ELECTRICAL PHENOMENA. 103 



the polarization of metallic electrodes. Whenever a metal conductor and a 

 liquid conductor come into contact there is apt to be polarization. What 

 takes place may be represented by the following diagram, in which a current 

 "(s supposed to be passing 



^ 



+ + + + 



Na Na Na Na 



CI CI CI CI 



-^ 



between the poles A and C tlirough a solution of sodium chlorid. During 

 the passage of the current the cations, Na, with their positive charges 

 move toward the cathode; at the cathode the free sodium ion acts upon 

 the water, HHO, forming NaOH and hberating hydrogen, which accumu- 

 lates upon the cathode in the form of gas. The anions, CI, with their negative 

 charges move toward the anode; there the chlorin acts upon the water, form- 

 ing HCl and liberating oxygen. In consequence of this accumulation of 

 gases upon the poles a gas battery is formed, in which the direction of current 

 is against that of the main current, that is, from C to A. It is obvious that 

 in quantitative studies of the electrical currents of animal tissues polari- 

 zation will destroy the accuracy of the results; the demarcation current 

 will show a diminution due not to changes in the nerve, but to physico- 

 chemical changes at the leading-off electrodes. To prevent polarization 

 du Bois-Reymond devised the non-polarizable electrodes consisting of zinc 

 terminals immersed in zinc sulphate. Theoretically any metal in a solution 

 of one of its salts may be used, but experience shows that the zinc-zinc sulphate 

 electrode is most nearly perfect. Each electrode where it comes into contact 

 with the tissue is made of one of these combinations. Various devices have 

 been used. For instance, the electrode may be constructed as shown in the 

 diagram (Fig. 45). A short glass tube of a bore of about 4 mms. is well 

 cleaned — one end, which is to come into contact with the nerve — is filled, as 

 shown, by a plug of kaolin made into a stiff putty with physiological sahne 

 solution of NaCl (0.7 per cent.). The kaolin should have a neutral reaction 

 and unless good kaolin is obtainable it is better to use a plug made of clean 

 filter paper macerated in physiological saline and packed tightly into the end 

 of the tube. Above this plug the tube is filled in for a part of its length with 

 a saturated solution of zinc sulphate into which is immersed a bar of amal- 

 gamated zinc with a copper wire soldered to its end. A more convenient form 

 of electrode is the so-called "boot electrode,"* made of baked clay. The in- 

 terior of the boot is partly filled with the solution of zinc sulphate and the 

 amalgamated zinc rod is dipped into this solution. With a pair of such 

 electrodes the conduction of the current through the nerve or muscle to the 

 metallic part of the circuit may be represented as follows: 



^ 



+ + + + + + + I 



Zn Zu Na Na Na Zu Zn 



Zn 



SO, so, CI CI CI so, so, 2n 



The liquid part of the circuit comes into contact with the metallic part 

 at the junction of Zn and ZuSO,. At the cathode it may be supposed that 

 the Zn cation instead of acting upon the water and liberating hydrogen, 

 deposits itself upon the zinc electrode; at the anode the sulphion (SO,) 

 attacks the zinc instead of the water, forming ZnSO,. In this way polarization 

 is prevented, and by the construction of the electrode the living tissue la 

 brought into contact only with the plug of kaolin moistened with physio- 

 logical saline. Such electrodes are indispensable in studying the electrical phe- 

 nomena of living tissues, and also in all investigations bearing upon the polar 

 * Sold by the Harvard Apparatus Company, Boston. 



