84 EXPERIMENTAL PHYSIOLOGY 



inside the borer. Solder an insulated wire to one end of a straight piece of pnre 

 zinc wire of about three inches length, and then thoroughly amalgamate the 

 zinc wire by dipping it into the amalgamating fluid and rubbing it on a 

 clean duster. Now pass the zinc wire through the hole in the cork made by the 

 bradawl until it projects well on the lower side. Next take some powdered 

 kaolin and make it into a stiff clay with normal saline, and force a little of this 

 up the constricted end of the glass tube to act as a plug to close that orifice. 

 Then fill the tube with saturated zinc sulphate solution, and fit it into the 

 large hole in the cork, so that the zinc wire dips into the solution. All that is 

 now required to complete the electrode is to fix to the lower end a plug of 

 china clay whose apex may be moulded to any desired shape (see fig. 74). 

 In many cases it will be found convenient to fix in the centre of this china- 

 clay guard a coarse thread soaked in normal saline, which can then act as 

 the terminal part of the electrode. These electrodes can be kept, and all 

 that they will require at a future time is a fresh guard of clay, when they will 

 at once be ready for use. 



Experiment 11. — Arrange the apparatus exactly as in experiment 10, 

 fig. 72, but employ these electrodes in the place of the wire ones of that experi- 

 ment. The nerve does not now become polarised. 



Great as is the importance of using unpolarisable electrodes when 

 a current is to be sent through a nerve, their use is of still greater 

 necessity when we require to examine the currents produced by a 

 tissue, and for this purpose are making use of a sensitive galvano- 

 meter. 



