GLASS MICROCAPILLARY ELECTRODES 



the resistance between the two channels with the tip in sahne and in oil. 

 To some extent leaks of this kind can be avoided by using tubing with 

 thick intermediate partitions. An added glass partition was found necessary 

 by del Castillo and Katz^^. 



Some methods of modifying microcapillaries 



Occasionally specially shaped tips may be required in one or more channel 

 microcapillaries (Kennard^"). A tube may have to be broken or the inter- 

 mediate wall removed a short distance, and it may be necessary to close 

 an orifice partially. The ease of such operations will depend on the size 

 of the tip. 



The simplest method of breaking off pieces from the tip is by means of 

 a fine steel microneedle carried in a micromanipulator: however, a more 

 readily controlled method is a vibrating needle. This is mounted on the 

 diaphragm of an earphone — the best being the balanced reed type — or 

 directly on the electromagnet of an earphone. It is of advantage to have 

 the needle vibrating in a plane rather than a cone. The microcapillary is 

 supported as close to the tip as possible and the vibrator is carried on a 

 micromanipulator. Power is supplied from an oscillator and the best 

 frequency found by trial. The orifice can be closed by applying heat with 

 a loop of fine platinum wire under microscopic control. For this purpose 

 the operation can be carried out with the capillary mounted vertically and 

 viewed end on through transmitted light. The contrast — and therefore 

 the image — is improved by surrounding the capillary stem with plasticine. 



Construction of capillaries with three or more channels 



Glass capillaries with many channels can be made (Elson^^, Vis^). Those 

 with 3, 7 and 12 tubes are most readily drawn as these numbers of tubes 

 form stable arrangements: the principle of the method is similar to that 

 used for two-channel capillaries. Care must be taken to immobilize the 

 tubes, and they can be pushed into tight fitting rings for this purpose. Heat 

 should be applied gradually and kept to a minimum in order to avoid 

 sharp gradients of temperature. To blow numbers of tubes in an array it is 

 necessary to connect each tube individually by means of soft rubber 

 tubing and a multi-way connection. To avoid lateral hindrance each tube 

 is first drawn down to about half its diameter for approximately 1 in. 



A capillary formed of three tubes has an additional channel in the centre, 

 and the larger groups have many more channels than parent tubes though 

 some may be severely distorted. Capillaries of this kind are fragile and not 

 readily worked. 



Fused quartz tubing with several compartments is manufactured commer- 

 cially. It is used chiefly for purposes of insulation. 



Multiple-bore tubes can be used for spacing arrays of electrodes and for 

 extensive study of a small region of active tissues. 



Clusters of capillary electrodes 



Groups of electrodes may be required to measure the potential at points 

 a short distance apart, arranged to cover an area or in some definite array. 

 If such an area be at the surface of a tissue several micromanipulators can 



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