ELECTRICAL APPARATUS IN USE IN PHYSIOLOGICAL WORK 7 



by rubbing its surface with mercury after it has been cleaned by 

 dipping into dilute sulphuric acid. 



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 

 partition ; that of Bunsen, which is similar to Grove's, but with a negative plate 

 of carbon ; that of Leclanche (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 usually termed the 

 electrodes. 1 They are often made of platinum set in a 



vulcanite holder ; but a FIG. 5. PIN- 

 r ,-, ,2 ELECTRODE. 



.-^'. : .:^:--y;^i P air * P ms with nne 



^* wires soldered to their heads, which 



FIG. SAMPLE CORK ELECTRODE-HOLDER, can on occasion be passed through 



a small cork, with their points pro- 



jecting for a few millimeters, constitute a readily improvised and 

 efficient pair of electrodes for most class purposes. 



To determine which of the two electrodes in any case is the anode and which 

 the kathode, they may be placed in contact with a piece of blotting-paper moist- 

 ened with starch solution containing iodide of potassium (pole-testing paper). 

 Iodine is set free at the anode and turns the starch blue. Feeble differences of 

 electrical potential are determined and estimated by other methods (galvano- 

 meter, electrometer), which will be studied later. 



Non-polarisable electrodes. Like the plates of the battery 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 and to employ electrodes 



1 The term electrode means literally the " path " of the electric current, 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). 



