. Portable Capillary Electrometer. 403 



polarizing electromotive forces upon the surface-tension be- 

 tween mercury and sulphuric acid solution of density 1*17 : — 

 E.M.F. Volts. Surface-Tension. 



y 



0-0202 1-021 7 



0-0404 1-040 y 



0-0605 1-059 7 



0-0807 1-080 y 



0-1009 1-097 y 



7 is the " natural" surface-tension between the mercury and 

 the solution. Its approximate value is 300 ergs per sq. cm. 

 Judging from these numbers, it would seem that the surface- 

 tension is altered by about one part in 10,000 by a polarizing 

 E.M.F. of -0001 volt. In the case in which the capillary- 

 tube is horizontal and the wide tubes are vertical, the relation 

 between the motion of the meniscus 8a and the corresponding 

 change in surface-tension By is expressed approximately by 



c?p g hx=KcBy 

 if we assume that the capillary and wide tubes are of uniform 

 cross-section — a being the area of cross-section of the capil- 

 lary, a its circumference, A the area of cross-section of the 

 wide tubes, and p the sum of the densities of the mercury 

 and the solution. From this formula it would appear possible 

 to produce a much larger movement of the meniscus than 

 *01 mm. by means of a potential-difference of "0001 volt by 

 using a very narrow capillary, especially if the cross- 

 section of the capillary were elliptical instead of circular. 

 It is found in practice, however, that the motion of the 

 meniscus under minute polarizing forces is controlled very 

 largely by stickiness and accompanying changes in the 

 angle of contact between the liquids and the glass, and 

 by variation in the cross-section of the tube at the place 

 where the meniscus rests. The sensitiveness of the instru- 

 ment to small electromotive forces is practically as great 

 when the diameter of the capillary is 1 mm. as when it is 

 very much less. A very narrow capillary is less easy to 

 manipulate than a comparatively wide one (about 1 mm. in 

 diameter) and is only advantageous when it is desirable that 

 the capacity of the instrument should be as small as possible. 

 or that its action should be as rapid as possible. In the latter 

 case the length of the capillary should also be small. 



If the capillary-tube, instead of being horizontal, is inclined 

 at an angle 6 to the horizontal, the equation given above 

 becomes 



a g{(Pi + P%) a l^ • cos + (P1—P2) sin 9} bx=c By, 

 in which p,. is the density of the mercury, and p a that of the 



