144 



J. R. Pratt, Jr. — Capillary Electrometer. 



containing mercury and dilute sulphuric acid ; a and. /? are 

 platinum wires which serve as conductors for the electric cur- 

 rent. They pass through the glass tubes in vessels G and V, 

 and are fused into the tubes at the end ; the platinum elec- 

 trodes, accordingly, have only a small area. The end of A B 

 itself is filled with sulphuric acid to M, the mercury meniscus. 

 S is a leveling screw. The whole is mounted on a wooden base 

 to give it stability. C D is a millimeter scale on which the 

 readings are made. 



For most of the work done the unaided eye, or a small 

 magnifying glass was sufficient to make the readings, — as the 

 differences of deflection for slight differences in E. M. F. were 

 comparatively great. For instance, the difference of deflection 

 between - 02 Daniell and "03 Daniell was, for a given adjust- 

 ment, 2 "4 millimeters ; hence, 'l mm (which can easily be read) 

 showed a difference of about *0004 Daniell. For the experi- 

 ments in quantity of electricity a micrometer was used, by 

 which readings could easily be made to -g Jo" mm with accuracy. 



I. Lippmann states * that the value of the capillary con- 

 stant in his instrument increases for increasing potentials to 

 about -9 Dan. and then diminishes. The same results are ob- 

 tained with the form here given. 







rn 







4 d> 



° R ° 









B 









<*. 













Two Darnell's cells are connected as shown in figure 2. 

 E, is a resistance-box of 10,000 ohms, through which the cur- 

 rent passes. By the wires a. and ft, fractions of the current are 

 made to pass through the electrometer. The accompanying 

 table and plot (No. I, fig. 4) will exhibit the results. Column 

 I gives the fraction of the E. M. F. of a Daniell cell which pro- 

 duces the deflections given in column II. 





Table 



I.— March 1, 1887. 









E. M. F. Deflect. 



E. M. F. 



Deflect. 



E. M. F. 



Deflect. 



E. M. F. 



Deflect 



•008 Dan. l mm 



•16 Dan. 



28*2 mm 



•60 Dan. 



77.4mm 



1-2 Dan. 



80 mm 



•016 2-3 



•20 



34-7 



•80 



88-0 



1-4 



64 



•04 7-2 



•30 



49-5 



•892 



90-3 



1-6 



50-2 



•08 14-5 



•40 



60'7 



1-0 



89-0 



1-8 



42-5 



In taking the deflections for the higher of these potentials 

 1*2 to 1*8, electrolytic action was noticed and bubbles of gas 



* Annales de Chim. et de Phys., Ser. Y, vol. v: E. Mascart, Traite d' Electricite 

 Stat.,ii, p. 550. 



