154 



EXPERIMENTAL PHYSIOLOGY 



THE CAPILLARY ELECTROMETER 



The capillary electrometer consists of a piece of glass tubing with its end 

 drawn out to a fine capillary of an internal diameter of about 30 ft. This is 

 filled with mercury and then immersed in diluted sulphuric acid (1 part to 

 6 of water) contained in a second tube, in the bottom of which is some 

 mercury which serves by a platinum wire fused in the glass to make 

 contact with the acid. The mercury in the upper tube is connected by a 



Fig. 121 Lippmann's Capillary Electrometer. 



1. Pressure apparatus ami microscope, on the stand of which the capillary tube is fixsd. 



2. Capillary tube dipping into ILSO^ in a surrounding tube, and in connection with pressure 

 apparatus (the mercury in the lower part of the surrounding tube serves only to establish connection 

 with the platinum wire). 



3. The capillary tube and column of mercury as seen in the field of the microscope. (Scale 

 in Tooth mm.) (Waller.) 



second platinum wire which forms the other electrode. The changes of 

 position of the mercury surface in the capillary are watched under a micro- 

 scope. The upper tube is connected by stout rubber tubing to the lower of 

 two reservoirs containing mercury, by altering the relative heights of which 

 pressure can be exerted upon the surface of the mercury. With a constant 

 pressure the mercury in the capillary is brought to a certain position in 

 which the capillarity is exactly balanced by the pressure. If a current be 



