MEASUREMENTS WITH MICROCAPILLARY ELECTRODES 



fibre, and so will depend on the fibre diameter as well as the viscous and 

 elastic properties at the prevalent temperature. 



The last 10 // of the electrode from the orifice are important to physi- 

 ologists as it determines the area of membrane disturbed. This part of the 

 electrode often shows great variability, usually an increased rate of taper, 

 and may sometimes form a 'shoulder' just before the ultimate portion. 

 Such a shoulder may be rather exaggerated when examined microscopically 

 in air as a result of optical artefacts. The degree of taper at the terminal 

 portion of the tip used by most workers appears to lie in the range between 

 1 : 8 to 1 : 10 (Nastuk and Hodgkin'^; Alexander and Nastuk^^; Tasaki, 

 Policy and Orego^^; Frank and Fuortes^^; Fatt and Katz^^). This figure 

 is derived chiefly from measurements of published photographs of micro- 

 electrodes, and it is not known in fact whether greater variation is found in 

 other satisfactory electrodes. Some figures which have been quoted showing 

 a more gradual taper refer to portions of the electrode away from the tip. 



The physical conditions under which the final submicroscopic tip is 

 formed probably do not vary greatly when similar tubing is used. The 

 shape of the tip will be determined by the initial temperature of the glass, 

 the wall thickness and the velocity of pull. With thick walls the temperature 

 to which the tube is heated will be higher, it will cool slowly and thus the 

 electrode will be formed at a higher temperature. The velocity of pull will 

 control the time allowed for cooling. As the tip is formed at a relatively 

 low temperature the discontinuity in properties near the transition point 

 may be responsible for the discontinuity of taper which is found in varying 

 degree near the tip. 



The actual proportions of wall thickness to lumen remain remarkably 

 constant throughout the whole electrode. Thus the choice of the initial 

 tube helps to determine the final dimension of the tip. 



The mechanical strength of the tip may be important for some types of 

 experiments. When greater strength is required the wall thickness may be 

 increased by using thick-walled tubing. This is drawn at as high a tempera- 

 ture as possible which itself strengthens the glass. If the tip can be made 

 without a terminal shoulder the electrode is probably stronger. 



MEASUREMENTS WITH MICROCAPILLARY ELECTRODES 



D.C. measurements 



The physical size of a microelectrode tip gives it a high resistance which 

 effects its functioning in several ways. A capillary tube with a lumen of 

 0-5 fx diameter will have a resistance of 5 mCi for each fi of length when 

 filled with Ringer's solution. Such electrode resistance will be placed in 

 series with the resistance of the tissues and of the measuring instrument. 

 When a voltage V is developed by the tissue, the value measured at d.c. will 

 be determined by the proportion of the input resistance of the measuring 

 instrument to the total resistance. Measurements of d.c. voltage within 

 tissues are usually made in a bath of Ringer's fluid or in situ, when either the 

 bath or the animal is earthed. The voltage can be measured with the aid of 

 directly coupled differential amplifiers which possess good stability and an 

 indicator. 



547 



