622 



A MANUAL OF PHYSIOLOGY 



the tube, or simply inserted through its upper end, dips into the 

 mercury. Another, passing through the cork, or, better, fused 

 through the bottom of the bottle, makes contact with the sulphuric 

 acid through some mercury. The bottle is fastened on the stage 

 of a microscope, the capillary brought into focus, and the 

 meniscus adjusted by raising or lowering the reservoir. When the 

 platinum wires are connected with points at different potential, a 

 current begins to pass through the instrument, and the meniscus of 

 the mercury in the capillary tube, where the current density is the 

 greatest, becomes polarized by the ions separated from the sulphuric 



B, bottle containing sulphuric acid ; 

 Hg, mercury ; E, E', platinum wires. 

 E dips into the mercury in the vertical 

 tube, and E' is fused through the 

 bottom of B, so as to make contact with 

 the mercury in B, the other end of it 

 passing out through a small hole in the 

 wooden platform F, on which B rests. 

 F is fastened to the stage of the micro- 

 scope, S, by a pin, G, passing through 

 one of the clip-holes, and to the wooden 

 upright, D, by the pin, H. D fits tightly 

 over the microscope stage, but can be 

 moved laterally a little so as to bring 

 the capillary into the middle of the field. 

 /, stem of glass T-tube passing through 

 a hole in D. L, rubber tube connecting 

 the capillary point with the vertical 

 portion of the T-tube. A is a reservoir 

 containing mercury connected by the 

 rubber tube M to /. A can be raised or 

 lowered by sliding it in the clips K. 

 In the figure the capillary tube appears 

 as if the mercury extended to the very 

 point of it. This should not be the 

 case ; the sulphuric acid should rise for 

 some distance in the capillary, so that 

 the mercury shows a finely-bounded 

 meniscus in the tube as is represented 

 in C, the magnified image of the capillary 

 as seen with the microscope. 



FIG. 211. A SIMPLE CAPILLARY 

 ELECTROMETER. 



acid at the surface of contact between the acid and the mercury, so 

 that the meniscus is no longer in equilibrium in the tube. The 

 surface tension is diminished when the direction of the current is 

 from mercury to acid (mercury at a higher potential than acid), and 

 is no longer able to counterbalance the hydrostatic pressure of the 

 mercury. The meniscus therefore moves down in the tube. With 

 the opposite direction of current (mercury at a lower potential than 

 acid) the surface tension is increased, and the meniscus moves up. 

 The polarization develops itself almost instantaneously, and thus an 

 electromotive force is at once established in the opposite direction 

 to that between the points connected with the electrometer, and 

 equal to it so long as the external electromotive force is not sum- 



