150 J. H. Pratt — Capillary Electrometer. 



meniscus. The deflections for equal successive intervals of 

 time were thus obtained. Series were made for various E. M. F. 

 from 0*2 D. to 1*0 D. with accordant results. The table and 

 curve IY (fig. 4) will show the results when a current of 6 Dan. 

 was used. With a large resistance in the circuit, the mercury 

 never reaches the point to which a current of the same poten- 

 tial with small resistance would bring it. The deflections of 0'6 

 Dan. without large resistance is 81 mm . That found after thirty 

 minutes passage of the current with resistance was 78 mm . 



Table IV. -May, 1887. 

 Deflect. Time. Deflect. Time. Deflect. Time. Deflect. Time. 



24-8 mm 1 min. 66-2 mm 5 min. 75-2 mm 9 min. 76-8 mm 13 min. 

 43-0 2 69'8 6 76"2 10 



54-0 3 72-2 7 76-5 11 



61-0 4 74-0 8 76-8 12 



Since the movement of the mercury is slow we may assume 



that, during the excursion of the mercury, the E. M F. at the 



surface of polarization varies according to the curve of the 



potentials, and that at any instant the position of the meniscus 



determines the E. M. F. We have, thus, a means of computing 



the capacity of the instrument for a given potential. By 



E Et 



Ohm's law 1 = ^. Also C (capacity) = — . By reference to 



li. XV 



curve IY, it will be seen that a portion of the curve between 

 any two points taken sufficiently near to each other will differ 

 but slightly from a straight line ; hence the mean of the E. M. 

 Forces at the beginning and end of any short interval of time 

 will give the mean potential of that interval. If E 1 equals E. 

 M. F. at the beginning of the first interval of time ; E 2 that at 

 the beginning of the second, etc., the capacity is represented 

 by the equation 



c =k{~^ +E ' +E -+ +K -) 



R is the inserted resistance plus the resistance of the electro- 

 meter. The latter was found to be, approximately, 10,000 

 ohms ; so that R equals 250,000 ohms. The interval of time (t) 

 was 60 seconds Computation gives for potential - 2 Dan., C 

 equals 314 mfds.; 04 Dan., C equals 445 mfds.; 0'6 D., C equals 

 605 mfds.; 0'7 D., C equals 648 mfds.; 1-0 D., C equals 730 

 mfds. The values for the capacity are approximate and will 

 apply only to the instrument in question. In this the radius 

 equals 0*64 mm about, and hence the surface of the mercury, re- 

 garded as hemispherical equals about 2*572 sq. mm. The radius 

 of the tube v was found by measuring the length of a column of 

 mercury (for the hemispherical shape of whose ends due allow- 

 ance was made) in the tube, and weighing the mercury. From 

 these two factors the radius of the tube was computed. 



