7 o2 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



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

 trometer has been much employed in physiology A convenient form 

 of it is shown in Fig. 227. A simple form, suitable for students work- 

 ing 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 mercury. 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. 228. 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 m it. When 



Fig. 227. Capillary Electro- 

 meter (after Frey), as arranged 

 for mounting on the Microscope 

 Stage. The electrometer consists 



(1) of a small table carrying a 

 parallel-sided glass vessel contain- 

 ing mercury and sulphuric acid. 



(2) The capillary tube, which can 

 be moved in two directions at 

 right angles to each other, and so 

 adjusted in the field of the micro- 

 scope. (3) A pressure-vessel, and 

 a manometer connected with it 

 for measuring the pressure. 

 (4) Two binding-screws connected 



by wires to the mercury in the 

 capillary tube and in the parallel^ 

 sided vessel. The binding-screws 

 can be short-circuited by closing 

 the friction -key shown at the 

 right side of the figure, thus pre- 

 venting any difference of elec- 

 tromotive force between two 

 points connected with the screws 

 from affecting the electrometer. 



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 sul- 

 phuric 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 



* 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. 



