PRELIMINARY DATA 



7<>3 



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 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 (p. 423) 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 coun- 

 terbalance the hydrostatic pressure of the mercury. The meniscus there- 

 fore 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 in- 

 stantaneously, and thus an electromotive force is at once established 

 in the opposite direction to that between 

 the points connected with the electro- 

 meter, and equal to it so long as the 

 external electromotive force is not 

 sufficiently great to cause continuous 

 electrolysis of the acid that is, so long 



Fig. 228. A Simple Capillary Electro- 

 meter. 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 microscope, 

 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 capil- 

 lary into the middle of the field. /, stem 

 of glass T-tube passing through a hole in 

 D. L, rubber tube connecting the capil- 

 lary 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. C, magnified 



portion of the capillary tube showing ' *-^ -r 



the meniscus. % c, <^ / 



*' *^t Hn 



as it is below about 2 volts. The external current is therefore at once 

 compensated, and after the first moment no current passes through the 

 instrument, which is accordingly not a measurer of current, but of 

 electromotive force. 



Induced Currents. When a coil of wire in which a current is flowing 

 is brought up suddenly to another coil, a momentary current is developed 

 in the stationary coil in the opposite direction to that in the moving 

 coil. Similarly, if instead of one of the coils being moved a current is 

 sent through it, while the other coil remains at rest in its neighbour- 

 hood, a transient oppositely-directed current is set up in the latter. 

 When the current in the first coil is broken, a current in the same 

 direction is induced in the other coil. 



Du Bois-Reymond's Sledge Inductorium (Fig. 229). This consists of 

 two coils, the primary and the secondary, the former having a com- 



