METHODS OF ELECTRICAL STIMULATION 39 



upper end of the tube is joined to a cylinder in which 

 a piston is moved by a screw, thus making pressure 

 on the mercury column. The end of the capillary dips 

 in a reservoir containing twenty per cent sulphuric 

 acid. Platinum wires lead from the acid reservoir 

 and the mercury in the capillary to convenient bind- 

 ing posts. The platinum wire should never touch 

 the acid, but should be protected by a covering of 

 mercury. When mercury is placed in the vertical 

 tube it enters the capillary until the weight of the 

 column of mercury is balanced by the surface tension, 

 which is inversely proportional to the diameter of the 

 tube. If the capillary be now dipped in the reservoir 

 containing the sulphuric acid, and the piston driven 

 upward by its screw, mercury will be forced out of the 

 capillary into the acid ; and 011 lowering the pressure 

 the mercury will retreat within the capillary, drawing 

 the acid after it. Numerous advantages are presented 

 by this form of electrometer. It tits the stage of the 

 microscope. The microscope need not be tilted very 

 far, and the observer is therefore in a comfortable 

 position. The position of the electrometer on the 

 stage may readily be changed. All the parts near the 

 acid are of hard rubber, thus excluding currents that 

 might arise from acid touching metal parts. The 

 acid tube is flanged so that the acid cannot creep 

 out along the capillary tube. The capillary can 

 easily be brought against the wall of the acid tube. 

 The tube from which the capillary springs descends 

 within the acid tube, thus protecting the capillary 



