MUSCLE 



621 



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 ex- 

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

 piece of rubber-tubing with a glass T-tube as in Fig. 211. 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 



FIG. 210. CAPILLARY ELECTROMETER (AFTER FREY), AS ARRANGED FOR MOUNT 

 ING ON THE MICROSCOPE STAGE. 



The electrometer consists (i) of a small table carrying a parallel -sided glass 

 vessel containing 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 microscope. (3) A pressure- vessel, and a manometer 

 connected with it for measuring the pressure. (4) Two binding-screws con- 

 nected 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 preventing any difference of electro-motive force 

 between two points connected with the screws from affecting the electrometer. 



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 

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

 meniscus. 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. 



