ELECTRICAL APPARATUS IN USE IN PHYSIOLOGICAL WORK 7 



"amalgamated " by rubbing its surface with mercury after it has been 

 cleaned by dipping into dilute sulphuric acid. 



Other constant cells are Grove's, where the positive plate is platinum 

 and is plunged into strong nitric acid, separated from the sulphuric acid 

 containing the zinc plate by a porous partition ; Bunsen's, which is 

 similar to Grove's, but with a positive plate of carbon ; Leclanche's 

 (Fig. 4), in which the acid is replaced by chloride of ammonium and 

 the place of the positive plate is taken by carbon, which is surrounded 

 by manganese dioxide; and Grenet's, where carbon again forms the 

 positive plate, but with a single fluid (bichromate of potassium dis- 

 solved in dilute sulphuric acid), in which both plates are immersed. 

 The so-called " dry " cells are modified Leclanches. The 

 negative plate in every one of these cells is zinc. 1 



Electrodes. The wires used in physiological experi- 

 ments must always be insulated with gutta-percha or 

 rubber, or with silk or cotton ; in the latter case the in- 

 sulation 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 F I G - 5. Pin- 

 tissue which is to be investigated ; these ends are usually 



termed the electrodes.' 2 ' They are often 

 . J ,,. ; .. J .,^., made of platinum set in a vulcanite 



'-':-- ?+*'Si.'::&r holder ; but a pair of pins with fine 



wires soldered to their heads, constitutes 

 FIG. 6. Sim^plecork electrode- ft readily i mprov ised and efficient pair 



of electrodes for most class purposes. 



uch pin-electrodes can be passed through a small cork, or joined in 

 parallel fashion with Chatterton's cement, with their cleaned points 

 projecting for a few millimetres. Such an arrangement enables them 

 t) be conveniently handled. 



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

 aid which the kathode, they may be placed in contact with a piece of 

 Hotting-paper moistened with starch solution containing iodide of 

 ptassium (pole-testing paper). Iodine is set free at the anode and 

 turns the starch blue. Feeble differences of electrical potential are 



1 The accumulator or storage cell, with plates of lead and lead covered by lead 

 leroxide, is also often used in physiological work. 



2 The term electrode means literally the " path" of the electric current, and 

 n 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 

 \ given object (such as an animal tissue). 



