Chap, xxn.] THE CAPILLARY ELECTROMETER. 245 



which renders it easy for any one to make a capillary 

 electrometer for himself of sufficient delicacy to in- 

 dicate the muscle current, negative variation, the 

 current from the isolated beating heart of the frog, 

 etc. It is represented in Fig. 115. A small piece of 

 narrow glass tubing is taken, and drawn into a fine 

 capillary in the middle. Each end is bent up. Some 

 clean mercury is placed in a glass and covered by 

 dilute sulphuric acid (1 to 20 of water by volume). 

 One end of the tube is dipped under the surface of 

 the mercury, and suction applied by the mouth at the 

 other end till the mercury appears at the wide end 

 next the mouth. By raising the lower end, a little 

 acid is permitted to enter the tube, and then a little 

 more mercury is sucked in. After a little practice 

 one is able so to fill the tube that mercury occupies 

 each end, and a fine thread of mercury passes from 

 each end into the capillary, the centre of which is 

 occupied by acid. In the figure the dark portion in- 

 dicates the mercury, the clear part c in the middle 

 of the capillary is the dilute acid. No air-bubble must 

 be permitted in the tube. The tube should be sup- 

 ported in a frame, which can be laid on the stage of a 

 microscope. A platinum wire dips into the mercury 

 at each end a b of the tube ; and the other end of 

 the wires should be attached to binding screws on the 

 frame. The capillary is easily made fine enough to 

 be viewed by a lens magnifying 300 to 500 diameters. 

 To put the electrometer in circuit with the non- 

 polarisable troughs, all that is necessary is to connect 

 the binding screws of the frame to the troughs by 

 wires, one screw to one trough, a key being interposed 

 in the circuit. After placing a muscle on the troughs 



J. O C 1 



in the usual way (see page 117), on looking down the 

 microscope, and then closing the key, the movement 

 of the mercury will be seen. To obtain a very sensi- 

 tive instrument, clean mercury, clean glass tubing, and 



