ELECTRICAL PHENOMENA. 



95 



wire fused into its wall and dipping into the mercury By regulating the 

 pressure on the mercury the point of contact between the threacf of mercury 

 and the sulphuric acid in the capillary, d, can be brought to any desired posi- 

 tion. An equilibrium is then established which will remain constant as long 

 as the conditions are not changed. If now the circuit from a battery or 

 other source of electricity for example, the excised nerve or muscle is closed, 

 the current entering by wire g, if this represents the anode, traverses the sul- 

 phuric acid and mercury in the capillary and returns by the wire h. At the 

 moment of the establishment of the current the equilibrium of forces that holds 

 the mercury at a certain point in the capillary is disturbed, the end of the mer- 

 cury thread moves upward with the current for a certain distance, depending 

 on the strength of the current and the delicacy of the capillary. If the cur- 

 rent be passed in the opposite direction the mercury will move downward a 

 certain distance. The meniscus of contact moves up 

 or down with the direction of the current, owing, it is 

 supposed, to a change in the surface tension at this 

 point. The capillary tube as used for physiological 

 purposes is too small for the movements of the mer- 

 cury to be detected with the eye. It is necessary to 

 magnify it either with a microscope or a projection 

 lantern. Ordinarily the electrometer is so made that it 

 can be placed upon the stage of the microscope and 

 the capillary be brought into focus at the meniscus, 

 as shown in d, Fig. 40. By means of proper apparatus 

 the movement can be photographed and thus a per- 

 manent record be obtained of the direction and extent 

 of movement of the mercury. 



N on-polarizable Electrodes. In connecting a 

 muscle or nerve to an electrometer or galvanometer 

 it is necessary that the leading off electrodes that is, 

 the points of contact between the wires and the 

 muscle or nerve shall be iso-electrical and non-polar- 

 izable. By iso-electrical is meant that the two elec- 

 trodes shall have the same electrical potential, and it 

 is obvious that the leading off electrodes must fulfill 

 this condition approximately at least, since otherwise 

 the current obtained from the muscle or nerve could 

 not be attributed to differences in potential in the 

 tissue itself; it would be shown by any other moist 

 conductor connecting the two electrodes. Two clean 

 platinum electrodes would fulfill this condition. A 

 more serious difficulty is found in the polarization of 

 metallic electrodes. * Whenever a metal conductor 

 and a liquid conductor come into contact there is apt 

 to be polarization. This polarization expresses itself 

 by changes at the metal poles during the passage of 

 the current, changes of such a character that a cur- 

 rent is set up between the poles in the opposite di- 

 rection to the main current, thus weakening the lat- 

 ter. This polarization current is due to the accumulation of hydrogen gas 

 at the negative pole or cathode and of oxygen at the anode. What takes 

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

 supposed to be passing 



Na Na Na Na 

 Cl Cl Cl Cl 



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

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



Fig. 41. To show 

 the structure of a non- 

 polarizable electrode: 

 1, The pad of kaolin or 

 filter paper moistened 



with physiological sa- 

 (NaCl, 0.7 per 

 cent.) (this is placed on 



line 



the tissue) ; 2, the sat- 

 urated solution of zinc 

 sulphate; (3) the bar 

 of amalgamated zinc. 



