6 5 2 



A MANUAL OF PHYSIOLOGY 



5. Demarcation Current and Current of Action with Capillary 

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

 meter (Fig. 153, p. 538). 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. 156, p. 542). Fill the glass tubes about one-third full of 

 kaolin mixed with normal saline 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 



FIG. 221. MOIST CHAMBER. 



E, unpolarizible 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 induction machine. 



firm plug. Next put some saturated zinc sulphate solution in the 

 tubes, above the clay, with a fine-pointed pipette. Fasten the tubes 

 in the holder fixed in the moist chamber (Fig. 221). Now amal- 

 gamate the small pieces of zinc wire (p. 173), which are to be con- 

 nected with the binding-screws of the chamber. 



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

 sulphate. A piece of clay or blotting paper moistened with normal 

 saline 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 electro- 

 meter. If the movement is large, the electrodes are polarized,' 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 



