UNPOLARISABLE ELECTRODES 



83 



connect it with the electrodes. These do not cause any polarisation, 

 and at the same time are fairly good conductors. They are spoken 

 of as clay -guards. In many cases this form of electrode is too large, 

 and it becomes impossible to bring them into contact with any two 



-Several Models of Unpolaeisable Electrodes 



1 and 2, Dn Bois-Reymond's ; 3, Burdon-Sanderson's ; 4, von Pleischl's ; 5, d'Arsonval's. 



In 1, 2, 3, and 4 the component parts are zinc, zinc sulphate, and saline clay ; in 5 a silver rod 

 coated with fused silver chloride dipping in normal saline contained in the tube from which a thread 

 projects. (Waller.) 



desired points of the tissue to be experimented upon ; and to over- 

 come this many forms of electrode have been employed by different 

 workers. Some of these are shown in the accompanying figure (73). 



Make a pair of unpolarisable electrodes in the following way. Take a piece 

 of glass tubing of fairly thick walls and with an internal diameter of about 

 \ inch. Rotate this with its centre in a blowpipe flame without draw- 

 ing it out until the central part becomes of a less diameter ; then draw it 

 out slightly and allow to cool. Cut it 

 through the centre of the constricted 

 part and round off these ends in a 

 flame. The bore at this end should 

 now be about ^ inch. Cut the tubes 

 so that they are 2^ inches long and 

 round off the upper ends in a flame. 

 This glass tube is shown at a, fig. 74. 

 Take a cork and bore half through it 

 with a cork-borer of such size that the 

 upper end of the glass tube is held firmly 

 in the hole bored. With a fine bradawl pierce the cork from tbe upper end, and 

 this will remove the lower bored piece of cork if that has not already come away 



g2 



Fig. 74. — Simple Form ok Unpolaris- 

 able Electrodes. 



