PRACTICAL PHYSIOLOGY 5 



Other constant batteries which are frequently 

 used in physiology are that of. Grove, where the 

 negative plate is platinum and is plunged into 

 strong nitric acid, separated from the sulphuric 

 acid containing the zinc plate by a porous parti- 

 tion ; that of Bunsen, which is similar to Grove's, 

 but with a negative plate of carbon ; that of Le- 

 clanche (Fig. 4), in which the acid is replaced 

 by chloride of ammonium and the place of the 

 negative plate is also taken by carbon, which is 

 surrounded by manganese dioxide ; and that of 

 Grenet, where carbon again forms the negative 

 plate, but where a single fluid is used (bichromate of 

 potassium dissolved in dilute sulphuric acid), in 

 which both plates are immersed. The so-called 

 " dry " cells are modified Leclanches. The positive 

 plate in every one of these cells is amalgamated zinc. 



Electrodes. The wires used in physiological 

 experiments must always be insulated, either with 

 gutta-percha or with silk or cotton ; in the latter 

 case the insulation is rendered more effectual by 

 dipping the covered wire into molten paraffin. 

 For experimental purposes it is usual to place the 

 ends of the wires (which must be clean and free 

 from the insulating material) in some sort of holder, 

 so that they can be more readily applied to the 

 tissue which is to be investigated ; these ends are 

 then termed the electrodes. 1 - They are often made 



1 The term electrode means literally the ' ' path " of the electric cur- 

 rent, and in this sense the wires throughout are electrodes. But it 

 has come to mean technically the ends of the wires which are used 

 to apply the electric current to a given object (such as an animal 

 tissue). 



