540 A MANUAL OF PHYSIOLOGY 



capillary in contact with the meniscus of the mercury. A platinum 

 wire fused through 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 

 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 suffi- 

 ciently great to cause continuous electrolysis of the acid, that is, so 

 long 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. It is very suitable for detecting 

 and measuring such small differences of potential as occur in animal 

 tissues. 



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 neighbourhood, 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. 155). This consists 

 of two coils, the primary and the secondary, the former having a 

 comparatively small number of turns of fairly thick copper wire, 

 the latter a large number of turns of thin wire. The object of 

 this is that the resistance of the primary, which is connected with 

 one or more voltaic cells, may not cut down the current too much ; 

 while the currents induced in the secondary, having a high electro- 

 motive force, can readily pass through a high resistance, and are 

 directly proportional in intensity to the number of turns of the 

 wire. 



By means of various binding-screws and the electro-magnetic 

 interrupter, or Neefs hammer, snown in the figure and explained 



