OTHER ELECTRODES 



Any break in the insulation will be detectable by the bubbles of gas which 

 will rise from it. This method of testing is useful for any resin-insulated 

 metal electrodes. For testing insulation on very fine intracellular electrodes 

 it is convenient to measure the resistance of the electrode as it is lowered 

 into a saline bath. Any abrupt change in resistance is an indication of a 

 flaw in the insulation. 



Tungsten microelectrodes of this type have a resistance of 25 to 200 MQ. 

 in different electrodes at low frequency (100 c/s) and low current density. 

 At higher frequencies (5-10 kc/s) the impedance falls to values around 

 0-5 MD. 



CONCENTRIC ELECTRODES 



These are useful for recording from or stimulating relatively small areas 

 of tissue in whole animals which may be conscious or anaesthetized. Their 

 chief application is in the field of electromyography where they can be used 

 to detect activity in motor units for clinical or experimental purposes. 

 They are very robust and can be inserted into dense tissues through intact 

 skin. A simple concentric electrode can be constructed as follows : 



Pass about 70 cm of Eureka resistance wire (gauge 44) through a 26 

 gauge Solila type hypodermic needle of the desired length so that about 

 10 cm of the wire projects from the point of the needle and about 60 cm 

 from the back of it. Dip the protruding 10 cm of wire into Damarda 

 lacquer (Grade L3128, Bakelite Ltd.) thinned with equal proportions of 

 Bakehte S 11210 thinner. Pull the needle over the lacquered wire until 

 only about 0-5 cm protrudes from the tip and then place in an oven at 

 150°C for 3-4 minutes. Too hot an oven will cause bubbling of the lacquer 

 and roughness on the electrode. Solder another piece of wire on to the 

 base of the needle, degrease in Trilene and dip needle into lacquer. Leave, 

 point upwards, to drain for 5 minutes which will allow any bubbles to flow 

 to the base of the needle, and bake for 3 to 4 minutes. Repeat the degreasing 

 and lacquering five or six times. The tips of the wire and needle are then 

 ground flush on a fine hard stone and the electrode tested for insulation 

 and conduction. 



ELECTRODES FOR MEASURING OXYGEN TENSION 



If a small potential of the order of 0-6 V is applied to a noble metal electrode 

 in animal tissue the current that flows is approximately proportional to the 

 oxygen concentration in the tissue (Davis and Brink^", Cater, Phillips and 

 Silver^^). Two types of electrodes can be used for such measurements: 

 (a) the simple 'open ended' electrode which is merely a gold or platinum 

 wire insulated with resin and ground to a flat end, or (b) the 'recessed' 

 electrode which consists of platinum wire fused into glass capillary tubing 

 so that the tubing projects beyond the metal (Figure 36.4). 



The open-ended electrodes are useful for making continuous records 

 of changes in oxygen tension while the recessed type will make intermittent 

 readings of absolute oxygen concentration. 



Currents of the order of 10~^ to 10~^^ amps are obtained depending on 

 the size of the electrode and the oxygen tension in the tissue. 



576 



