method can also be used. The tube of 1-2 mm diameter is 

 first drawn down to form a short length about 0-3 mm thick; then, using 

 a very small flame this tube is heated and drawn apart very rapidly. This 

 method gives electrodes with a wide stem, thus eventually reducing the 

 resistance to some extent; it requires, however, more skill. 



If the tube at all stages is prevented from overheating and collapsing, 

 the ratio of lumen to wall thickness remains constant throughout. It was 

 found by direct measurement, from the end-on view, that this ratio varied 

 less than 5 per cent commencing with the initial parent tube down to about 



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