102 Mr. G. J. Bureh. Contributions to [Sept. 26, 



" Contributions to a Theory of the Capillary Electrometer. IL— 

 On an Improved Form of Instrument." By George J. 

 Burch, M.A. Oxon., F.B.S., Lecturer in Physics, University 

 College, Beading. Beceived September 26, — -Bead November 

 20, 1902. 



During the sixteen years that I have worked with the capillary 

 electrometer I have had occasion to make between 150 and 200 instru- 

 ments, and have therefore given naturally a good deal of thought to 

 the problem of its construction. I have used eleven different forms, 

 three of which are figured in my little book on the ' Capillary Elec- 

 trometer in Theory and Practice,' reprinted from ' The Electrician,' 

 1896. A fourth is in use in the Physiological Laboratory, Oxford, for 

 the research on , nerve in which Professor Gotch and I have been 

 engaged, and is in fact the improved form referred to on page 9 of my 

 book. 



It may be of interest to indicate briefly the points that must be 

 observed in the design of the instrument. 



It must be simple, easy to adjust and to clean, and with reasonable 

 care not liable to be broken. Above all it must be suitable for use 

 with objectives of short focus and wide-angle condensers. For this 

 reason I adopted in my first projection-electrometers the plan of 

 placing the capillary within a piece of thick-walled burette tubing of 

 1 mm. bore, half ground away so as to form a trough of semi-circular 

 section. A piece of thin cover-glass serves as a front to this trough, 

 the lower end of which dips into the dilute sulphuric acid, the liquid 

 rising in it by capillary attraction to a sufficient height above the level 

 of the U-tube to enable the microscope to be focussed on the capillary 

 within. 



In the first instrument of this type the trough was ground to fit 

 the mouth of the U-tube like a stopper, and rested loosely in it. It 

 was found, however, that as the acid loses or absorbs water with the 

 changes of weather, the variations of level in the L T -tube affect the 

 adjustment of the' trough, bringing the capillary sometimes too far 

 from the cover-glass for good definition and sometimes dangerously 

 near it. 



Hence in the later improved type I fused the upper end of the glass 

 trough on to a glass rod which was fixed firmly with adjusting screws 

 to the brass support that held the capillary, the U-tube containing the 

 acicl being independently supported so that the end of the trough 

 dipped into it. The short limb of the U-tube was made wide in order 

 that the trough should not be likely to touch against it in the event 

 of any accidental pressure on the instrument, and also because experi- 



