PRACTICAL PHYSIOLOGY 



of platinum set in a vulcanite holder; but a pair 

 of pins with fine wires soldered to their heads, 

 which can on occasion be passed through a small 

 cork, with their points projecting for a few milli- 

 metres, constitute a readily improvised and efficient 



FIG. 5. PAIR OF ELECTRODES IN CORK OR VULCANITE HOLDER. 



pair of electrodes for most class purposes. Like 

 the plates of the battery itself, such metallic elec- 

 trodes 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 ex- 

 periments it is necessary to obviate this 

 polarisation of electrodes and to employ 

 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 modelling clay moistened 

 with salt solution, filling the tube with 

 saturated solution of zinc sulphate, and 



. . A 



plunging an amalgamated zinc rod (to 

 which one of the wires of the circuit is soldered 

 or otherwise attached) in the zinc sulphate. 



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 



