PRACTICAL EXERCISES 



739 



4. Demarcation Current and Current of Action with Capillary 

 Electrometer. (a) Study the construction of the capillary electro- 

 meter (Fig. 210, p. 621). Raise the glass reservoir by the rack and 

 pinion screw, so as to bring the meniscus of the mercury into the 

 field. Place two moistened fingers on the binding-screws of the 

 electrometer, open the small key connecting them, and notice that 

 the mercury moves, a difference of potential between the two binding- 

 screws being caused by the moistened fingers. 



(b) Demarcation\Current. Set up a pair of unpolarizable elec- 

 trodes (Fig. 213, p. 1 625). Fill the glass tubes about one-third lull 

 of kaolin mixed with physiological salt solution till it can be easily 

 moulded. To do this, make a piece of the clay into a little roll, which 

 will slip down the tube. Then with a match push it down until it 

 forms a firm plug. Next put some saturated zinc sulphate solution 

 in the tubes, above 

 the clay, with a 

 fine - pointed pi- 

 pette. Fasten the 

 tubes in the holder 

 fixed in the moist 

 chamber (Fig. 294). 

 Now amalgamate 

 the small pieces of 

 zinc wire (p. 182), 

 which ~ are to be 

 connected with the 

 binding-screws of 

 the chamber. (Or 

 use Porter's ' boot ' 

 electrodes. These 

 are made of un- 

 glazed potter's 

 clay. In use the 

 leg of the boot is 

 half -filled with 

 saturated zinc sul- 

 phate solution, in- 

 to which dips a 

 thick amalga- 

 mated zinc wire. 

 In the foot of the 

 boot is a hollow 



(or well) which is filled with physiological salt solution and serves 

 to keep the feet well moistened with the salt solution. The nerve 

 is laid on the feet of the boots. When not in use the boots should be 

 kept in physiological salt solution.) 



The zincs are now placed in the tubes, dipping into the zinc sulphate. 

 A piece of clay or blotting-paper moistened with physiological salt 

 solution is laid across the electrodes to complete the circuit between 

 their points, and they are connected with the electrometer to test 

 whether they have been properly set up. There ought to be little, 

 if any, movement of the mercury on opening the side-key of the 

 electrometer. If the movement is large, the electrodes are ' polar- 

 ized,' and must be set up again. The second pair of binding-screws 

 in the chamber are connected with a pair of platinum-pointed 

 electrodes on the one side, and on the other, through a short-cir- 



472 



FIG. 294. MOIST CHAMBER. 



E, unpolarizable electrodes supported in the cork C ; 

 M, muscle stretched over the electrodes and^kept in 

 position by the pins A, B, stuck in the cork plate P ; 

 B, binding-screws connected with galvanometer or 

 capillary electrometer. The other pair of binding- 

 screws serves to connect a pair of stimulating electrodes 

 inside the chamber with the secondary coil of an induc- 

 tion machine. 



