a 



EXPERIMENTAL PHYSIOLOGY 



determined and estimated by other methods (galvanometer, electro- 

 meter), which will be studied later. 



Non-polarisable electrodes. Like the plates of the cell itself, 

 metallic electrodes are capable of becoming polarised when they are 

 in contact with the moist tissues and a current is passed continuously 

 between them in one direction. For some experiments it is necessary 

 to obviate this polarisation of electrodes by employing electrodes 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 : an amalgamated zinc rod (to which one of 



Zinc wire. 

 Cork. 



Saturated solution 

 of zinc sulphate. 



Clay plug moistened 

 with salt solution. 



FIG. 7. Non-polarisable 

 electrode. 



Clay plug in 

 small de,ach 

 able glass mbe. 



Zinc sulphati 

 solution. 



FIG. 8. Sanderson's pattern of non- 

 polarisable electrode. 



the wires of the circuit is soldered or otherwise attached) 

 plunged into the zinc sulphate. 



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

 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 soakec 

 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 divertin 

 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 

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

 through a pool of mercury (mercury key Figs. 9 and 10) ; or by con- 

 tact between a platinum plate and platinum point (contact key Fig. 



