the Range of the Capillary Electrometer. 69 



Thomson's Balance Capillary 



Cell. Method. electrometer. 



Silver Chloride 1-020 volts. 1*025 volts. 



Dry Cell ] -446 " 1-440 " 



Leclanehe.. 1-453 " 1-458 " 



. Daniell 1-076 " 1-081 " 



The deflection produced by the Daniell cell was 2 - 59 mm while 

 the standard Clark cell moved the mercury column 3*20 mm . 



Thus it is seen, that even with a cathetometer reading only 

 to hundredths of a millimeter and having a telescope of low 

 magnifying power, values correct to one-half of one per cent 

 may be obtained. 



The mercury columns of the electrometer move almost 

 instantaneously to the position of equilibrium, and the exact- 

 ness with which measurements may be made depends chiefly 

 upon the magnifying power of the telescope of the cathetom- 

 eter, and the precision with which its scale is graduated. It 

 is doubtless possible to determine with this electrometer, the 

 electromotive force of a cell correctly to 0-001 of a volt. 



The accuracy with which measurements may be made may 

 be increased somewhat by increasing the number of cells which 

 are used in series. When four cells are employed the part of 

 the curve corresponding to electromotive forces, which are in 

 the neighborhood of 1/5 volts, is very nearly a straight line, 

 and the curve ascends more rapidly than that characteristic of 

 three cells. Hence a change in the deflection produced by a 

 small variation in the electromotive force is more readily ob- 

 served. 



There is another advantage in using more cells than seem to 

 be absolutely necessary. If the electrometer is used repeatedly 

 to measure electromotive forces which are nearly equal to that 

 which produces continuous electrolysis, the curve of the instru- 

 ment is less constant from day to clay, so that if great accuracy 

 is wished for the curve should be redetermined. 



If after long continued use it becomes desirable to refill the 

 instrument, it is easily accomplished ; since ordinarily the 

 mercury need not be replaced, but can be sufficiently cleansed 

 by drawing some acid of- the standard solution through the 

 electrometer by the aid of the aspirator. 



The results of this investigation may be briefly stated as 

 follows: The arrangement of capillary cells in series does not 

 alter essentially their behavior, when they are subjected to an 

 electromotive foice. Hence, in each cell of a series of like 

 cells, the fall of potential is the same. Moreover, when a cur- 

 rent of electricity passes through this apparatus, the displace- 

 ment of the mercury columns varies with the electromotive 

 force, according to the same law as in a single cell, so that the 



