ELECTRICAL PHENOMENA. 



95 



instrument known as the capillary electrometer is employed. The principle 

 of the construction of this instrument is illustrated in Fig. 40. A glass tube, 

 a, is drawn out at one end into a very fine capillary, the end of which dips 

 into some diluted sulphuric acid contained in the vessel (/) . At the bottom 

 of this vessel is a layer of mercury connecting with a wire, g, fused into the 

 glass vessel. The tube a is partially filled with redistilled mercury, which 

 penetrates for a short distance into the capillary. By means of pressure ap- 

 plied from above c, the mercury can be forced through the capillary. Then 

 by diminishing the pressure the mercury can be brought back into the capil- 

 lary a certain distance, drawing after it some of the dilute sulphuric acid. 

 The mercury in tube a is connected with the other pole of the battery by a 

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

 pressure on the mercury the point o contact between the threacT 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 cer- 

 tain distance, depending on the strength of the current 

 and the delicacy of the capillary. If the current 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 physiologi- 

 cal 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. 



Non-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. What takes place may be represented by the following 

 diagram, in which a current is supposed to be passing 



-2 



Fig. 41. To show 

 the structure of a non- 

 polarizable electrode : 

 1, The pad of kaolin or 

 filter paper moistened 

 with physiological sa- 

 line (NaCl, 0.7 per 

 cent.) (this is placed on 

 the tissue); 2, the sat- 

 urated solution of zinc 

 sulphate; (3) the. bar 

 of amalgamated zinc. 



