CAPILLARY ELECTROMETER 405 



platinum wire sealed through the bottom of the tube. This platinum wire 

 and the one coming from the mercury in the capillary are short circuited 

 through a spring key. The whole instrument is placed on a microscope 

 stage set vertically. 



Details, (a) Mercury. Pure dry mercury must be used. To clean 

 mercury : shake for 10-20 minutes with a solution of mercurous nitrate 

 acidified with nitric acid. Wash thoroughly with distilled water and dry 

 with filter paper. 



(b) Sulphuric acid. The pure (boiled) acid in six times its volume of 

 distilled water is skaken up with a little pure mercury and is best kept, in 

 contact with some mercury. 



(c) The glass parts must be free from grease and the rubber connections 

 from French chalk. 



Filling and Setting' the Electrometer. Fill the pump-reservoir with 

 mercury, allowing free access to the capillary. Before inserting the plun^r. 

 cover the mercury surface with a film of thin oil (balance oil). The 

 insertion of the plunger will cause mercury to be forced through the 

 capillary. Fix the capillary in position in the test tube, which should be 

 half full of acid. A slight turn of the plunger screw will force a little 

 mercury into the test tube to cover the platinum contact. Adjust by means 

 of the plunger screw till the Hg-H 2 S0 4 interface lies in the middle of the 

 microscopic field. The definition of the mercury meniscus may be im- 

 proved by cementing a cover glass to the test tube with Canada balsam. 

 An eyepiece micrometer provides a scale whereby the movements of the 

 mercury may be measured. (1) Connect the terminals by non-polarisable 

 electrodes, one to the cut and the other to the uncut surface of a muscle. 

 Depress the short-circuiting key and so bring the electrometer into circuit 

 with the muscle E.M.F. Note the movement of the mercury. (2) Similarly 

 attempt to demonstrate the diphasic electrical variation in the beating 

 heart (frog). (3) Take off leads (salt solution) from the human heart (arms, 

 see p. 296). (4) If the electrometer is used to find the point of balanced 

 E.M.F. on a Wheatstone bridge (Expt. 39), it is essential to arrange that 

 the direction of the positive current is such that the capillary mercury is 

 the cathode, otherwise mercurous sulphate might be formed in the 

 capillary tube on account of the passage of a rather large current. For the 

 same reason, the short-circuiting key should only be opened for as brief periods 

 as possible. 



Fine instruments may be purchased filled, adjusted and sealed ready 

 for use. They are of a somewhat different type, and while of value for fine 

 measurements in the hands of a careful worker, are unsuited for class 

 work. 



The current of injury, etc., of muscle (pp. 134 and 143) is usually measured 

 by compensation. A cell of constant known E.M.F. is put in the same circuit 

 as the tissue giving rise to the current but sending its current in the opposite 

 direction (Fig. 78 and Expt. 45). By moving the jockey along the slide 

 wire (RfRz) till the E.M.F. from the cell exactly balances the demarcation 

 current, i.e. till the meniscus at the mercury-acid interface becomes steady, 

 one may determine what relationship the constant E.M.F. bears to the 

 muscle E.M.F. Non-polarisable electrodes must be used. 



19. Preparation of Membranes. I. Collodion membranes are easily 

 made in any size or shape, and, as they are transparent, arc very convenient 



