8 EXPERIMENTAL PHYSIOLOGY 



which are not polarisable. These are usually made by taking two 

 small pieces of glass tubing open at both ends, either straight (fig. 7) 

 or curved (fig. 8), and having plugged one end of such tube with 

 china clay made into a paste with salt solution, the tube is filled 

 with saturated solution of zinc sulphate, and an amalgamated zinc 



Zinc wire. 

 Cork. 



Saturated solution 

 of zinc sulphate. 



Clay plug, moistened 



with salt solution. 



FIO. 7. NON-POLABISABLE 

 ELECTRODE. 



- Clay plug in 

 small glass 

 tube. 



Zinc sulphate 

 solution. 



PIG. 8. SANDERSON'S PATTERN OP NON- 

 POLARISABLE ELECTRODE. 



rod (to which one of the wires of the circuit is soldered or otherwise 

 attached) is plunged into the zinc sulphate. 



The rod is amalgamated by dipping it for a few seconds into a solution of 

 mercury in nitric acid, washing under a tap, and polishing with cotton- wool. 



A convenient form of non-polarisable electrode is that of Porter, 

 who uses a boot-shaped tube of unglazed porcelain which is soaked 

 with normal saline and filled with saturated solution of zinc sulphate ; 

 the amalgamated zinc rod is passed into the leg of the boot. 



Keys. Any apparatus which is used for interrupting or diverting 

 the course of a current is called a key or switch. The keys used in 

 physiological experiments are arranged to close and open a circuit 



FIG. 9. DIAGRAM OP MERCURY KEY. 



(make and break the current) by connecting two wires together either 

 through a pool of mercury (mercury key figs. 9 and 10) ; or by contact 

 between a platinum plate and platinum point (contact key fig. 11), 



