728 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



that is, differences of electric potential. Lippmann's capillary elec- 

 trometer has been much employed in physiology. A simple form, 

 suitable for students working in a class where a considerable number of 

 copies of the instrument is needed, can be conveniently made as follows : 

 A glass tube is drawn out to a capillary at one end and filled with mer- 

 cury. The tube is inserted into a small glass bottle,* and fastened in 

 its neck by a cork or a plug of sealing-wax which does not quite fill the 

 opening, so that the interior of the bottle is still in communication with 

 the external air. The upper end of the tube is connected by a short 

 piece of rubber tubing with a glass T-tube as in Fig. 235. The bottle 

 is partially filled with 5 to 10 per cent, sulphuric acid, under which the 

 capillary dips. By means of a small reservoir made from a piece of 

 glass tubing filled with mercury, and connected with the stem of the 

 T-tube, a little mercury is forced through the capillary so as to expel 

 the air in it. When the pressure is lowered again, sulphuric acid is 

 drawn up, and now lies in the 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 reser- 

 voir. 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 mer- 

 cury, so that the meniscus is no longer in equilibrium in the tube. 

 The surface tension (p. 429) 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 sufficiently 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. 



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- 



* A parallel-sided bottle is best, as it gives the clearest image of the menis- 

 cus. But it is easiest to make a cylindrical bottle from a piece of wide glass 

 tubing, and to insert a platinum wire into it before closing it at the bottom in 

 the blow-pipe flame. The tube can then be firmly fastened with sealing-wax 

 in a depression in a piece of wood, the wire being brought out through a 

 hole in the wood. Once the instrument is arranged, there is little chance 

 of the capillary getting broken, and there is very little evaporation of the 

 acid. 



