)20 



ILL USTRA TI VE EXPERIMENTS 



mercury 

 acidified 



witli 



21. The Capillary Electrometer. 



This iustruinent tor measuring differences of electric potential depends for 

 its action upon the alteration of surface tension between mercury and 

 sulphuric acid with alterations of the potential diiTerence at the interface 

 (see p. 50, and Experiments 16 and 17). For class use the simplest satis- 

 factory form is that made by the Harvard Apparatus Co. It consists of 

 a capillary tube containing mercury which is continuous with a reservoir in 

 the form of a plunger pump. The position of the mercury in the capillary 

 may be altered by adjusting the plunger by means of a micro-screw. The 

 glass capillary dips into a small test tube containins; dilute sulphuric acid 

 and a drop of mercury to make good contact with the 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 

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

 nitric acid. Wash thoroughly with distilled water and dry 

 with filter paper. 



(If) Sulphuric acid. The pure (boiled) acid in six times 

 its volume of distilled water is shaken i;p 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. 



Filli)ifi and Settinr/ the Electrometer. Fill the pump-reservoir 

 with mercury, allowing free access to the capillary. Before 

 inserting the plunger, 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 niercury into the test tube to cover the platinum 

 contact. Adjust by means of the plunger screw till the 

 Hg-H.SOj interface lies in the middle of the microscopic 

 field. The definition of the mercury meniscus may be improved 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. 



For finer work, the closed type of electrometer (Fig. 106) gives excellent 

 results, and, once adjusted, needs no further attention for months. 



Before this piece of apparatus is set u]), tilt the tube until the greater ])art 

 of the mercury has run over from B to T (Fig. 12). Set the tube with the bulb 

 limb {B) sloping slightly downwards and allow mercury to pass along f7and A 

 from T and to drip into B until, on suddenly righting the tube, the mercury 

 stands midway in A as in the figure. Fix in the stand and focus the micro- 

 scope on the meniscus. Affix the leads E and F as in the diagram (Fig. 12), 

 taking care to see that the short-circuiting key is across them and is kept 

 closed. 



After any capillary electrometer has been readjusted so that a new surface 

 has been formed at the meniscus, it is essential to allow this surface to come 

 into electrical equilibrium with the glass and the sulphuric acid. (See Ageing 

 of Surfa,ces.) For rough work a few hours' rest is enough, but about 24 hours 

 is a preferable interval. Test the sensitivity of the electrometer by opening 

 the short-circuiting key and noting the movement, if any, of the meniscus. 



Fig. 106.— Capillary 

 electrometer. 



